National Solar Observatory

Glossary of Terms

National Solar Observatory

This glossary lists some of the difficult words that have been used in the Press Release pages, including various thing you might see in pictures of the Sun. If you want to know where a difficult word in these pages comes from, or what it means, then look it up in this list.

The division into syllables of the key word or words may be given between parentheses after the key word(s), with the syllables divided by periods (.). A quote indicates that the next syllable should be pronounced with more emphasis. If the first line of the description is written with italic letters, then that line contains extra information about the words: a guide to their pronunciation and/or information about their origin. Pronunciation hints are given between parentheses, introduced by the word pronounced:. Often the hints consist of a number of short words separated by dashes (-): the key word or words are pronounced something like what you get if you pronounce each of the words in the guideline separately and then string them together. The language that a word comes from is written between square brackets, (usually [Greek] or [Latin]).

A B C D E F G H I L M N O P R S T U V W X Z.


Absorption Line (ab.'sorp.tion line)
(pronounced: ab-sorp-shun line) absorptio = [Latin] absorption
An absorption line is a spectral line (a very narrow set of colors) at which an object shines less brightly than at nearby colors.

Active Region ('ac.tive 're.gion)
(pronounced: ek-tiff ree-jun) actif = [French] active, from [Latin] agere = to act; regio = [Latin] region, from regere = to direct, govern
An active region is an area on the Sun with a lot of magnetic field in the form of sunspots, pores, and plage. Big active regions may get to be 100,000 miles (160,000 km) long and stay around for two months or more, but small ones may appear and disappear again in a matter of days. The full disk H-alpha image shows an active region.

Albedo (al.'; plural: albedos)
(pronounced: al-bee-doh) albus = [Latin] white
The albedo of an object is a number between 0 and 1 that indicates the fraction of light that is reflected by the object. The albedo of the Earth is about 0.3: the Earth and its atmosphere reflect about 30 percent of the sunlight that hit them. All other things being equal, a planet looks brighter and is colder if its albedo is higher.

In geometry, an angle is a difference between directions. Angles are commonly measured in degrees. A full circle is equivalent to 360 degrees: If you turn around completely and end up looking in the same direction as you did before, then you've turned over 360 degrees. A right angle is equal to 90 degrees, and the angles between the sides of an equilateral triangle (one with three sides of equal length) is 60 degrees.

A degree is further divided into 60 arcminutes, and an arcminute into 60 arcseconds, so one degree is equal to 3600 arcseconds. Astronomical objects often appear so small in the sky that their apparent angular sizes are expressed in arcminutes or arcseconds. For instance, the Sun and Moon have angular diameters of about 30 arcminutes, the planet Jupiter has an angular diameter of about 40 arcseconds, and the planet Pluto of 0.1 arcseconds.

The smallest detail a telescope can possible see (which is called its resolution) is also often measured in arcseconds. It is about equal to 0.13 arcseconds divided by the diameter of the primary entrance of the telescope in meters, or to 130 arcseconds over the diameter in centimeters, or to 5.0 arcseconds divided by the diameter in inches. With a perfect 6-inch telescope, you may be able to see details as small as 0.8 arcseconds - so in such a telescope Jupiter would appear as a small disk, but Pluto would look like a point, just like the stars. Often, the resolution of telescopes is worse than what this formula yields, because the mirrors or lenses are not perfect and because the atmosphere of the Earth tends to blur the images.

Anomalistic (''lis.tic)
Anomalistic refers to a time period measured relative to the apsis of an orbit. The anomalistic month is the time period between two perigees of the Moon.

Anomaly (an'
In astronomy, anomaly is used to indicate several angles that are of interest in calculating positions along orbits of objects in the solar system.
  1. The true anomaly is the angle between the current position of the object and the perihelion of its orbit, as seen from the Sun (or, rather, the barycenter of the solar system, which is very nearly the same thing). If the true anomaly is equal to 0 degrees, then the object is nearest to the Sun. If the true anomaly is equal to 180 degrees, then the objects is furthest from the Sun (in the apoapse of its orbit).
  2. The eccentric anomaly is an angle that is related to both the true anomaly and the mean anomaly. It is encountered when solving Kepler's equation.
  3. The mean anomaly is what the true anomaly would be if the object orbited the Sun in a perfectly circular orbit (with eccentricity equal to 0) at a constant speed. The mean anomaly is 0 in the perihelion and 180 degrees in the apohelion, just like for the true anomaly, but in other points along the orbit the two have different values (unless the orbit is circular). The mean anomaly at a given time is often used as one of the orbital elements of an orbit.

Apoapsis (apo.'ap.sis; plural: apoapses)
apo = [Latin] related to; apsis = [Latin] orbit
The apoapsis of an orbit of one object around another is the point at which the one object is furthest away from the other object. Apoapsis is the general term for such a point, but there are also many specific terms for specific cases: the aphelion is the furthest point from the Sun in an orbit around the Sun. Likewise, apoastron is linked to other stars, apogee to the Earth, and apojove to Jupiter. The opposite is periapsis. The word apofocus is sometimes used instead of apoapsis, and apse instead of apsis.

Apofocus ('apo.'fo.cus; plural: apofoci)
apo = [Latin] away from; focus = [Latin] hearth
The apofocus is the same as the apoapsis.

Apsis ('ap.sis)
apsis = [Latin] orbit
An apsis is a position in an orbit that is at an extreme distance (either a minimum or a maximum) to the central object. The minimal distance is attained in the periapsis and the maximal distance in the apoapsis. When referred to particular celestial bodies, the "apsis" part may be replaced by the (greek) name of the body. For instance, the position in an orbit that is closest to the Earth is called the perigee. Instead of apsis, sometimes apse is used.

AU or Astronomical Unit
An AU is very close to the average distance between the Sun and the Earth. Distances between the planets and the Sun are often expressed in AU. 1 AU equals 93 million miles or 150 million km. The AU is not exactly equal to the average distance between the Sun and the Earth because that average distance varies a bit under the influence of the gravity of the other planets, while the AU is a fixed distance. The difference between the two is very small, however.


Barycenter ('ba.ry.'cen.ter)
barus = [Greek] heavy
The barycenter is the same as the center of mass. It is spelled barycentre in British English.


Celestial (ce.'les.tial)
caelestis, from caelum = [Latin] sky
Celestial means having to do with the sky. For instance, a celestial object is an object in the sky (or in space).

Center of Mass
The center of mass of a system is the average of the positions of all objects in the system, each position weighted with the mass of the object. The "objects" in the "system" may be anything at all: atoms in the Earth, or planets and the Sun in the Solar System, or stars in a galaxy. If outside forces act on the system, then in many cases the system reacts as if all of its mass is concentrated in the center of mass. If a system is symmetric, then its center of mass lies in the plane, line, or point of symmetry. For instance, the center of mass of a spherically symmetric planet or star lies in its very center. Forces between the objects in the system cannot influence the position of the center of mass.

Coelostat ('coe.lo.stat)
(pronounce: seal-o-stat) coelo = [Latin] sky; sta- = [Greek] to stand
A coelostat is a construction with two movable mirrors that (if the mirrors are in the right place and orientation) reflect sunlight along a particular path (for instance, down a fixed telescope tube) such that the image at the other end of the telescope does not appear to move or rotate when the Sun moves along the sky. One of the mirrors rotates around an axis (along its surface) that is parallel to the Earth's axis of rotation, at half the solar rate. The orientation of the other mirror is adjustable in all directions. The Evans Facility in Sunspot has a coelostat.
A coelostat is similar to a heliostat, but has a more complex design and (unlike a heliostat) yields an image in a fixed orientation.

Chromosphere ('
(pronounce: kro-mo-sfear) chrooma = [Greek] color; sphairos = [Greek] ball
The chromosphere is a layer in the Sun between about 250 miles (400 km) and 1300 miles (2100 km) above the solar surface (the photosphere). The temperature in the chromosphere varies between about 4000 K at the bottom (the so-called temperature minimum) and 8000 K at the top (6700 and 14,000 degrees F, 3700 and 7700 degrees C), so in this layer (and higher layers) it actually gets hotter if you go further away from the Sun, unlike in the lower layers, where it gets hotter if you go closer to the center of the Sun. The density in the chromosphere is much, much smaller than the density of air at sea level on Earth. At the top of the chromosphere there are only about 10 thousand million atoms in each cubic centimeter (100 thousand million atoms per cubic inch). The chromosphere shows up in images taken in the center of the H-alpha spectral line and also (briefly) near the beginning and end of a total solar eclipse. You can see the chromosphere in the Solar Layer Image.

Conjunction (con.'junc.tion)
(pronounce: kon-junk-shun) con = [Latin] together; junctio = [Latin] to join
When two heavenly bodies are in conjunction, then they are very close together in the sky. When astronomers say something like "Jupiter is in conjunction" without mentioning a second heavenly body, then they mean "with the Sun". In such a case Jupiter is not visible at any time of the night. The planets that are further away from the Sun than the Earth (the superior planets) have one conjunction each synodical orbital period. The planets that are closer to the Sun (the inferior planets, Mercury and Venus) have two conjunctions per synodical orbital period: one when they pass between the Sun and the Earth (the inferior conjunction), and one when they pass behind the Sun (the superior conjunction).

Continuum (con.'; plural: continua)
(pronounce: kon-tih-new-um) continuum = [Latin] something without breaks
The continuum is the name astronomers use for the combination of all colors that an object such as the Sun emits, and also for the broad variation from color to color in how much light is emitted. Broad means: without looking at the little details, such as spectral lines. The continuum is determined mostly by the temperature of the object. The hotter the object is, the brighter it shines. The color at which an object shines brightest also depends on the temperature. Hot objects such as the Sun shine brightest in yellow light; less hot objects shine most in red light, and cool objects shine brightest in (invisible) infrared light. The full-disk continuum image is an example of an image observed in the continuum.

Convection (con.'vec.tion)
(pronounce: kon-vek-shun) convectio = [Latin] bring together, from con- = with, and vectio = carry
Convection is a form of energy transport in which the material with the energy in it moves. The hotter material moves toward the cooler area and the cooler material toward the hotter area. On Earth, you can see convection in a pan of boiling water (where hot water moves up and cooler water moves down), and also in a thunderstorm (where warmer, moist air moves up and forms clouds). You can also find convection in the convection zone of the Sun, and in granulation.

Convection Zone (con.'vec.tion zone)
(pronounce: kon-vek-shun zown) convectio = [Latin] bring together, from con- = with, and vectio = carry; zona [Latin] from zoone = [Greek] girdle
The convection zone is a layer in the Sun that reaches from just below the surface (the photosphere) down to about 130,000 miles (183,000 km) below the surface. The convection zone contains about 2/3 of the Sun's volume (up to the visible surface) but only about 1/60 of the Sun's mass. In this layer the energy of the Sun is transported to the surface by convection. Thetemperature inside this layer is thought to vary between 2.0 million and 6500 K (4 million and 12,000 degrees F, 2.2 million and 6200 degrees Centigrade), and the density between 100 times more and 4,000 times less than that of air at the Earth's surface. You can see the convection zone in the Solar Layer Image.

The core of the Sun is the centermost part of the Sun, where all the Sun's energy is produced by nuclear processes. It has a radius of about 86,000 miles (140,000 km). It contains about 1/120 of the Sun's volume (up to the visible surface), and about 1/3 of the Sun's total mass. At the very center of the Sun, the temperature is thought to be about 16 million K (28 million degrees F, 16 million degrees Centigrade), and the density about 150 times that of water. At the outer edge of the core, the temperature is thought to be 9 million K (17 million degrees F, 9 million degrees C), and the density 34 times that of water. You can see the core in the Solar Layer Image.

Corona (co.'
corona = [Latin] from koroone = [Greek] crown
The corona is the outermost layer of the Sun, starting at about 1300 miles (2100 km) above the surface (the photosphere). Thetemperature in the corona is 500,000 K (900,000 degrees F, 500,000 degrees C) or more, up to a few million K. The corona is very dilute indeed (less than 1000 million atoms per cubic centimetre or 10,000 million atoms per cubic inch) and cannot be seen with the naked eye except during a total solar eclipse, or with the use of a coronagraph. The corona does not have an upper limit: you could say that the Earth moves through the solar corona, though the density of the material near the Earth is usually only a paltry few particles per cubic centimeter . You can see the position of the corona in the Solar Layer Image, and an image of the corona in the Corona Image Map.

Coronagraph (co.'
corona = [Latin] from koroone = [Greek] crown; graphia = [Greek] description
A coronagraph is an instrument that can look at the faint outer layers (the corona) of the Sun by covering the bright solar disk. The Evans Facility contains a coronagraph.

Coronal Hole (co.'ro.nal hole)
corona = [Latin] from koroone = [Greek] crown
A coronal hole is an area in the corona of the Sun that appears dark in pictures taken with coronagraphs or during total solar eclipses or in X-rays. There are often coronal holes around the north and south poles of the Sun, especially near the minimum of the solar cycle. Material can get from the solar surface into space through the coronal holes. You can see some coronal holes in the Corona Image Map.

Coronal Mass Ejection (co.'ro.nal mass e.'jec.tion)
corona = [Latin] from koroone = [Greek] crown, ejectus = [Latin] thrown out
Also called Coronal Mass Eruption or Coronal Transient, and generally abbreviated to CME. A CME is a huge eruption of material from the solar corona into interplanetary space. They can look like bubbles or loops or stranger shapes. When seen close to the Sun, these CMEs can be bigger than the Sun itself, but they are also extremely dilute and do not contain much material. Near the Earth, the material typically has a density of at most about 1600 protons per cubic inch (100 per cm^3), which is equivalent to about 19 orders of magnitude less than the mass density of air. See the Solar Eruption Page for more information about CMEs.

Coronal Streamer (co.'ro.nal
corona = [Latin] from koroone = [Greek] crown
A coronal streamer is a wisp-like stream of particles traveling through the corona of the Sun. Such streamers may be visible in pictures taken with a coronagraph, or during a total solar eclipse. They can be longer than the diameter of the Sun, but are very dilute and do not contain much material. Coronal streamers are thought to be associated with active regions and/or prominences and are most impressive near the maximum of the solar cycle. The material in such streamers moves away from the Sun and finally becomes part of the solar wind. You can see some coronal streamers in the Corona Image Map.

Cosmic Rays ('cos.mic rays)
kosmos = [Greek] order
Cosmic rays are actually particles (mostly helium nuclei, protons, and electrons) that travel through space with a relatively very large amount of energy. Because of their large amount of energy, cosmic rays are about as dangerous as X-rays or gamma rays. Cosmic rays have many sources. Some are formed in solar flares. Others come from beyond our solar system, such as the galactic cosmic rays.


Declination (''na.tion)
(pronounce: dek-lih-nai-shun) declinatio = [Latin] turning aside
The declination is the coordinate in the equatorial coordinate system in the sky that is similar to latitude on Earth. It ranges between -90 degrees at the southern celestial pole and +90 degrees at the northern celestial pole and is zero at the celestial equator. The other equatorial coordinate is the right ascension.

Doppler Velocity ('dopp.ler ve.'loc.i.ty)
(pronounce: dop-ler vay-loh-sih-tee) Doppler = [German] Christian J. Doppler
Doppler velocity is the speed in the direction of the line of sight, i.e. directed toward you or away from you. The Doppler velocity is named after the Doppler effect (named after the discoverer, Mr Doppler), which is the effect that the frequency of a tone changes when the speed of the source in the line of sight changes. The same effect occurs in frequencies of light and other electromagnetic radiation and enables us to measure velocities on the Sun. An image showing Doppler velocities is called a dopplergram. The Narrow-Band Image page shows dopplergrams. See also at redshift. Some people sometimes write Doppler with a small d (as in "doppler velocity").


Eccentricity ('ec.cen.'tric.i.ty)
(pronounce: ex-sen-trih-sih-tee) eccentricus, from ex = [Latin] out, and centrum = [Latin] center
The eccentricity of an orbit is one of the orbital elements. It is a number that indicates how much the orbit deviates from a circle. A circular orbit has an eccentricity equal to zero. An elliptical orbit has an eccentricity between zero and one. In this case, the eccentricity is equal to the difference between the lengths of the long and short axes of the ellipse, divided by the sum of those lengths. A parabolic orbit has an eccentricity of one, and a hyperbolic orbit has an eccentricity larger than one.

Orbits with eccentricities less than one are closed, so the objects in such orbits return to the same position regularly. Orbits with eccentricities greater or equal to one are open, which means that objects in such orbits never return to the same position.

Ecliptic (e'clip.tic)
ecliptica linea, from eclipsis = [Latin] to omit, fail, and linea = [Latin] line
The ecliptic is the path that the Sun appears to take between the stars, as seen from the Earth. All the planets and the Moon also stay close to the ecliptic.

Ecliptical Coordinates (e' co.'or.di.nates)
ecliptica linea, from eclipsis = [Latin] to omit, fail, and linea = [Latin] line, and coordinate = [French] position
Coordinates of objects in the sky relative to the ecliptic. The coordinates are: ecliptic longitude and latitude. The ecliptic latitude is zero on the ecliptic, +90 degrees at the north ecliptic pole, and -90 degrees at the south ecliptic pole. The ecliptic longitude is zero on the meridian that passes through the point where the Sun is during the vernal equinox, and is commonly expressed in degrees to the west of the equinox, between 0 and 360 degrees. The planets, Sun, and Moon have an ecliptic latitude (ignoring the sign) of less than 20 degrees as seen from Earth.

Electromagnetic Radiation (e'lec.tro.mag.'net.ic ra.di.'a.tion)
(pronounce: ee-lek-tro-mag-net-ik ray-dee-ay-shun) elektron = [Greek] gold or amber; he Magnetos lithos = [Greek] stone of Magnesia; radiatio = [Latin] radiate
Electromagnetic radiation is any kind of radiation that consists of alternating electric and magnetic fields and that can propagate even in a vacuum. Electromagnetic radiation is characterized by its wavelength (or, equivalently, its frequency or energy). Some different types of electromagnetic radiation, in increasing order of frequency and energy and decreasing order of wavelength, are: radio waves, microwaves, infrared light, visible light, ultraviolet light, X-rays, and gamma rays. In general, the later in the list the type is, the more dangerous it is. Visible light is the only form of electromagnetic radiation that we can see with our eyes.

Emission Line (e'mis.sion line)
emissio = [Latin] send away
An emission line is a spectral line (a very narrow set of colors) at which an object shines more brightly than at nearby colors.

Energy Transport (' 'trans.port)
energeia = [Greek] ability to do work; trans- = [Latin] over, portare = [Latin] carry
Energy transport can be done in three ways:
  1. by radiation.
  2. by convection (motion of a material).
  3. by conduction (transport through a material).

If you stick one end of a strip of metal in a box such that no light can get in, and then stick the other end of the strip in a fire, then all three kinds of energy transport occur. The hot gases rise, and so transport the energy (heat) by convection. The gases shine brightly both in the visible colors and in the infrared, and so transport energy by radiation. You can feel the infrared radiation on the exposed skin of your face and hands, and if you hold your hand in front of your face, then your face cools off because the radiation no longer reaches it. Conduction occurs in the metal strip. The enclosed end of the strip heats up, even though the end is inside a closed box, so that neither the hot gases nor the radiation can get to it. Instead, the heat moved through the metal.

The Sun is not solid, and conduction does not work very well in it. It has to choose between radiation and convection, and picks whichever method works best in a given situation. Basically, if thetemperature gradient (change with position) is small enough, then transport by radiation works best, as in the radiative zone. If the temperature gradient is too great, then transport by convection works better, as in the convection zone.

Something similar happens in a pan of water that is heated up: At first the temperature difference between top and bottom is small, so conduction and radiation work well and the water is at rest. After a while the temperature gradient becomes too large and then convection takes over: then the water boils.

Equator (e'qua.tor)
aequatio = [Latin] make equal
The equator is a plane equally far from both geographical poles and divides the Earth into a northern and a southern part. By extension, there is also an equator in the sky, which divides the sky into a northern and a southern part.

Equatorial Coordinates (equa.' co.'or.di.nates)
aequatio = [Latin] make equal, coordinate = [French] other distance
Coordinates of objects in the sky relative to the celestial equator. The coordinates are: right ascension (similar to longitude on Earth) and declination (similar to latitude on Earth). Observers in the northern hemisphere of the Earth cannot see the south celestial pole, and observers in the southern hemisphere cannot see the north celestial pole.

Equatorial Mounting (equa.'
aequatio = [Latin] make equal
An equatorial mounting is a way of mounting a telescope so that it can follow the Sun or Moon or star you're looking at by moving around one axis only. The movement of the telescope is then parallel to the equator. The Evans Facility has its telescopes mounted using an equatorial mounting.

Equinox ('equi.nox)
aequinoctium = [Latin] equinox, from aequatio = [Latin] make equal, and noctium, nox = [Latin] night
[1] An equinox is a moment when the Sun crosses the celestial equator. Close to an equinox, day and night have nearly the same length (12 hours) everywhere on Earth. The equinoxes signal the beginning of the seasons of spring and autumn. The March equinox is the spring equinox in the northern hemisphere, and the autumnal equinox in the southern hemisphere. The march equinox is also called the vernal equinox. The beginning of the other seasons is governed by the solstices.

[2] In celestial coordinate systems, the equinox is the point where the Sun is during the vernal equinox (see item [1]). The ecliptical and equatorial coordinate systems are linked to the equinox. Because of the precession of the equinoxes, the equinox slowly moves with time, so when one quotes ecliptical or equatorial coordinates one has to indicate relative to which equinox these coordinates are measured. Three equinoxes that are commonly used in stellar atlases and planetary calculations are those of 1950, 2000, and the equinox of the date (i.e., the equinox of the same date as the coordinates themselves).

Exponential Notation ('ex.po.'nen.tial no.'ta.tion)
(pronounce: ex-poh-nen-shal no-tay-shun)
Astronomers and other scientists often encounter very large and very small numbers in their work. Writing all of those zeros is not very convenient, so scientists often use exponential notation for such numbers. In exponential notation, you first write a number (possibly fractional, and generally between 1 and 10) called the mantissa, then an exponent marker (usually an "E" or an "e" in computer-related work), and then an integer number called the exponent. The exponent indicates over how many places you are supposed to shift the decimal point in the mantissa to get the indicated number. A positive exponent means "shift to the right", and a negative exponent "shift to the left". On calculators, the button that you must press before entering the exponent is usually labeled "EXP" or "EE".

For example, 2.3e-3 is the same as 0.0023, and 5e10 is the same as 50,000,000,000. In general, people use either exponential or normal notation, whichever is shorter. Exponential notation is convenient for describing numbers that are many orders of magnitude different from 1.


Facula ('; plural: faculae)
fax, facis = [Latin] torch
Faculae are individual small bright patches (of at most about 200 km or 100 miles in diameter) that are visible near the edge of the Sun in continuum observations, and also near the center of the solar disk in observations recorded in the centers of spectral lines. In groups, faculae form plage. They are thought to be places where small flux tubes are.

Filament ('fil.a.ment)
filum = [Latin] thread
Filaments are prominences seen against the solar disk. Because the solar disk is brighter than the filaments, the filaments appear dark. Filaments can be seen only in the centers of strong spectral lines, such as Ca K or H alpha. The full-disk H-alpha image shows several filaments.

Filigree ('fil.i.gree)
filum = [Latin] thread; granum = [Latin] grain
Filigree is the name of the strings of small bright points that are sometimes visible in intergranular lanes in continuum images. The bright points are thought to be places where flux tubes penetrate the photosphere. You can see filigree in the very-high-resolution continuum image.

Flux Tube
fluxus = [Latin] flow
Flux tubes are tube-like spaces where the magnetic field is strong. Small flux tubes (diameter less than about 200 miles / 300 km) form filigree and plage and may appear bright. Bigger flux tubes appear dark and are called pores (up to about 1500 miles / 2500 km diameter) or sunspots.


Gamma Rays (' rays)
gamma = [Greek] third letter
Gamma rays are a form of electromagnetic radiation with a large amount of energy. They have a large penetrating and destructive power.

Geocentric ('geo.'cen.tric)
geo = [Greek] Earth
Geocentric means relative to the center of the Earth. The geocentric coordinates of some celestial object are its coordinates as seen from the Earth.

Geographical ('geo.'
geo = [Greek] Earth, graphoo = [Greek] to write
Geographical means relative to the coordinates on Earth that are based on the rotation axis of the Earth. The geographical poles are the places where the rotation axis of the Earth penetrates the surface; those are commonly called the north and south poles. Geographical coordinates are the longitude and latitude.

Granule ('gran.ule)
granulus = [Latin] small grain, from granum = [Latin] grain
Granules are regions where hot material comes to the solar surface (the photosphere from below. All granules and intergranular lanes together are called granulation. A typical granule is about 600 miles (1000 km) across, and the centers of two adjacent granules are typically about 900 miles (1400 km) apart. Granules appear bright incontinuum images and are also visible in dopplergrams. Granules typically last about 5 to 10 minutes before they fade away. You can see granules in the high-resolution continuum image and in the very-high-resolution continuum image.

Granulation ('gran.u.'la.tion)
(pronounce: gran-u-lay-shun) granulus = [Latin] small grain, from granum = [Latin] grain
Granulation covers most of the visible surface (the photosphere) of the Sun. It looks a bit like rice pudding, with individual bright "rice kernels", called granules, separated from each other by a connected network of dark paths, called intergranular lanes. Granulation can be seen in the high-resolution continuum image and in the very-high-resolution continuum image.


H Alpha (h 'al.pha)
H-alpha is the historical name of a very strong spectral line of hydrogen (H) in the solar spectrum. Because the line is the first in a sequence of hydrogen lines, it has the qualifier "alpha". The wavelength of the spectral line is 6563 Å or 656.3 nm. An example of an image observed in H-alpha is the H-alpha Image Map.

Heliostat ('he.lio.stat)
helios = [Greek] Sun; stat = [Greek] to stand
A heliostat is a construction with one fixed mirror and one movable mirror that (if the mirrors are in the right place and orientation) reflect sunlight along a particular path (for instance, down a fixed telescope tube) such that the image at the other end of the telescope does not appear to move when the Sun moves along the sky. The movable mirror is mounted equatorially.
A heliostat is similar to a coelostat, but has a simpler design and (unlike the coelostat) makes the image of the Sun rotate as the Sun moves along the sky.


Inclination ('in.cli.'na.tion)
(pronounce: in-klin-ay-shun) inclinare = [Latin] to lean
In astronomy, inclination is an angle between some direction or plane and some reference direction or plane.
[1] Inclination is the name of the orbital element that indicates the angle between the plane of the orbit and the plane of the ecliptic.
[2] Inclination is also used for the angle between the equator of a planet and the plane of the ecliptic.

Infrared ('in.fra.'red)
infera = [Lat] below
Infrared radiation or light is a kind of electromagnetic radiation with wavelengths just beyond and frequencies just below those of visible light. Humans perceive infrared radiation as "heat radiation", which is one way of energy transport.

Intergranular Lane ('inter.'gran.u.lar)
inter = [Latin] between; granulus = [Latin] small grain, from granum = [Latin] grain
Intergranular lanes form a connected network around and between granules. Intergranular lanes appear dark in continuum images and mark places where cool solar material is going down below the surface. You can see intergranular lanes in the high-resolution continuum image and in the very-high-resolution continuum image.


Lagrange Point (La.'grange point)
Lagrange = [French] Joseph-Louis Lagrange A Lagrange point is a place relative to two large bodies (e.g, the Sun and the Earth, or the Sun and Jupiter) where the gravitational and centrifigal forces on a small body (e.g., an asteroid or a space ship) that remains at the same point relative to the two large bodies are balanced. There are five Lagrange points in any system of two large bodies, but generally only two of them are stable so that a small body near such a point will remain near it.

Latitude ('lat.i.tude)
latus = [Latin] wide
Latitude is a coordinate that is used to specify positions on a sphere. The latitude of a place is the distance of the place to the equator, measured in degrees along a circle between the two poles. Places on the equator have zero latitude, the north pole has latitude +90 degrees (or 90 degrees north), and the south pole has latitude -90 degrees (or 90 degrees south). In the sky, latitude is used in the ecliptical and galactic coordinate systems. The corresponding coordinate in the equatorial coordinate system is called declination. The second coordinate needed to specify a position on a sphere is the longitude.

Lightyear ('light.year)
A lightyear is the distance that light travels in one year. It is not an amount of time, even though it ends in "year". Since there are various definitions for the length of a year, e.g., based on the seasons (tropical), or on the motion of the Earth relative to the stars (sidereal), or on the regular calendar year, or on the average calendar year, there are correspondingly slightly different values for a lightyear. One lightyear corresponds to about 9.461e15 m, 5.879e12 mi, or 63239.7 AU, or 0.3066 pc. The nearest star (Proxima Centauri) is about 4 lightyears away. The center of our galaxy is some 30,000 lightyears away, and the Andromeda Nebula, our sister galaxy, is about 2 million lightyears away from us.

Line of Sight
The line of sight is the line that connects the observer's eye with the thing the observer is looking at. This expression is often used with measurements of doppler velocity and magnetic field in magnetograms, where only that part is measured that is directed along the line of sight. To know the complete velocity or magnetic field, the components that are at right angles to the line of sight must also be measured. Such components may be called the field-of-view components, though this expression is not commonly used.

Longitude ('
Longitude is a coordinate that is used to specify positions on a sphere. The longitude of a place is the distance of the place to the prime meridian, measured in degrees along a circle at a fixed distance to the poles of the sphere. All places with the same longitude lie on a half-circle between the poles. On Earth, the prime meridian of the common geographical coordinate system is the one passing through Greenwich in England. In the sky, longitude is one of the coordinates used in the ecliptical and galactic coordinate systems. The corresponding coordinate in the equatorial coordinate system is called right ascension. The second coordinate needed to specify a position on a sphere is the latitude.

Lunar Eclipse ('lu.nar e'clipse or 'lu.nar 'eclipse)
Luna = [Latin] MoonA lunar eclipse occurs when the Moon moves through the shadow of the Earth. The shadow of the Earth points in exactly the opposite direction from the Sun and the Moon only rarely moves through it. Lunar eclipses occur only when the Moon is full (but not every time the Moon is full). When the Moon is partly in the shadow of the Earth, then you can see that the shadow of the Earth is round, which means that the Earth itself must be round, too. There are also solar eclipses.


Magnetic Field (mag.'net.ic field)
he Magnetos lithos = [Greek] stone of Magnesia
The magnetic field is a force field that is linked to moving electrical charge and affects charged particles and certain metals by either attracting or repulsing them. Almost all solar material is affected by magnetic field. Magnetic field behaves as if it consists of closed magnetic field lines (such as might be observed when a magnet is held under a glass plate with iron filings on it). Magnetic field in the Sun appears to exist in only two forms: Either it is so weak that it is passively swept along by the material, or it is so strong that it hinders movement (e.g. convection) of the material.

In the latter case the magnetic field exists in the form of flux tubes: isolated tube-like structures in which the magnetic field is strong, while it is weak or absent outside the tube. Most of the interesting features on the Sun are associated with magnetic field: sunspots, pores, plage, filaments, solar flares, and prominences. A famous quote in solar physics is

If the Sun had no magnetic field, then it would be as boring as most people think it is.

If you know who first said this, then please let me know.

Solar astronomers measure the strength of magnetic field in gauss (G). The magnetic field of the Earth is at most 1 G strong. The magnetic field inside a sunspot at the visible surface (the photosphere) of the Sun can get up to 3000 G strong. You can see magnetic field in magnetograms such as that on the Narrow-Band Image page. You can read about variations in the solar magnetic field and the effects this may have on the Earth in the Polarity page.

Magnetogram (mag.'net.o.gram)
A magnetogram is a kind of image that shows a measure of the strength of the magnetic field in the direction of the line of sight. Magnetograms are constructed from two narrow-band images taken in a spectral line that is sensitive to the magnetic field. Grey areas in a magnetogram indicate that there is no magnetic field in the direction of the line of sight, while black and white areas indicate regions where there is such magnetic field. The narrow-band image page shows a magnetogram.

Mercury ('
Mercurius = [Latin] messenger god, same as [Greek] Hermes
The Romans and Greeks of long ago believed in many Gods, and Mercurius/Hermes was the messenger of their Gods. Mercurius delivered his messages very fast, and so other fast things were named after him, such as the planet Mercury, which goes fastest around the Sun of any planet, and the metal mercury, which is liquid (like water) at room temperature and so moves very fast. The metal mercury is used in old thermometers and barometers. Another name for the metal mercury is quicksilver, which means "live - i.e. moving - silver".

Microwaves ('mi.cro.waves)
mikros = [Greek] small
In physics and astronomy, microwaves are a form of electromagnetic radiation with wavelengths between those of ultraviolet radiation and those of radio waves.

A month is a period of time that is connected with the motion of the Moon around the Earth. Various kinds of months are in use:
  • calendar month: a period of between 28 and 31 days that divides up calendars. In some calendars the months are no longer related to the actual motion of the Moon, and in others the beginning of months are linked to particular phases of the Moon.
  • sidereal month: the period after which the Moon returns to the same position (longitude) relative to the stars. On average 27.32 days.
  • synodical month: the period after which the Moon has the same phase (e.g., new moon). On average 29 days, 12 hours, 44 minutes, and 3 seconds.
  • tropical month: the period after which the Moon returns to the same position relative to the vernal equinox. On average 27 days, 7 hours, 43 minutes, and 12 seconds.
  • anomalistic month: the period after which the Moon returns to the same position relative to its perigee. On average 27 days, 13 hours, and 19 minutes.


Narrow-Band Image (' 'im.age)
A narrow-band image is an image that was created using light of only a very narrow range of wavelengths of the electromagnetic spectrum, usually in a spectral line or in the continuum. The Narrow-Band Image page shows a number of narrow-band images.


Opposition ('op.po.'si.tion)
(pronounce: op-poh-sih-shun)
Two celestial bodies (planets, Sun, Moon, asteroids, etcetera) are in opposition when they are in opposite directions in the sky. In that case, the two bodies are never visible at the same time. When astronomers say something like "Jupiter is in opposition" without indicating with what other body Jupiter is in opposition, then they mean "with the Sun". When a planet is in opposition, it is above the horizon all night long.

Orbital element (' 'el.e.ment)
The orbits of objects around much heavier objects (e.g., a planet around the Sun, or a moon around a planet, or a spacecraft around a moon, planet, or Sun) are usually close to what are called in geometry conic sections, i.e., circles, ellipses, parabolas, hyperbolas, or straight lines. Five numbers are needed to fully describe the size, shape, and orientation of such an orbit, and a sixth is necessary to specify the position of the object in the orbit. These numbers are called the orbital elements.

In the most commonly used set of orbital elements, the semimajor axis and the eccentricity specify the size and shape of the orbit. Three more, the inclination, the longitude of the ascending node, and the argument of periapsis (together called the Euler angles), determine the orientation of the orbit in space, and the remaining one, for which usually either the mean anomaly at a specified time or else a time of periapsis is taken, determines the position of the object along the orbit.

Order of Magnitude ('or.der of '
An order of magnitude is about a factor of ten. This phrase is often used by astronomers and physicists, who investigate very small and very big things and have to deal with mind-boggling numbers that may be quite inaccurate.

If an astronomer says that thing A is three orders of magnitude greater than thing B, then the astronomer means that thing A is about 1,000 times greater than thing B. It may be a bit bigger than that (perhaps 3,000 times greater), and also a bit smaller (maybe only 500 times greater), but it is not 100 times greater, or 10,000 times greater. For each extra order of magnitude, you have to multiply by an extra factor of 10. The order-of-magnitude phrase is especially useful for really big numbers. For instance, it is easier to speak of 18 orders of magnitude (which is about the ratio between the diameter of the Earth and the diameter of an atom, or between the diameter of the Universe and the diameter of the Earth) than it is to speak of the number one million million million, or 1,000,000,000,000,000,000, or 1e18.


Parallax ('
parallaxis = [Greek] change
Parallax is the effect that nearby objects seem to move relative to far-away objects when you move to another viewpoint. For instance, if you hold your finger in front of your nose and look at it with just one eye at a time (without moving your finger), then you'll notice that your finger blocks different parts of the background depending on which eye looks at it.

The parallactic angle is the angle over which the nearby object seems to move relative to the background. In astronomy, the maximum parallactic angle of a star due to the motion of the Earth in its orbit around the Sun is called the star's parallax. Because stars are very far away, their parallaxes are very small: the nearest star has a parallax of only 0.76 arcseconds.

Parsec ('par.sec)
from parallactic arcsecond
A parsec (abbreviated pc) is the distance at which a star or other object shows a parallax of one arcsecond due to the motion of the Earth around the Sun. One parsec is exactly equal to 648000/pi AU, and approximately equal to 206265 AU, 3.086e16 m, 1.917e13 mi, and 3.2616 lightyears. A kiloparsec (abbreviated kpc) is 1000 pc (or about 3262 lightyears), a megaparsec (abbreviated Mpc) is 1,000,000 pc (or about 3 million lightyears), and a gigaparsec (abbreviated Gpc) is 1,000,000,000 pc.

Penumbra ('; plural: penumbrae or penumbras)
paena = [Latin] almost; umbra = [Latin] shadow
A penumbra is an annulus (thick ring) around the umbra of a sunspot. The penumbra is less dark than the umbra, but is darker than granulation. The penumbra appears to be built up of many fibers that point away from the center of the sunspot. The high-resolution continuum image shows a big sunspot with a penumbra.

Periapsis (peri.'ap.sis; plural: periapses)
peri = [Latin from Greek] around; apsis = [Latin] orbit)
The periapsis of an orbit of one object around another is the point at which the one object is closest to the other object. Periapsis is the general term for such a point, but there are also many specific terms for specific cases: the perihelion is the closest point to the Sun in an orbit around the Sun. Likewise, periastron is linked to other stars, perigeum to the Earth, and perijove to Jupiter. The opposite is apoapsis. The word perifocus is also sometimes used.

Perifocus (peri.'fo.cus; plural: perifoci)
peri = [Latin from Greek] around; focus = [Latin] hearth
The perifocus is the same as the periapsis

Photosphere ('
phootos = [Greek] light; sphairos = [Greek] ball
The photosphere is the deepest layer of the Sun that we can observe directly. It reaches from the surface visible at the center of the solar disk to about 300 miles (500 km) above that. The radius of the Sun (from its center to the visible surface) is 432,000 miles (696,000 km). The temperature in the photosphere varies between about 6500 K at the bottom and 4000 K at the top (11,000 and 6700 degrees F, 6200 and 3700 degrees C) and a density which is between about 4000 and 200,000 times smaller than the density of air at sea level on Earth. Most of the photosphere is covered by granulation. You can see the photosphere in the Solar Layer Image.

(pronounce: plaa-dzj) plage = [French] beach, from plagios = [Greek] tilted
Plage is any area at the solar surface (the photosphere that looks brighter than its surroundings when observed in the center of a spectral line. The extra brightness indicates the presence of small magnetic flux tubes that stick through the surface there. Plage cannot be seen in continuum images, except near the limb of the Sun, but even there the contrast between plage and its surroundings is quite low. Plage consists of facula. Examples of plage can be seen in the full-disk H-alpha image.

Polarity (po.'lar.i.ty)
polarité = [French] polarity, from polus = [Latin] pole, from polos = [Greek] turning pointPolarity is a word that indicates one of two opposing choices. In solar physics it is mostly used to distinguish whether magnetic field is pointing (somewhat) toward you or away from you. This is called the magnetic polarity. Read more about magnetic polarities on the Polarity Page.

poros = [Greek] small entrance
Pores are like small sunspots but without a penumbra. Pores get up to about 1500 miles (2500 km) in diameter and are less dark than sunspot umbrae. The high-resolution continuum image shows some pores.

Precession of the Equinoxes (pre.'ces.sion of the '
The Earth wobbles slowly around the poles of the ecliptic because of gravitational effects of the Sun and the Moon on the equatorial bulge of the Earth. One wobble takes about 26,000 years. One result of the wobble is that the point among the stars where the Sun is at the beginning of (northern hemisphere) spring (the vernal equinox) moves, by about one degree every 71.6 years. The position of the Sun at the beginning of the other seasons (the equinoxes and solstices) moves similarly. This movement is called the precession of the equinoxes. The vernal equinox has been in the modern constellation of the Fishes (Pisces) since about 68 BC, and will move into the constellation of the Waterman (Aquarius) around 2597 AD.

Prominence ('prom.i.nence)
prominentis = past participle of prominere = [Latin] to stick out, be prominent
Prominences are clouds of solar material that sit up to about 30,000 miles (50,000 km) above the solar surface (the photosphere). They can be observed in the center of strong spectral lines, but not in the continuum. When seen beyond the limb of the Sun, these clouds appear bright. When seen against the solar disk, the clouds appear dark and are called filaments. Filaments and prominences can stay around for up to about two months, though some of them disappear much faster. Some seem to appear as a result of a solar flares.

Proper Motion ('pro.per 'mo.tion)
The proper motion of a star or other celestial object is its motion along the sky, measured in units of angle per time period (e.g., arcseconds per year). If the object's distance is known also, then proper motion can be translated to a speed perpendicular to the line of sight. If the object's doppler velocity is known, too, then one has all components of its motion.


Radiative Zone ('rad.i.a.tive zone)
radiatio = [Latin] radiate; zona = [Latin] zone, from zoone = [Greek] girdle
The radiative zone is the layer of the Sun just outside of the core. It reaches between about 86,000 miles (140,000 km) and 319,000 miles (513,000 km) from the center of the Sun, which is the same as between 346,000 miles (556,000 km) and 113,000 miles (183,000 km) below the surface (photosphere) of the Sun. It contains about 1/3 of the Sun's volume (up to the visible surface) and about 2/3 of its mass. Energy transport in this layer is by radiation, so the material itself is at rest. Thetemperature inside the radiation zone is thought to vary between 9 million K and 2.0 million K (17 million and 4 million degrees F, 9 million and 2.0 million degrees C). The density varies between 34 and 0.14 times that of water. You can see the radiative zone in the Solar Layer Image.

Radio Waves ('ra.dio waves)
radius = [Latin] stick
A form of electromagnetic radiation with wavelengths of a few meters (yards) or more. Some different type of radio waves are, in order of increasing wavelength: FM radio waves, AM radio waves, shortwave radio.

Redshift ('red.shift)
Redshift is the displacement of electromagnetic radiation such as light to longer wavelengths because the source of the radiation is moving away from the observer. Redshift is a kind of doppler shift. All material in the Universe, except for the closest few galaxies in our own Local Group, are moving away from us and therefore their light shows redshift. From the measured redshift we can determine the distance of the source, through Hubble's Law.

Right Ascension (right as.'cen.sion)
(pronounce: as-sen-shun) ascensio = [Latin] going up
The right ascension is the coordinate in the equatorial coordinate system in the sky that is similar to longitude on Earth. It is commonly measured in hours, minutes, and seconds, between 0 and 24 hours. The vernal equinox has right ascension zero. The other equatorial coordinate is the declination.


Semimajor axis ('semi.'ma.jor '
semi = [Latin] half; maior = [Latin] great; axis = [Latin] axis
The semimajor axis is one of the orbital elements that determine the shape and orientation of a simple orbit in space. It is a measure for the size of the orbit of a celestial object. For an ellipse, it is equal to one half of the length of the longest axis of the ellipse. For a circle, it is equal to the radius of the circle. For an object orbiting around a much heavier object (such as a planet or spacecraft around the Sun, or a moon or spacecraft around a planet), the semimajor axis of the orbit is equal to the average of the smallest and the largest distance between the light and the heavy objects.

Sidereal (si.'de.real)
sideris = [Latin] star
Sidereal refers to something (usually a time period) measured relative to the stars. The sidereal day is the time period after which the same star returns to the same position in the sky. The Earth's sidereal day is about 4 minutes shorter than the common (so-called tropical) day between successive noons. The sidereal month is the time period after which the Moon returns to the same position relative to the stars. It is about 2 days shorter than the synodical month between two successive Full Moons. Another time period is the synodical one.

Solar Constant ('so.lar 'con.stant)
The solar constant is the amount of energy from the Sun that reaches the vicinity of the Earth per unit area, measured perpendicular to the direction to the Sun. This would be how much energy would hit the ground on Earth where the Sun is in the zenith if the Earth did not have an atmosphere to absorb or reflect part of the energy. The "solar constant" is not, in fact, constant, but varies slightly with time around an average value of about 1372 W/m^2.

Solar Cycle ('so.lar cy.cle)
sol = [Latin] Sun; kuklos = [Greek] circle
The Solar Cycle is an approximately 11-year cycle in the magnetic activity of the Sun. The most obvious effect is the variation in the number of sunspots visible on the Sun, as can be seen in a plot on the Sunspots page of Mr Sunspot's Answer Book. There are also other effects, which are further described on the mentioned Sunspots page and on the Polarity page.

Solar Eclipse ('so.lar e'clipse or ' 'eclipse)
Sol = [Latin] Sun; eclipsis = [Latin] eclipse, from ekleipsis, ekleipoo = [Greek] be delayed
A solar eclipse occurs when the Moon covers the Sun in the sky. Solar eclipses are fairly rare. A picture of the Sun's corona taken during a solar eclipse can be seen on the Evans Facility Pictures page.

Solar Flares ('so.lar flare)
Sol = [Latin] Sun
Solar flares are "explosions" (violent events) on the Sun in which tension in the magnetic field is released, which generates a lot of energy that heats up material and shoots it into outer space. Solar flares are always associated with active regions.

Solar Physics ('so.lar 'phys.ics)
Sol = [Latin] Sun; phusike = [Greek] of Nature, from phusis = [Greek] Nature
Solar physics is the physics of the Sun: the study of how the Sun works. Solar physics is part of astronomy.

Solar Wind ('so.lar wind)
Solar wind is the name astronomers use for the steady stream of particles (mostly hydrogen and helium ions and electrons) that flows away from the Sun at all times. The solar wind is very dilute: In a laboratory on Earth it would be considered a vacuum. You can see the solar wind in the Corona Image Map.

Solstice ('sol.stice)
solstitum = [Latin] solstice, from sol = [Latin] Sun, and stitum = [Latin] unmoving
A solstice is a moment when the Sun reverses its motion between the stars (as seen from the Earth) from northward to southward or the other way around. The longest and shortest days and nights occur near the solstices. These moments signal the beginning of the seasons of winter and summer. The December solstice is the winter solstice in the northern hemisphere, and the summer solstice in the southern hemisphere. The other two seasons are governed by the equinoxes.

spatium = [Latin] space
Space or Outer Space are the seemingly empty places (vacuum) between planets and stars. Space is not really empty, but the material in space is so dilute that it is hard to detect it. Space looks black. A particular camera or detector cannot observe all kinds of radiation (such as all colors of light, infrared and ultraviolet radiation, radio waves, and X rays), so if a bright object happens not to emit radiation of the kinds that the camera can detect, then the picture looks like a picture of outer space.

Spectral Line ('spec.tral line)
spectral = [French] of spectrum, from spectum = [Latin] to watch
A spectral line is a very narrow range of colors at which an object such as the Sun shines less brightly (usually) or brighter than at nearby colors. A spectral line in which an object shines less brightly is called an absorption line, and a line in which the object shines brighter is called an emission line. Each type of atom or ion has its own set of associated spectral lines, so spectral lines act as a fingerprint of the associated material. The strength of a spectral line (the amount of absorption or emission in it) depends on many things, including thetemperature and pressure of the material, and for some lines also the strength of the magnetic field, so astronomers often use filters to look at only one particular spectral line to measure one or more quantities to which the spectral line is sensitive.

Spectrum ('spec.trum; plural: spectra or spectrums)
spectum = [Latin] to watch
A spectrum in general is a wide variety of things. In astronomy, the (electromagnetic) spectrum is how the energy is divided among all wavelengths of electromagnetic radiation. The visible part of the spectrum includes all colors of the rainbow.

Sunspot ('
Sunspots are regions where the magnetic field is very strong. Sunspots appear darker than their surroundings in almost all kinds of observations because they are a few thousand degrees cooler than their surroundings, and because they are fairly big. Sunspots range in diameter between about 1500 miles (2500 km) and more than 30,000 miles (50,000 km). A sunspot is roughly circular in shape, though some are have a very irregular shape. Sunspots have two distinct parts: the umbra and the penumbra. You can tell a sunspot apart from a pore because a pore has no penumbra. Several sunspots can be seen in the full-disk continuum image. For more information, see the Sunspots page of Mr Sunspot's Answer Book.

Synodical (syn.'
sunodos = [Greek] meeting, from sun = [Greek] together, hodos = [Greek] roadA synodical time period (month, period, year) refers to the time period as it appears from a moving viewpoint - usually the Earth. For instance, the time period between two successive Full Moons is a synodical month. The time period after which a particular sunspot returns to the center of the solar disk as seen from the Earth is the synodical rotation period of the Sun. The time period between two oppositions of a planet is its synodical period. A different time period is the sidereal period.


Telescope ('tele.scope)
telescopio = [Italian] telescope, from tele = [Greek] far; skopeoo = [Greek] to see
A telescope is an instrument that can "see far": make faraway things appear close by. Some of the telescopes in Sunspot are the Evans Facility, the Vacuum Tower Telescope, and the Hilltop Dome. You can also read Mr Sunspot's Answer Book pages about Earth-bound telescopes.

Transit ('tran.sit)
The transit of a celestial object is when it crosses the prime meridian in the sky (which goes from the north celestial pole through the zenith and the south point on the horizon to the south celestial pole). The time when the object is at the greatest height above the horizon is practically the same as the time of its transit.

Transition Region (tran.'si.tion 're.gion)
transitio = [Latin] change, from trans = [Latin] over, ire = [Latin] to go
The transition region is a very narrow (60 miles / 100 km) layer between the chromosphere and the corona where thetemperature rises abruptly from about 8000 to about 500,000 K (14,000 to 900,000 degrees F, 7700 to 500,000 degrees C). You can see the transition region in the Solar Layer Image.

Tropical ('
In time-keeping, tropical refers to a time period measured relative to the vernal equinox, i.e., relative to the seasons. We have, for instance, the tropical year and month.

Tropical Year (' year)
The tropical year is the year as defined by the seasons: the mean interval between vernal equinoxes. At the moment it is 365.24219 days long and decreases by 5.3 seconds per thousand years. Many calendars try to follow the tropical year.


Ultraviolet ('
ultra = [Latin] beyond; violet = [Old French] little viole, from viola = [Latin] violet (the flower)
Ultraviolet "light" (abbreviated: UV) is a form of invisible electromagnetic radiation with wavelengths just below those of visible light. UV radiation is more powerful than visible light and the more powerful forms of it may cause damage to your skin.

Umbra ('um.bra; plural: umbrae)
umbra = [Latin] shadow
The umbra is the centermost, darkest part of a sunspot. It can get over 12,000 miles (20,000 km) in diameter, though most umbrae are at most about half that size. The magnetic field in umbrae is strong: from 1500 up to about 3000 G. Umbrae are darker than their surroundings because they are cooler. Most of the visible solar surface has an averagetemperature of some 5700 K (10,200 degrees F, 5400 degrees C), but the visible parts of big umbrae have temperatures of only about 3700 K (6600 degrees F, 3400 degrees C). A big umbra can be seen in the high-resolution continuum image.


Vacuum ('; plural vacua or vacuums)
vacuum = [Latin] empty space, from vacuus = [Latin] empty
Something that contains a vacuum is completely empty, and does not even have any air in it. In practice, it is very hard to make a pure vacuum, so the name is also used when there is hardly any air or other material. The Vacuum Tower Telescope has a vacuum in its main tube.

Visible Light ('vis.i.ble light)
Visible light is a form of electromagnetic radiation which humans can perceive with their eyes and which has wavelengths between those of infrared and ultraviolet radiation. The Sun emits most of its radiation as visible light, which is probably why our eyes are sensitive to it. So-called white-light images show observations of visible light.


Wavelength ('wave.length)
The wavelength of a wave pattern is the distance from one wave crest to the next one. All things that vary periodically have a wavelength associated with them, including light and other electromagnetic radiation, sound, AC electricity, spectral lines, and water waves.

White Light
Astronomers speak of white-light observations or images if all colors of light were allowed into the eye or camera, so that the thing you are looking at appears in its natural colors. So, the "white light" is about what colors were allowed, not about what colors are actually in the picture: a white-light picture of a green ball would show a green ball. You can see a white-light picture of the Sun on the Full-disk Continuum Image page.


X rays
X rays are a form of electromagnetic radiation with wavelengths shorter than those of ultraviolet light. X rays carry a lot of energy and therefore have a relatively large penetrating (and destructive) power.


Zenith ('ze.nith)
samt ar-ra's = [Arabic] way over your head
The zenith is the direction that is straight up. When the Sun is in the zenith, then shadows are commonly the shortest.

Dave Dooling and Ruth A. Kneale, Webmaster | February 21, 2005