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In order to cast a shadow on the Earth's surface after sundown, how bright must a celestial object appear -- as bright as the full Moon, the 1/4 Moon, Jupiter? Which objects qualify as shadow makers?

I think we've all been outside on a clear moonlit night and have been surprised just how bright the landscape can appear when illuminated by reflected sunlight -- moonlight. When the Moon is full, it's even possible to read a newspaper outdoors. Of course, the brightness of the full Moon literally pales in comparison to that of the Sun, but its nonetheless sufficiently bright to cast shadows. Next to the Sun, our lone natural satellite is by far the brightest object in the sky. The Sun's apparent magnitude is -26.7, and the full Moon's apparent magnitude is about -12.7. This translates to sunlight being approximately 400,000 brighter than the light from the full Moon.

Are the brightest stars bright enough to produce a shadow on moonless nights? The answer is no. While it's possible to see the reflection of several bright (first magnitude) stars off the surface of a still pond or lake, even the brightest star (Sirius - the Dog star) is too dim to generate a shadow. However, it's indeed possible to see shadows cast by Venus!

This month Venus gleams like a polished diamond in the pre-dawn sky. Approximately every 19 months, Venus reaches its maximum brilliance. Formerly referred to as our "Sister Planet," Venus is usually the third brightest object in the night sky. Note that Jupiter can be brighter on occasion. Since Venus orbits the Sun more rapidly than the Earth, it passes us every 584 days or 19 months. However, when Venus is closest to us, it's invisible since it's obscured by the Sun's overwhelming glare. Therefore Venus is brightest, from our perspective, not when it's nearest but when it flanks the Sun (just before dawn or just after sunset. At these times, it's takes on a crescent or ¼ phase and attains a -4 magnitude.

If skies are clear, and if you're outside of the city and away from street-lights, see if you can detect your shadow in the light cast by Venus. Though it's feeble compared to the brightness of the full Moon, or any phase of the Moon for that matter (the full Moon is about 1,500 times brighter than Venus at its brightest), it's possible to see your shadow from Venus's reflected sunlight if conditions are right. If you're in a darkened location, you should be able to see your Venus shadow without any problem. It helps if the surface isn't too dark because it's easier to see subtle shadowing on light-colored surfaces such as snow, a sandy beach or a white sheet spread out on the ground.

Shadows cast by Venus are much more distinct than one might imagine. The reason is that the light from Venus appears to originate from a point rather than a disk, such as light that comes from the Sun or the Moon. The bigger or closer the disk, the blurrier the shadow becomes.

I mentioned earlier that Jupiter can be brighter than Venus on occasion, but it never achieves a magnitude of -4, which it turns out is the magic number needed to be a shadow maker. Though usually not as bright as Venus, Jove at its brightest has a magnitude of about 2.8 -- more than twice as bright as Sirius (magnitude 1.46). Venus at its brightest is 3-4 times brighter than Jupiter at its maximum magnitude.

Are there any other celestial objects capable of throwing shadows? Certainly, a fireball is bright enough to briefly produce shadows, and even the Great Comet of 1910 during its breathtaking 1910 visit likely cast shadows. This magnificent apparition was visible for a time even when the Sun was up! Halley's Comet also graced the skies with its presence the same year - certainly one of the best ever for stargazers. However, Comet Halley reached a peak magnitude of about 0.0, bright, but not stupendously so. On Halley's subsequent loop around the Sun in 1985/86 it was barely visible for residents in the Northern Hemisphere, and though some of us will have flamed out long before Halley comes around again in 2061, it's predicted to be quite dramatic but still won't be as bright as the Great Comet.

One other possible shadow maker is a supernova. The supernova of 1604 was almost as bright as Jupiter. This was the last supernova discovered in the Milky Way (visible with the unaided eye), and it was approximately 6,000 light years distant. A supernova even more brilliant than this one or one much closer (maybe too close for comfort) would have to ignite in order to be capable of casting shadows.

Venus and Jupiter are both stunning in the morning sky this month. In fact, if you keep track of Venus as the Sun rises, you'll be able to see it in broad daylight.

For more about this see the Skywatcher's Diary - http://www.pa.msu.edu/abrams/diary.html

Star Magnitudes -

Also see the Science Question of the Week for February 22, 2001.

This week's question comes from Dr. James Foster. Dr. Foster originated this series and did it as a solo project for the GSFC website for SEVEN YEARS! Two years ago, Dr. Foster has decided to share the enthusiasm he has for this project with other Goddard scientists and will be posing questions on a semi-regular basis.