17 March 2006

In the UK, Monday 20 March 2006 marks the Spring Equinox. The day also traditionally marks the first day of spring, but what is it all about?

Q: What is the equinox?

A: The Equinox is one of the two points in the year which occurs on or around 20/21 March and 22/23 September when the astronomical day and night are equal.

Q: What is the solstice?

A: The Solstice in astronomy is the period the Sun reaches its furthest angular distance to the equator. So solstices are the points in the year when the Sun reaches its southernmost or northernmost position and occurs around the 20/21 June and 21/22 December. The position of the Sun as seen from the Earth moves north and south through the year, when it changes direction it stands still momentarily, this is the solstice.

Q: What decides the date of the equinox and solstice?

A: Put simply, it is the position and movement of the sun.

• The equinox is when the sun crosses the celestial equator.
• The time of the solstices is decided by the moment at which the sun reaches its lowest or highest angle relative to the celestial equator. These are the longest or shortest days (in terms of hours of daylight) of the year, depending on which hemisphere you are in.

Q: So why does the actual day of the solstice and equinox vary?

A: The actual day of the equinox or solstice varies because of variation in the rotational axis of the Earth. The axis is not fixed, it exhibits a slow, but steady precession through the years.

Q: Is it the first day of spring?

A: Astronomically yes, but climatologically, because of the variation in the days on which the equinox and solstice falls, it is more convenient to use whole months. The Met Office therefore classifies the spring months as being March, April and May.

Earth's rotation around the Sun:

The satellites images show how the shadow across the Earth varies through the year: