The equinoxes

Because of the angle between the celestial equator and the ecliptic the path of the Sun through the sky varies from one time of year to another.

The equinox is a point where the ecliptic crosses the celestial equator – it does this twice a year as you can see from Figure 1. At the Spring (vernal) equinox the Sun crosses the celestial equator from the south to the north. At the autumnal equinox the Sun crosses the celestial equator from the north to south.

At the equinoxes the Sun rises due east and sets due west and day and night are the same length. In summer and winter the Sun follows a higher or lower path through the sky. The position of the Sun at midwinter is called the winter solstice and that at midsummer is the summer solstice.

There are two equinoxes in a year – the spring equinox (on or near 21st March) and the autumnal equinox (on or near 23rd September). The spring, summer, autumn and winter periods are not the same length because the Earth's axis is elliptical and not circular.

The two following diagrams show where the ecliptic crosses the celestial equator at the Spring and Autumnal equinoxes.

Note: The point where the ecliptic crosses the celestial equator in the Spring is called the First point of Aries. You will notice that it is not in the constellation of Aries at all but in Pisces – that is because of precession – the change in the direction of the Earth's axis over the centuries. The same applies to the autumnal equinox – it is called the first point of Libra although it is now in the constellation of Virgo.