Equinoxes happen twice a year when our Sun crosses Earth’s Celestial Equator.
This is when the centre of our Sun is on exactly the same plane as Earth’s equator. The Earth’s axis isn’t tilted toward the Sun one way or the other and the hours of night and day are almost equal in length.
Our Earth is quite laid back, about 23.5 degrees! The axis of rotation of our Earth is roughly 23.5 degrees, meaning that as the Earth orbits the Sun, the angle a point on the surface makes with the Sun varies. For example the Southern Hemisphere is angled towards the Sun during summer and away from it in winter. This axial discrepancy leads to changes in the amount of daylight any given place on the Earth receives.
Hours of daylight are greater in summer and less in winter. This means there has to be a ‘tipping point’ where hours of daylight are just about the same as the hours of night.
These dates are known as equinoxes, and there are two a year. The autumnal equinox occurs around March 21, and the vernal equinox occurs around September 23 each year.
On the dates of the equinox, the Sun is exactly above the Earth’s equator, causing the equal length of night and day. For 2021, the autumnal equinox occurs on the 20th March 2021 at 5:37pm, and the vernal equinox occurs on the 23rd September 2021 at 3:20am.
By the way, the Celestial Equator is just a projection of Earth’s equator onto the imaginary sphere that we picture surrounding our planet when we look up at the night sky. This imaginary sphere is called the Celestial Sphere.
Interesting facts about Equinoxes
- There are two equinoxes every year.
- In the Southern Hemisphere, the equinox in March is sometimes called the autumnal equinox and the equinox in September can be called the vernal or spring equinox.
- In the Northern Hemisphere, the names of the equinoxes are reversed.
- The Sun is half way between its most southern position in the sky and its most northern position.
- At equinox, sunrise is due east and sunset is due west.
- The number of daylight hours is almost equal to the number of hours of darkness.
ARE YOU GETTING INTO ASTRONOMY?
Stargazers Club WA is especially for beginners! We’re a wonderful community of stargazing and astronomy lovers.