As the Earth travels around the Sun in its orbit, the north to south position of the Sun changes over the course of the year due to the changing orientation of the Earth's tilted rotation axes. The dates of zero tilt of the Earth's equator correspond to the Spring Equinox and Autumn Equinox.
What are Equinoxes?
Equinoxes occur when the axis of rotation of the earth (i.e. the line form the N to S poles) is exactly parallel to the direction of motion of the earth around the sun. This happens on just two days of the year, the spring and autumn equinoxes. This means that day length is exactly the same (12 hours) at all points on the earth's surface on these days (except right at each pole, where it will be about to change from permanent light to dark, or vice versa).
Where does the name Equinox come from?
The name is derived from the Latin aequus (equal) and nox (night), because at the equinox the night and day are nearly equally long.
How many times a year does a Equinox occur?
Equinoxes occur twice a year.
Day and night have approximately the same length.
Spring Equinox is near 20 March.
The posh name for Spring Equinox is Vernal Equinox.
Did you know?Equinoxes do not always occur on the same day each year, and generally will occur about 6 hours later each year, with a jump of a day (backwards) on leap years.
Why do the equinoxes not always occur on the same day each year?
The reason is due to the time the Earth takes to go around the Sun and our calendar.
The Earth takes approximately 365.25 days to go around the Sun, yet our year is 365 days. Every 4 years, we have a leap year where another day is added to our calendar to make up for the 4 missing quarters. It is important to do this so that there is not a gradual drift of date through the seasons.
For the same reason the precise time of the equinoxes are not the same each year.