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.
Where does the name Equinox come from?
How many times a year does a Equinox occur?
Equinoxes occur twice a year.
The Spring Equinox
The Spring Equinox is the first day of spring season and occurs when the sun passes the equator moving from the southern to the northern hemisphere. The North Pole begins to lean toward the sun again.
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.