Earth and the Moon

The Seasons

What are equinoxes and when do they happen?

An equinox is a time of the year when, in the course of Earth’s orbit, our planet is at a location where the equatorial plane and the ecliptic plane intersect. In other words, the tilt of Earth’s axis is pointed perpendicular to the line between Earth and the Sun at an equinox—Earth’s poles are tilted neither “toward” nor “away” from the Sun, but tilted at a tangent off to the “side.” On the day of an equinox, there are as many minutes of daylight as there are of night—hence the term “equinox,” meaning “equal darkness.” In the northern hemisphere, the vernal (spring) equinox occurs around March 21 of each year, and the autumnal (fall) equinox occurs around September 21.


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