There are two equinoxes every year – in September and March – when the sun shines directly on the equator and the length of day and night is nearly equal. Seasons are opposite on either side of the Equator, so the equinox in September is also known as the autumnal (fall) equinox in the Northern Hemisphere, and is considered the first day of fall.
In the Southern Hemisphere, it is known as the vernal (spring) equinox and marks the first day of spring. The September equinox occurs the moment the Sun crosses the celestial equator – the imaginary line in the sky above Earth’s Equator – from north to south. This happens either on September 22, 23, or 24 every year.
Earth's axis is tilted at an angle of about 23.4° in relation to the ecliptic, the imaginary plane created by Earth's path around the Sun. On any other day of the year, either the southern hemisphere or the Northern Hemisphere tilts a little towards the Sun. But on the two equinoxes, the tilt of Earth's axis is perpendicular to the Sun's rays, like the illustrations show.
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What is an Equinox?
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An equinox is one of the two times of the year when the amount of daylight and nighttime hours are just about of equal length. The two equinoxes occur around March 20–21 and September 22–23.
The equinoxes occur on these days, but an equinox is not the whole day—it is the moment when the sun is directly above Earth’s equator. The word equinox can also refer to the position of the sun at this moment. This can also be called the equinoctial point. The equinoxes are traditionally considered to mark the start of spring and fall. In the Northern Hemisphere, the vernal equinox (or spring equinox) occurs in March and the autumnal equinox occurs in September. In the Southern Hemisphere, it’s the reverse.
In contrast, a solstice is one of the two times of the year when the positioning and tilt of Earth relative to the sun results in the most amount of daylight time or the least amount of daylight time in a single day. There are two solstices during the year: one that occurs around June 20–22 (usually June 20 or 21) and one that occurs around December 20–23 (usually December 21 or 22). In the Northern Hemisphere, the summer solstice occurs in June and the winter solstice occurs in December. In the Southern Hemisphere, it’s the reverse.
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