Fall starts at 9 p.m. ET Thursday, a day officially known as the autumn equinox.

Both equinoxes and solstices only happen twice a year — the first days of fall and spring are equinoxes, while the first days of summer and winter are solstices.

How are they different?

What is an equinox?

On the day of an equinox, the Earth is tilting neither toward or away from the sun, and therefore receives almost an equal amount of daylight and darkness, according to the National Weather Service.

At places along the equator, the sun is directly overhead at about noon on these days. Day and night appear to be equal due to the bending of the sun's rays, which makes the sun appear above the horizon when it is actually below it.

During an equinox, days are slightly longer in places with higher latitudes. At the equator, daylight may last for about 12 hours and seven minutes. But at a place with 60 degrees of latitude, such as Alaska's Kayak Island, a day is about 12 hours and 16 minutes.

What is a solstice?

On the day of a solstice, the Earth is at its maximum tilt, 23.5 degrees, either toward or away from the sun.

During a summer solstice, the sun is directly above the Tropic of Cancer in the Northern Hemisphere and is tilting toward the sun, causing the longest day of the year. It is winter in the Southern Hemisphere, where the Earth is tilting away from the sun.

Likewise, a winter solstice happens in the Northern Hemisphere when the hemisphere is tilting away from the sun, making it the day with the least amount of sunlight. The sun is above the Tropic of Capricorn in the southern hemisphere, making it summer there.