Fall equinox marks first day of fall in Northern Hemisphere

The first day of fall coincides with the autumn equinox, when day and night are nearly equal.

“However, that day/night length depends on where you are on Earth,” Mitzi Adams, NASA solar scientist, said in NASA press release. “For example, at the North and South  Poles, the length of the day and night is six months!”

At the North Pole, the Sun will sink below the horizon, entering a twilight mode until sometime in October when it will be completely dark. The sun rises above the horizon again a few weeks before the equinox in March.

Equinox only happens twice each year in the Earth's trip around the Sun. At all other times of year, the sun rises unequally over the Northern and Southern hemispheres because of the tilt of the Earth's axis. On equinox days, the sun shines equally on the north and the south.

In the Northern Hemisphere, the beginning of fall marks when nighttime hours last longer than daylight hours. It's the opposite in the Southern Hemisphere; the September equinox marks the beginning of spring.