Goodbye summer, hello fall!
Fall officially begins on Monday, September 22 which is the exact time of the autumnal equinox.
This marks the moment when the sun is positioned directly above the Earth's equator at noon.
During the equinox, both the northern and southern hemispheres experience nearly equal lengths of day and night.
At this time, the sun rises exactly in the east and sets exactly in the west everywhere on Earth, except at the North and South Poles.
When viewed from space, the Sun light up the Earth evenly, so both the northern and southern hemisphere receive nearly equal amount of sunlight.
The same situation happens during the vernal equinox, which occurs around March 21 and signals the start of spring in the northern hemisphere.
Earth's tilt and its orbit around the Sun cause the seasons to change.
Because of this tilt, the northern and southern hemispheres receive different amount of sunlight at different times so when it is fall or summer in one hemisphere, the opposite hemisphere experiences spring or winter.
In the fall, the Sun stays lower in the sky at noon and the night get longer and cooler.
