Fall officially begins this weekend, though parts of the US have already been hit with the cooler temperatures that the season brings this month.

Here are five things you should know about the autumnal equinox:

1. The start time of fall on Sunday September 22 depends on where you live: If you are in the Eastern Time Zone, fall officially begins at 8:43 a.m. local time. Fall begins at 7:43 a.m. in the Central Time Zone, 6:43 a.m. in the Mountain Time Zone, and 5:43 a.m. in the Pacific Time Zone.

(MORE: Fall Leaves Tracker)

2. This is why there is a very specific time that marks the beginning of fall: Twice a year, around March 20 or 21 and September 22 or 23, the sun’s rays shine directly over the equator. In the Northern Hemisphere, these are known as the vernal equinox and the autumnal equinox.

The specific time, in this year’s case 8:43 a.m. EDT, marks the passage of direct sunlight across the equator from the Northern Hemisphere to the Southern Hemisphere.

As a result, day and night are in balance around the world, at nearly 12 hours each. You can see this in the satellite image of the September 23, 2019 equinox, which is half sunny and half nocturnal (dark areas).

From this point on, daylight in the Northern Hemisphere will gradually shorten until the winter solstice, which occurs on December 21, 2024. This is because the Earth in the Northern Hemisphere tilts further and further away from the Sun.

4. For the southern hemisphere everything is opposite: South of the equator, the September equinox marks the beginning of spring.

In countries like Australia and South America, daylight will last longer until the start of summer in December, as that part of the Earth receives the most direct sunlight then.

5. Autumn and fall are used interchangeably, but there is a difference in what they meanFind out more here with our full explanation.

Chris Dolce has been a senior meteorologist at weather.com for more than 10 years, after beginning his career at The Weather Channel in the early 2000s.