When we think of the start of fall, we often picture cooler temperatures, changing leaves and shorter days, but when exactly fall begins depends on whether you're using the astronomical or meteorological calendar.

Astronomical fall is based on the position of the Earth in relation to the sun. It begins with the autumnal equinox, usually around Sept. 22 or 23 in the Northern Hemisphere, when day and night are nearly equal in length. Astronomical seasons shift slightly from year to year because they follow Earth’s orbit around the sun, not the calendar.

On the other hand, meteorological fall is defined by the calendar and climate data. It starts on Sept. 1 and ends on Nov. 30 — a neat, three-month period that helps meteorologists and climatologists organize and compare seasonal weather data more consistently over time.

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This is because the coldest three months of the year in the Northern Hemisphere are usually from the beginning of December through the end of February, while the warmest three months are typically from the beginning of June through the end of August.

So while the equinox may be the "official" start of fall in many minds, your local weather might have already begun to change. So what side are you on?

Jennifer Gray is a weather and climate writer for weather.com. She has been covering some of the world's biggest weather and climate stories for the last two decades.