Fall begins in the Northern Hemisphere on Monday, Sept. 22, when Earth experiences equal day and night, known as the autumnal equinox.

While fall officially begins with the autumnal equinox, if you ask a meteorologist, fall starts three weeks earlier on Sept. 1 .

Meteorological fall is based on Earth's annual temperature cycles or seasons. The three months in the Northern Hemisphere between the coldest and the warmest months, summer and winter, are what define meteorological fall from Sept. 1 to Nov. 30.

FALL FOLIAGE TRACKER: WHERE AND WHEN TO SEE THE MOST BRILLIANT COLORS ACROSS THE U.S.

Astronomical seasons are labeled as such because of Earth’s tilt, either away or toward the Sun.

Dr. Tyler Richey-Yowell, a postdoctoral fellow at Lowell Observatory in Arizona , helps break down why

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