It's official. Autumn, fall, whatever you want to call it, arrives on Sept. 23 with the autumnal equinox. Pumpkin spice lattes, flannels and boots are now acceptable. 

Here are five things to know about the autumnal equinox. 

1. The sun will be directly in line with the equator. 

That's what makes it the equinox. The alignment officially occurs at 3:50 a.m. EST on Sept. 23.

2. It marks the astronomical first day of autumn. 

Meteorologically speaking, fall is the months of September, October and November. Astronomically, fall is from Sept. 23 until Dec. 21 this year, when the Southern Hemisphere starts to receive more light than the Northern. 

3. The Northern and Southern Hemispheres will both experience an equal amount of daylight. 

But for the Southern Hemisphere, this equinox marks the start of spring. 

4. Days are about to get shorter. 

After the 12 hours of daylight, the Northern Hemisphere starts to see shorter days associated with fall and winter. After the winter solstice on Dec. 21, the days will start to get longer again. 

5. Expect the leaves to start changing soon. 

According to the Farmers' Almanac, the changing of the leaves actually depends on the amount of sunlight and not weather conditions. With shorter days starting, the trees will start to react with colorful leaves.

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