Autumn Is Coming: September Equinox Arrives On Tuesday

Say goodbye to summer because autumn is set to officially begin this week with the arrival of the autumn equinox.

The autumn equinox typically arrives some time around the 22nd of September each year and this year's event is arriving Tuesday at precisely 9:31 am EDT. Astronomically, this marks the end of summer and the beginning of autumn.

During equinoxes, the sun shines directly on the equator, which means that the length of night and day will be nearly equal. In fact, the name "equinox" itself comes from the Latin aequus, which means equal, and nox, which means night. But since the Earth is continuously moving, this time of nearly equal amounts of day and night will quickly change.

For those in the Northern Hemisphere, the equinox marks the beginning of later dawns and earlier sunsets. During this time, people will begin to observe cooler weather, butterflies and birds migrating southward, and some animals beginning to grow coats for the winter, as EarthSky explains.

At this time, it is common in the Northern Hemisphere to observe the foliage change from bright greens to yellows, oranges and reds. Although it is typically believed that this change is caused by the weather conditions, with the temperatures beginning to drop, it is actually because of the change in the amount of daytime that causes the change in color. When the chlorophyll breaks down the greens disappear and the yellows and oranges of fall become visible instead.

It's worth noting that while the autumn equinox is the astronomical beginning of autumn, for meteorologists, autumn already began three weeks prior to the equinox, last Sept. 1.

The difference between the two is that the astronomical seasons are based on the position of the Earth in relation with the sun while the meteorological seasons are based on the annual temperature cycle, the National Oceanic and Atmospheric Administration (NOAA) explains.

On the other side of the globe, in the Southern Hemisphere, these events are all also happening on the same dates but the seasons are reversed. This means that places including Australia, South America, New Zealand and southern Africa will also experience nearly equal parts of day and night. However, the event will mark the first day of spring instead of the first day of autumn.