A colorful sunrise over the tidal basin on November 2, 2013. (Ian Livingston via Flickr )

Meteorologists consider September 1 the first day of autumn. But for those who prefer defining the seasons by astronomy, the autumnal equinox, which occurs Monday night, marks the end of summer and the beginning of fall in Earth’s northern hemisphere.

Here are six images that explain the fall equinox, which occurs September 22 at 10:29 p.m. EDT.

1. Nearly equal daylight in both hemispheres

On the equinox, incoming solar energy is equal in both hemispheres. (NOAA)

During the fall equinox, the sun can be seen at zenith over Earth’s equator before its direct rays shift into the Southern Hemisphere for the next six months. Neither of Earth’s hemispheres is tilted toward the sun, which results in roughly 12 hours of daylight and darkness at all latitudes (but not exactly, as explained below).

2. Sunrise at due east, sunset at due west 

Position of sunrise and sunset in Washington, D.C. on the fall equinox. Click for larger image. (suncalc.net)

Except at the North and South Pole, all latitudes on Earth see the sun rise due east and set due west on the September equinox. Until the winter solstice in December, the sun will continue to rise and set farther to the south. The screenshot above, from an interactive app called SunCalc, shows sunrise and sunset aligning with the east-west oriented Reflecting Pool on the National Mall.

3. Fastest loss of daylight

(Justin Grieser, data from timeanddate.com)

Mid-latitudes of the Northern Hemisphere see the greatest loss of daylight around the fall equinox. The table above shows D.C. loses one hour and 14 minutes of daylight during the month of September, or about two minutes and 30 seconds a day. At higher latitudes, the decrease in daylight is even more pronounced.

4. Not quite equal day and night

(timeanddate.com, adapted by CWG)

A common misconception is that the equinox means equal day and night everywhere on Earth. In reality, this isn’t quite true. This chart shows the sun is up for 12 hours and seven minutes in D.C. on the equinox, but not until September 26 are sunrise and sunset exactly 12 hours apart.

The main reason for this is atmospheric refraction, an optical phenomenon that allows us to see the sun even when it’s below the horizon. Most places in the Northern Hemisphere therefore don’t see an exact 12-hour day until after the autumnal equinox, typically between Sept. 25-28 (read more).

5. Fastest sunsets and shortest twilight of the year

Comparison of twilight length in four U.S. cities throughout the year. Civil twilight refers to the time before sunrise and after sunset when the sun is 6 degrees or less below the horizon. (Justin Grieser)

Careful sun observers may notice that around the fall equinox it gets dark not only earlier but also faster. And they’re right – the fastest transitions between day and night occur around the two equinoxes. This happens because the sun crosses the horizon at a slightly steeper angle than it does on the solstices. As a result, we see the sun appear and disappear from the horizon more quickly. The difference is subtle closer to the equator (see Miami in the chart above), but is much more noticeable in high latitudes.

6. A warmer version of the spring equinox

(Image by Justin Grieser, data from NOAA)

Due to seasonal temperature lag, the autumnal equinox in the Northern Hemisphere is generally much warmer than the spring equinox in March – even though both days receive the same amount of incoming solar energy. Since oceans cover 71 percent of the planet, and water takes longer than air to heat up and cool down, it takes a while for the Northern Hemisphere to respond to a drop in incoming solar energy. This is why even though the days are shorter, it’s not uncommon to get a period of summer-like weather in October.

Yet, while autumn is known for its pleasant weather, the September equinox is a melancholy reminder that we’ll be seeing much less of the sun for the next six months.

Related posts:

Autumn Equinox 2013
Autumn Equinox 2012
Autumn Equinox 2011