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Posted at 02:45 PM ET, 09/22/2011

Autumnal equinox marks the official start of fall Friday morning


Shorter days and cooler nights bring out the best in fall foliage (by author).
Unlike last September, cooler temperatures, frequent rain, and overcast skies have given this month a ‘fall-like’ feel. While meteorological fall already began three weeks ago, the autumnal equinox occurs Friday at 5:04 a.m. (EDT), signaling the official start of fall in the Northern Hemisphere.

Besides ushering in a new season, the fall equinox marks the time at which the sun shines directly overhead at the equator, which results in nearly equal periods of daylight and darkness at all latitudes. Until the winter solstice in December, the sun will continue its southward migration towards the Tropic of Capricorn, bringing us increasingly shorter days and a lower sun angle in the months ahead.

In Washington, D.C., the equinox sun will rise at 6:56 a.m. and set at 7:04 p.m. It is one of only two days of the year when the sun rises due east and sets due west of us. For the next three months, the sun will rise and set increasingly to our southeast and southwest respectively.


Chart of sunrise, sunset and day length beginning September 23 through the end of fall in Washington, D.C.
Disappearing daylight

If it seems like the days are rapidly getting shorter, it’s not just your imagination: on September 1, the sun rose at 6:37 a.m. and set at 7:39 p.m. By September 30, the sun will rise 26 minutes later and set 46 minutes earlier than it did at the start of the month.

Why the rapid change?

Mid-latitude locations see their greatest changes in daylight around the equinoxes. Washington, D.C. loses about 2½ minutes of daylight each day during the month of September. The simple explanation for this is that the sun’s declination (height above the horizon) changes most rapidly in March and September.

For example, in D.C. the sun reaches a maximum height of 59.3º above the horizon at solar noon on September 1. By the end of the month, the maximum sun angle is only 48.3º. Around the solstices, however, the sun’s maximum height changes little with respect to the horizon, and the change in daylight slows considerably. A more detailed explanation/analogy can be found here.

Why are sunrise and sunset times not changing at the same rate?

If you are an astute observer of the sun’s daily and monthly movements, you may notice that the sunrise and sunset times do not change at equal rates (during September, D.C. loses 46 minutes of evening daylight, but only 26 minutes in the morning). This inconsistency is due to a gradual, earlier shift in solar noon.

Solar noon is simply the time when the sun reaches its maximum height for the day. Many people are unaware that solar noon slowly oscillates by as much as 16 minutes throughout the year (this is true for all locations on earth). This happens because technically the time that elapses from one solar noon to the next can be slightly under or over 24 hours. During certain times of the year, it takes the sun a few seconds more than 24 hours to circle from its maximum noontime height on one day to its maximum height the next. At other times, the sun moves “ahead of the clock,” reaching its maximum height from one day to the next in slightly under 24 hours.

Currently, the sun is moving ahead of the clock, causing solar noon to drift earlier. On September 1, solar noon in D.C. was at 1:08 p.m. By October 1, it occurs at 12:58 p.m. This earlier shift in local noon means that the sun will rise and set just slightly “faster” from one day to the next. In the mornings, sunrise will not move later as quickly as one would expect. In the evenings, however, the sunset shifts earlier more dramatically because there is a compound effect of both shorter days and a progressively earlier local noon. By early November, solar noon begins to shift later by several minutes, allowing sunrise to change more noticeably than the time of sunset.

Equinox – but not equal day and night?

A common misconception about the equinox is that the entire world experiences exactly 12 hours of day and night on this date. Most Northern Hemisphere locations, however, do not see an exact 12-hour day until a few days after the fall equinox (and a few days before the spring equinox). Washington’s sunrise/set table above shows the sun above the horizon for 12 hours, 8 minutes on the equinox, but not until September 26 is the day exactly 12 hours in length. What causes this?

The main reason is atmospheric refraction: This bending of the sun’s light allows us to see the entire sun before and after it crosses the horizon. (By definition, actual sunrise occurs as soon as the upper edge of the solar disk appears above the horizon, while sunset occurs the moment the sun’s trailing edge disappears below it – though that’s not how our eyes see it.)

This helps explain why the day is slightly more than 12 hours long on the equinox. It also explains why places on the equator always see just over 12 hours of daylight year-round: It’s because of the angle from which they observe the sun.

The weeks ahead

As the daylight period continues to shorten, average temperatures drop in response. Currently, D.C.’s average high/low temperature is 77/60. By late October, averages drop to 64/46. Warm weather fans can rest assured that D.C. typically sees several mild, sunny days throughout the fall. But no matter what the weather, the sun is guaranteed to spend less time above the horizon in the coming months.

Additional reading

Sunrise and sunset in Washington, D.C.
The September equinox explained
Why the equinoxes do not have exactly 12 hours of daylight
How the location of sunrise and sunset changes throughout the year
Why twilight is shortest on the equinoxes
The summer solstice and other sun facts explained (Capital Weather Gang)

By  |  02:45 PM ET, 09/22/2011

Categories:  Latest, Local Climate, Astronomy

 
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