Eager to say goodbye to summer once and for all? Say hello to fall: the 2012 autumnal equinox occurs this Saturday, September 22 at 10:49 a.m. (EDT).
As the new season begins, Earth’s axis will be tilted neither away from nor towards the sun, allowing the sun’s direct rays to shine overhead at the equator before moving into the Southern Hemisphere.
With the exception of the poles, all locations on Earth experience slightly over 12 hours of daylight and see the sun rise due east and set due west along the horizon. Until the December solstice, the sun continues to rise and set increasingly to our southeast and southwest, respectively.
In Washington, D.C., the fall equinox sun rises at 6:56 a.m. and sets at 7:04 p.m. Due to atmospheric refraction the sun is actually above the horizon for a bit more than 12 hours – instead of exactly 12 hours as the Latin-derived ‘equinox’ might imply.
This unequal day and night discrepancy can be seen at all locations in the Northern Hemisphere. Most cities in the continental U.S. see the sun above the horizon for about 12 hours and 8 minutes at the equinox. Not until after the fall equinox do we see a day with exactly 12 hours between sunrise and sunset – which occurs anywhere from Sep. 25-28, depending on latitude (read more: Equal day and night - but not quite).
How quickly are we losing daylight?
As discussed last year, mid-latitudes of the Northern Hemisphere see their greatest loss of daylight around the time of the fall equinox. Washington, D.C. loses 1 hour and 16 minutes of daylight during the month of September, or about 2 minutes, 30 seconds a day.
Compare this with a far northern location like Fairbanks, Alaska, which loses nearly 7 minutes of daily sunlight at this time of year. That quickly adds up to 3 hours and 20 minutes less daylight in the month of September alone – not the best place for those with seasonal affective disorder!
Not just dark earlier but darker faster?
Careful sun observers might notice that around the fall equinox it gets dark not only earlier but also faster. How can this be?
Twilight on both the spring and fall equinoxes is indeed shorter than at any other time of year. However, the difference is subtle unless you live at a fairly high latitude (at least 45º from the equator). The reason for shorter twilight is related to where the sun rises and sets around the equinox: since the sun rises due east and sets due west, it actually crosses the horizon at a slightly steeper angle than it does on the solstices. This causes the sun to appear and disappear from the horizon more quickly.
Compare this with the solstices when the sun sets in the northwest during summer and southwest during winter. When setting farther from due west, the sun crosses the horizon at a shallower angle, which prolongs the duration of evening twilight (and morning twilight before sunrise). The graph below compares the length of twilight in four cities at different latitudes:
At all times of year, cities at higher latitudes experience longer twilight than those closer to the equator (compare Seattle with Miami, for example). Yet each location sees a decrease in twilight length around both the spring and fall equinox. In March, we usually don’t notice the shorter twilight because the days are getting longer. In September, on the other hand, we are more likely to observe not just how much earlier it gets dark, but also how quickly nighttime falls.
Rapidly decreasing daylight, quick twilight transitions, and cooler average temperatures are a sure sign that autumn has arrived.
Previous astronomical season posts by Capital Weather Gang