Winter solstice – and other interesting sun facts – explained

Reply Thu 22 Dec, 2011 11:25 am
All of the photos and graphs are here: http://www.washingtonpost.com/blogs/capital-weather-gang/post/winter-solstice-marks-the-shortest-day-of-the-year-thursday-morning/2011/12/21/gIQANxaG9O_blog.html?hpid=z4

Winter solstice – and other interesting sun facts – explained
By Justin Grieser - Washington Post

Compared with the summer solstice and equinoxes, the winter sun shines at a low angle and spends little time above the horizon. (Daniel V. Schroeder, Weber State University)If you’ve grown weary of short days and are looking forward to more sunlight, you won’t have to wait much longer. Tomorrow is the winter solstice in the Northern Hemisphere, bringing us the shortest daylight period and longest night of the year. Going forward, the days will gradually lengthen as the sun climbs higher in the sky over the next six months.

In 2011 (this year), the winter solstice occurs at 12:30 a.m. EST on December 22. As the first day of astronomical winter* in the Northern Hemisphere, the solstice marks the time at which the sun shines directly overhead at 23.5 degrees south latitude. With the North Pole tilted away from the sun, locations north of the equator see the sun take its lowest and shortest path across the southern sky.

For the Washington, D.C. area, this means:

* The sun is above the horizon for approximately 9 hours, 26 minutes
* Sunrise occurs at 7:24 a.m. and sunset at 4:50 p.m.
* The sun angle at solar noon (12:07 p.m.) reaches its minimum height of 27.7º above the horizon (compared with 74.6º above the horizon on June 21)
* The sun rises at its southeasternmost point and sets at its southwesternmost point along the horizon (120º and 240º from due north, respectively)

Table of sunrise, sunset, and day length from winter to summer solstice in Washington, D.C., *Reflects change to daylight saving time (Timeanddate.com, U.S. Naval Observatory, Gaisma.com)

Comparison of sunrise/set times and day length for select U.S. cities. On the winter solstice the amount of sunlight steadily decreases as one moves closer to the North Pole.

Why do the earliest sunset and latest sunrise not fall on the solstice?

Careful sun observers are quick to note that the earliest sunset and latest sunrise of the year do not coincide with the winter solstice. Washington, D.C. saw its earliest sunset at 4:46 p.m. from December 2 through December 13, which means the sun has already been setting a few minutes later for the past week. Yet the sun continues to rise later, with the latest sunrise occurring at 7:27 a.m. from December 31-January 10.

What causes this discrepancy? To begin, it helps to understand the two “competing” forces that determine local sunrise and sunset times: 1) Changes in the sun’s declination, or height above the horizon throughout the year, and 2) The changing time of solar noon.

Solar noon occurs when the sun reaches its maximum height in the sky on any given day. At any location on Earth, the time of noon slowly oscillates back and forth by several minutes throughout the year (in other words, a sundial would not consistently show noon occurring at the same time as your wristwatch). These shifts are due to the earth’s elliptical (non-circular) orbit and axial tilt, and are summed up in a complex relationship called the equation of time (for simplicity, let’s call it the “solar noon effect”).

The reason the earliest sunset occurs before the winter solstice has to do with the later shift in solar noon “outweighing” the effect of the sun’s decreasing height and length of time above the horizon. For example, in D.C. solar noon is at 11:57 a.m. on December 1, but drifts 14 minutes later – to 12:11 p.m. – by December 31. This forward shift means that it takes a few seconds more than 24 hours for the sun to complete a full circle between its maximum noontime height from one day to the next. Meanwhile, as we approach the solstice, the sun’s declination is no longer changing as rapidly, which causes the days to shorten at a slower pace.

Around the solstice, the height of the sun changes little with respect to the horizon. This means that the time of sunrise and sunset is more influenced by the Earth’s elliptical orbit than at other times of year. Click here for larger image. (PATTERSON CLARK, The Washington Post)

In late November, the effect of a later-shifting solar noon begins to counteract the effect that the sun’s lowering declination has on pushing sunset earlier. Eventually, sunset reaches a minimum during the first week of December. While we would expect the earliest sunset to occur closer to the winter solstice, the rapid forward shift in solar noon causes sunset to creep later more than a week before then.

The sun rises over the Tidal Basin and Jefferson Memorial in early January 2010. Not until January 11 will the sun begin to rise earlier in Washington. (Kevin Ambrose) This complex phenomenon also explains why the sun keeps rising later after the winter solstice**. Again, the time of solar noon is still moving later for several more weeks. Initially, this counteracts the gradually increasing sun angle and prevents sunrise from moving any earlier. By mid-January, however, the sun’s increasing height above the horizon “accelerates” enough to bring about earlier sunrises, even as solar noon continues to move later until early February. The combination of shifting solar noon and the sun’s changing declination explains why January mornings remain quite dark while evenings are already noticeably lighter by the end of December.

As the days lengthen, will the cold strengthen?

Despite the gradually increasing daylight, average temperatures in the D.C. area will continue to drop, with a minimum average high/low temperature of 43/28F occurring January 12-22. Given the above average conditions lately, the old adage, “as the days lengthen, the cold strengthens” may have questionable merit this year. But whether you are a snow lover longing for a winter wonderland, or just hoping that the mild weather continues, we can at least agree that more daylight is something to look forward to.

* Meteorologists and climatologists consider winter to have started on December 1. The December 22 solstice signals the ‘official’ start of winter by its astronomical definition.
** This “earliest sunset/latest sunrise” paradox is highly latitude-dependent. Places farther from the equator will see their earliest sunset and latest sunrise occur closer to the winter solstice because daily changes in the sun’s declination are more apparent. Northern Hemisphere locations in relatively low latitudes (e.g. Miami) often see their earliest sunset in late November and latest sunrise not until late January due to a greater “solar noon” effect. The U.S. Naval Observatory has a more detailed explanation of this here .
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