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The Sky This Week
U.S. Naval Observatory's Weekly Blog
The Long and the Short of Winter's Nights
by Geoff Chester, USNO Public Affairs
01 December 2020
The Full Beaver Moon and Halo, imaged 2020 November 29 from Alexandria, Virginia
HDR image captured with a Canon EOS Rebel SL2 DSLR
The Moon begins the week high above the rising stars of the Great Winter Circle, then wends her way into the morning sky as she wanes to Last Quarter, which will occur on the 7th at 7:37 pm Eastern Standard Time. By the end of the week pre-dawn skywatchers can see her among the rising stars of the springtime constellations.
December 1st is generally considered to be the start of climatological winter, and all month long the Sun plays out a series of extremes centered on the winter solstice. From now until December 11th we will experience the earliest sunsets of the year. Here in the Washington, DC area that means that Old Sol slips below the horizon at 4:46 pm EST. After the 11th he will gradually begin to set a tad later each day, and by Christmas he’ll set at 4:52 pm. However, the time of sunrise is still creeping later each morning and won’t reach its latest time until the year’s end. Fromm December 30th until January 10th sunrise in DC will be at 7:27 am. The dates of these extremes are bisected by the solstice itself, which falls on the 21st. This will be the shortest day of the year in the Northern Hemisphere. In Washington we will see only 9 hours 36 minutes of daylight on that date.
This seemingly lopsided swing in the dates of solstice phenomena is a reflection on our desire to keep precise time. If you were to measure time with a sundial, the dates of latest sunrise and earliest sunset would correspond to the solstice. For centuries this scheme was adequate, but as mechanical timepieces became more precise it became apparent that the time kept by the Sun varied in its uniformity throughout the year. Since the Earth travels on an elliptical orbit around Old Sol its orbital velocity changes, moving faster at perihelion and slower at aphelion. However, its speed of rotation remains essentially constant throughout the year. The time of “noon” as measured by a sundial occurs when the Sun crosses the sundial’s meridian, so as far as the sundial is concerned noon is always 12 o’clock. However, bring a clock of constant rate into the mix and you’ll discover that the apparent time of the Sun’s noon meridian transit is not precisely every 24 hours. Depending on the time of year the apparent noon transit of the Sun can be as much as 16 minutes ahead of or 14 minutes behind “clock time”, and the most rapid excursion between these points happens to occur between early November and mid-February, skewing the “clock times” of latest sunrise and earliest sunset. A similar effect occurs around the time if the summer solstice, but its amplitude is about half of what it is in the winter. The annual variation between apparent solar (i.e. sundial) time and clock time is known as the “equation of time”, and its graphical solution may be found in the form of the “figure 8” diagram that’s usually printed on Earth globes and maps in the middle of the Pacific Ocean. This figure is called the “analemma”, and provides you with a handy guide to correct your sundial to the proper clock time…at least for a while. The Earth is gradually slowing in its rotation, and the eccentricity of its orbit and its axial tilt are slowly changing as well. Eventually we’ll have to redraw the analemmas on our globes.
Early evening skywatchers can still catch Jupiter and Saturn in the southeastern sky as evening twilight settles over the landscape. Both planets are gradually losing ground to the Sun, setting about three minutes earlier each passing night. Jupiter has a slight advantage, though, as you can see by watching him inch closer to Saturn over the next few weeks
Mars is now the planet that will get your attention each night. He crosses the meridian shortly after 8:00 pm local time, and his pink tint distinguishes him from all other bright objects in the sky. He continues to fade gradually as the distance between him and Earth increases, and the telescope will show that his disc is shrinking rapidly as well. Details on his distant surface are becoming harder to see as he recedes from us, and the recent development of one of his infamous dust storms further hinders the view.
You can still find bright Venus in the pre-dawn sky, but she, too, is gradually inching closer to the glare of the Sun. Look for her in the southeastern sky in the gathering morning twilight.
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