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The Sky This Week, 2019 December 3 - 10

The solstice is almost here.
The Moon, Venus, and Jupiter, 2019 November 28, imaged from Alexandria, Virginia.
The Moon, Venus, and Jupiter, 2019 November 28.
HDR image made with a Canon EOS Rebel SL2 DSLR from Alexandria, Virginia.

The Moon brightens the long winter nights this week, waxing toward the full phase, which falls on the 12th at 12:12 am Eastern Standard Time.  December’s Full Moon is variously known as the Long Night Moon, Old Moon, or the Moon Before Yule.  Since this Full Moon occurs closest to the winter solstice is will be the most northerly one of the year, beaming down on the frosty landscape below.  Luna spends most of the week moving through the barren star fields of autumn, but as the week ends look for her near the bright star Aldebaran and the Hyades star cluster in the constellation of Taurus, the Bull.

This week we begin to see the phenomena associated with the winter solstice.  The solstice “season” begins with the year’s earliest sunsets, which occur all of this week.  Here in Washington the Sun disappears below the horizon at 4:46 pm EST.  He will gradually start to set later starting on December 13th.  Although this gives the illusion of the days getting longer, we won’t see our latest sunrise until early January.  In between the solstice occurs on the 21st, which will be the year’s shortest day.  This seemingly odd behavior is due to the way we now keep time.  In traditional times the hours were marked by sundials, which registered noon when the Sun was highest in the sky.  Today, however, our clocks are ruled by the oscillations of atoms, producing a uniform time-scale where each second in a day is identical.  Solar time, as measured by sundials, is subject to the ellipticity of Earth’s orbit and therefore must be corrected to a “Mean Sun”.  This Mean Sun differs from the “Apparent Sun” by up to 15 minutes at different times of the years.  If you look at a world globe you have probably noticed a figure-8 diagram that’s usually placed over the Pacific Ocean.  This is the Analemma, the graphical solution to the Equation of Time, which shows the Apparent Sun’s declination and difference from the Mean Sun during the course of the year.  So if you enjoy more time at the end of the day take heart, but if you hate getting up in the morning in the dark, you’ll need to wait a few more weeks.

By 10 pm the autumnal constellations are heeling over to the west while the bright winter constellations move westward to take their place.  The stars of the Great Winter Circle bring a bright touch to the long nights, adding to the multi-colored lights decorating more Earthly abodes.  The stars Aldebaran and Capella lead the Winter Circle entourage into the sky, seemingly announcing the arrival of Orion, arguably one of the most spectacular constellations in the heavens.  Orion is visible from all of the inhabited parts of the world and figures in the sky lore of almost every culture that has left us written records.  It is hard to not see a striding figure marching across the sky with a jeweled belt marked by the three prominent blue-hued stars Alnitak, Alnilam, and Mintaka.  The colors of these stars stand out particularly well in binoculars.  Their pure blue tint indicates that they are very hot and luminous, and modern research tells us that they are very massive and luminous.  The middle star in the belt, Alnilam, is located about 2,000 light-years from the solar system, and its apparent brightness over that vast gulf implies that it shines with the brilliance of over half a million Suns!  Many of Orion’s other bright stars are similar in distance and luminosity, but their brightness belies their fate.  They are consuming their supply of hydrogen at a prodigious rate, and they will all be distant memories in about 10 million years.  By contrast, our feeble Sun will last for another several billion years.

If you’re looking for bright planets, you’ll need to act quickly.  Last week we had a beautiful gathering of Venus, Jupiter, and the young crescent Moon.  This week Venus has left Old Jove in her wake, and is setting her sights on Saturn.  Jupiter now sets before the end of evening twilight and will pass behind the Sun just before the end of the year.  Venus closes the gap with Saturn during the course of the week, and the pair will be within two degrees of each other on the evenings of the 10th and 11th.  Saturn will follow Jupiter into solar conjunction by the year’s end, but Venus will remain in the evening sky well into the new year.

The only other naked-eye planet that will be visible is Mars, but you’ll have to get up early to see him.  He’s not particularly bright, but he should be easy to find as he closes in on the third-magnitude star Zubenelgenubi in the constellation of Libra, the Scales.  By the end of the week he will be close to the star, ultimately gliding less than a quarter of a degree by the star before dawn on the morning of the 12th.