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The Sky This Week
U.S. Naval Observatory's Weekly Blog
When a Planet Meets a Planet Comin' Through the Sky
by Geoff Chester, USNO Public Affairs
15 December 2020
Orion (with faded Betelgeuse), imaged 2020 January 1 from Mollusk, Virginia
captured with a Canon EOS Rebel SL2 DSLR on an Omegon MiniTrack LX2 mechanical star tracker
The Moon returns to the evening sky this week, waxing to First Quarter on the 21st at 6:41 pm Eastern Standard Time. Look for Luna’s slender crescent low in the southwest near the close pairing of Jupiter and Saturn in the twilight hours of the 16th and 17th. By the end of the week you will find her approaching ruddy Mars.
The 21st also marks the date of the winter solstice, which will occur at 5:02 am EST. This is the moment when the center of the Sun’s disc reaches an ecliptic longitude of 270 degrees relative to the center of the Earth. In more practical terms it is also the day when the Sun reaches its most southerly point along the ecliptic, providing Northern Hemisphere residents with the shortest day of the year. Here in Washington, DC that corresponds to 9 hours 26 minutes of daylight. At the time of the solstice Old Sol will be directly overhead about 250 kilometers (150 miles) north of Pretoria, South Africa.
Yet another event occurs on the 21st this year in the form of a sight last seen by people in the year 1226 CE. This will be the extraordinarily close pairing of the planets Jupiter and Saturn that will play out in the evening twilight sky. If you’ve been watching the two planets over the course of the past few months you have seen Jupiter slowly but surely inch up on Saturn, and on the 21st they will pass just six arcminutes apart from each other. While they won’t appear to merge together as seen with the unaided eye, that apparent distance is about one-fifth the apparent diameter of the Moon. They will produce a spectacular view in the low-power field of a telescope, especially as the sky darkens and their respective flocks of moons become visible. These two planets generally encounter each other about every 19.8 years, and when they meet it is often called a “Great Conjunction”. After 19 of these encounters, their 20th can be very close, as is the case here. They were actually a tad closer in 1623, but that conjunction took place on the far side of the Sun and so was not visible. The 1226 close conjunction, which occurred on March 5th, brought the two planets to within three arcminutes of each other in a dark sky, which must have caused great consternation to astrologers of the day. The next close encounter of the two gas giants will occur after 19 more Great Conjunctions. On August 24, 2417 they will once again pass very close to each other, just five arcminutes apart. Mark your calendars!
The long winter nights may be dark, but Mother Nature has seen fit to do her part to brighten the view. By the late evening the spectacular constellations of the Great Winter Circle are marching toward the meridian, led by perhaps the most splendid of all, Orion, the Hunter. Within the bounds of the circle you will find ten of the 30 brightest stars in the sky, four of which reside in Orion. You may recall that last year one of Orion’s stars lost much of its luster; Betelgeuse, the red-tinged star marking one of the Hunter’s shoulders, faded to about the same brightness as the three “Belt Stars”. Happily the star has returned to its former brightness this year, but astronomers are still baffled by its deep minimum.
A somewhat brighter red-hued object now dominates the early evening hours. Mars crosses the meridian at around 7:30 pm local time, set like a glowing coal among the faint stars of Pisces. Although he has lost much of the luster he had at opposition in early October, he still has the ability to get your attention. The distance between Earth and Mars is rapidly increasing now, and his apparent disc is now just half the size that it was back then. You will need at least a six-inch telescope to see details on his distant surface.
Venus is gradually dropping toward the Sun in morning twilight. She rises at around 5:30 am, just before the onset of morning twilight. Look for her bright glow in the southeastern sky about half an hour before sunrise.
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