This Week’s Sky at a Glance, 2020 December 19 – 26
~by Curt Nason
The planets orbit at different speeds according to their distance from the Sun. One closer to the Sun has a shorter distance to travel and also does so at a greater speed. Therefore, it is not very rare to see one planet catch up to and pass near another in the sky. This event is called a conjunction, specifically when they have the same celestial coordinate called right ascension. This also applies when a planet is seen near a star, is passed by the Moon, or moves behind the Sun. The inner planets, Mercury and Venus, also pass between us and the Sun. This is called inferior conjunction, and to avoid confusion they are said to be at superior conjunction when passing behind the Sun.
Jupiter and Saturn orbit the Sun in nearly 12 and 30 years, respectively, meeting up in conjunction every 20 years. This weekend Jupiter edges toward Saturn for an especially rare winter solstice conjunction. It is not the timing that makes this exceptional, but their apparent nearness. On Sunday Jupiter is just below Saturn, and on Monday they are in conjunction; one-fifth the width of the Moon apart or 12 times the width of Jupiter as seen through a telescope. This is their closest conjunction in four centuries and the closest in eight centuries that they have been far enough from the Sun to be seen easily, although by sunset here they will have separated slightly and appear as close as their morning conjunction in 1563 (see Bob King’s excellent article on the Sky & Telescope website). If you get a chance to see this event, bear in mind they are really 730 million kilometres apart, with Saturn twice as far from the Sun as Jupiter.
This Week in the Solar System
Saturday’s sunrise in Moncton is at 7:57 am and sunset will occur at 4:35 pm, giving 8 hours, 38 minutes of daylight (7:59 am and 4:43 pm in Saint John). Next Saturday the Sun will rise at 8:00 am and set at 4:39 pm, giving 8 hours, 39 minutes of daylight (8:02 am and 4:47 pm in Saint John). The Sun reaches its most southerly position, over the Tropic of Capricorn although it is within the borders of the constellation Sagittarius the Archer, at 6:02 am on Monday, marking the beginning of winter in the northern hemisphere.
The Moon is at first quarter on Monday and it passes a binocular width below Mars on Wednesday. Also on Monday, Jupiter appears about one-fifth the width of the Moon from Saturn. Check them out by 5:30 – 6 pm because they set around 7 pm. Reddish Mars remains brighter than all of the stars except Sirius, leading similarly coloured Aldebaran and Betelgeuse across the sky. Venus rises two hours before sunrise this weekend, slowly heading sunward, while Mercury is at superior conjunction on Sunday. The Ursid meteor shower peaks from Tuesday evening to Wednesday morning, with shooting stars seeming to emanate from the bowl of the Little Dipper in the north.
With astronomy meetings and outreach activities on hold, you can watch the local Sunday Night Astronomy Show at 8 pm and view archived shows.
Questions? Contact Curt Nason.