Rare celestial event visible Dec. 21
I'm going to jump ahead slightly to Dec. 21. I want to make sure you get to see a very special (actually rare) celestial event. If you have children or grandchildren, you may want to take them out to see it as well and you'll understand why shortly.
As you may have been noticing, the planets Jupiter and Saturn have been drawing closer to one another in the evening sky since last summer.
When two celestial bodies in space appear to be close to one another (although, in fact, they are still millions of kilometers apart), it is referred to as a conjunction. About every 20 years, Jupiter and Saturn experience what is called a great conjunction when they are at their closest to one another in the sky.
On Dec. 21, Jupiter and Saturn will be at their closest great conjunction (0.1 degrees, as seen from Earth) to each other since 1226. There was a great conjunction between the two planets in 1623, but that conjunction occurred when the two planets were very close to the sun, and the conjunction wasn't observed. This makes the great conjunction on Dec. 21 the closest one seen in nearly 800 years.
The next great conjunction won't be until March 15, 2080. Thus, my recommendation of taking your children or grandchildren out with you to see this one. You can then tell them that, when they see the great conjunction of Jupiter and Saturn in 2080, they can think of you, and remember sharing this rare celestial event. Now that is a timeless gift, indeed.
Although the great conjunction itself occurs at
2:30 p.m. on Dec. 21, you can still observe the conjunction (there won't be any appreciable difference in separation) in the early evening sky just after sunset. The two planets will appear to the unaided eye as a single "star" low (about 12 degrees) above the southwest horizon. Find an unobstructed view of the horizon if you can by about 5:15 p.m. Binoculars or a small scope will separate the two planets, with Saturn being the fainter one. You will have to be quick though, as both planets will disappear below the horizon shortly before 7 p.m.
Don't worry if you miss the near-actual great conjunction on Dec. 21. Jupiter and Saturn will remain visible (less than the diameter of the full moon apart), low above the southwest horizon just after sunset, until Dec. 25.
THIS WEEK'S SKY
Heading toward superior conjunction (passing behind the sun as seen from Earth) on Dec. 20, Mercury cannot be observed, but will reappear in the southwest evening sky in January 2021.
Venus (magnitude -3.95) is likewise shifting closer to the sun with each passing week, and is now only visible around 5:45 a.m. in the eastern sky before fading from view when dawn breaks around 7:30 a.m.
Mars (magnitude -0.70), though slowly fading, is still visible in the early evening sky around 4:55 p.m., reaching a height of 52 degrees in the southern sky before dropping below 8 degrees above the western horizon by
1:40 a.m.
Jupiter (magnitude -2.0) and Saturn (magnitude +0.64), moving ever closer to one another, are visible in the early evening sky. Jupiter is visible around 4:50 p.m., 16 degrees above the southwest horizon, and Saturn around 5:10 p.m., 15 degrees above the southwest horizon; both planets will have set by
7:15 p.m.
Watch the crescent moon slide past the planets Dec. 16-17 and don't forget their great conjunction on the evening of Dec. 21.
Until next week, clear skies.
EVENTS
• Dec. 15 — Moon at perihelion
• Dec. 21 — Great conjunction of Jupiter and Saturn (just after sunset)
Glenn K. Roberts lives in Stratford, P.E.I., and has been an avid amateur astronomer since he was a small child. His column, Atlantic Skies, appears every two weeks.