If you’ve been reading this newsletter or following our Facebook page in 2020, you’re aware that the bright planets Jupiter and Saturn have been drawing closer, approaching their “Grand Conjunction,” an alignment that only occurs every 20 years. (Check out our archive of previous issues to learn more.) The time is quickly approaching so keep your eyes on this pair every clear night in the coming weeks!
(For detailed explanations of planetary conjunctions and other aspects of the “clockwork of the heavens,” check out our Signs & Seasons astronomy curriculum.)
Since October these planets have been drawing closer and closer. However, even if you’re watching regularly, it can be hard to notice the change. From watching other conjunctions come together over the years, I believe the human mind tends to forget how things looked a month ago. Also, these planets are now lower in the sky than they were in the summer, which affects their appearance.
As this pair gets close to the horizon, the “Moon illusion” makes them appear farther apart, even though they are physically closer. This was astutely noted by Jacqui on the Facebook page. Just as the Sun and Moon appear larger when near the horizon, the distance between Jupiter and Saturn also appear larger when they are close to setting. But this is only an illusion and there is a way to observe the difference.
You can measure the planetary positions without relying on memory alone. Your can make approximate angle measurementswith just your fingers. Hold out your hand at arm’s length and see how many fingers will fit between the planets. At one point this year, I could fit 4 fingers between Jupiter and Saturn. I saw this pair just tonight and only one finger fit between and two fingers covered this pair. This separation will get a LOT closer by December 21.
During the month of December, Jupiter and Saturn will be closing the gap. As December 21 approaches, the changes should be readily apparent even from night to night. But when the distance becomes less than one finger, try holding a pencil at arm’s length. You can notice from night to night that there is less space between the planets and the pencil eraser.
On December 21, 2020, the closest alignment, Jupiter and Saturn will be quite low in the evening sky after sunset and night falls. You’ll need a clear, flat horizon to spot this pair. You can locate the pair a few days earlier when the waxing crescent Moon passes these planets on the evenings of December 16 and 17.
A pencil eraser held at arm’s length subtends an angle of about 1/2 degree, which is the angular size of the Moon in the night sky. It might be hard to believe that the Moon is only that small but sometime try holding up a pencil next to the Moon to see for yourself.
At closest conjunction, Jupiter and Saturn will be only 0.1 degrees apart, a mere 1/5 of a lunar diameter. At this time, the pencil eraser will completely cover over Jupiter and Saturn. It should be easy to see both of these planets together in the field of a telescope eyepiece, as shown in this picture below. (This image is adapted from Stellarium, a free open-source planetarium program. Check it out, it’s been greatly improved over the years.)
Even though Grand Conjunctions of Jupiter and Saturn happens every 20 years, this is the closest that most everyone alive today can expect to see in their lifetime. There will be Grand Conjunctions on November 5, 2040 and April 10, 2060, but these will not be as close. But there will one in another 60 years, on March 15, 2080, that will be slightly closer and higher in the sky as seen at sunrise. My grandson Jonah, who was born this year, will be a few months older than I am now on that day. So make the most of this very rare event!
As an aside, this Grand Conjunction is only visible from the Earth. Jupiter and Saturn were aligned with the Sun on about November 1, when Saturn would have been at opposition as seen from Jupiter, and Jupiter would have been at inferior conjunction as seen from Saturn. By December 21, these planets will have moved out of alignment with the Sun but will be aligned as seen from the line of sight of the Earth’s position in its orbit.
Friends, be sure to take a look in the coming weeks and witness this rare configuration of planets. Grand Conjunctions can be very inspiring. It’s been said that the 16th century astronomer Tycho Brahe witnessed a Grand Conjunction as a boy, and this inspired his lifetime passion for astronomy. Observe this event with your own children, maybe one of them will be the next Tycho!
For my part, my own expectations are low since our December weather here on the ice-encrusted shores of the Great Lakes is predictably cloudy in that season, as in many other temperate locales in North America and beyond. But we can expect to see excellent photos that will likely be shared on social media, which is better than nothing! If you have clear skies in your area, check it on behalf of those of under stuck under clouds!