Ernie Jacobs

The Great Conjunction of December 2020

Posted on by Ernie Jacobs

Have you been watching Jupiter and Saturn? If you have, you probably noticed they appear to be getting closer to one another. From our vantage point here on Earth, they are. Both planets are heading towards the Great Conjunction on December 21st, when they will appear closest to each other. Many of our members are genuinely excited about this special and rare event. BAA member Patrick Crants discussed the Great Conjunction during the November 21st “Looking UP!” virtual star gazing event (click here to view the video on our Facebook page: https://fb.watch/22lkkJo-rW/ ). The following was assembled from his presentation.

Live Event Alert: The BAA is planning on sharing this event live on Facebook, weather depending, between December 18 – 23. You can find out more on information about this live event on our Website (https://www.buffaloastronomy.com/events/) as the date approaches. All our videos and Facebook events can be accessed here https://www.facebook.com/BuffaloAstronomy/videos/?ref=page_internal.

The Jovian and Saturnian Systems as they will appear during the Great Conjunction on December 21, 2020. Image created using Sky Safari 6 Pro for Mac.

What is a Conjunction?

  • A conjunction is simply an optical alignment of two celestial bodies (normally solar system objects – planets, moon, comet, asteroids, etc.) as seen from our perspective on Earth.
  • Jupiter/Saturn conjunctions are called “Great Conjunctions” by astronomers.
  • A conjunction is only an optical alignment. It does not imply any proximity between the two bodies. (See Figure 1)
  • During the great conjunction of December 21, Jupiter will be 5.927 AU’s from Earth (555 Million miles) while Saturn will be 10.829 AU’s (1 Billion miles), meaning Saturn will still be in excess of 400 million miles from Jupiter.
Figure 1 – From our view here on Earth, both planets will appear to be very close to one another even though they will be hundreds of millions of miles apart.

How often do Great Conjunctions Occur?

  • A conjunction of the planets Jupiter and Saturn occurs once every 19.6 years on average.
  • Due to the nuances of planetary orbits, not every conjunction reaches the same degree of separation. The conjunction of 12/21/2020 will be the closest great conjunction witnessed in the last 400 years (1623).
  • Oddly enough, this degree of separation will be repeated in just another 60 years (in 2080)

How close will they appear to be?

  • At its closest conjunction (appulse), Jupiter and Saturn will be separated by an angle of just over 6 arc-minutes. An arc-minute is one 60th of a degree.
  • For comparison, the full moon has an angular size of 30 arc-minutes, 5 times greater than the separation we will see between Jupiter and Saturn.
  • This means that both planets and many of their respective moons will be visible (see figure 2) within the eyepiece of a medium length telescope at a medium magnification (1-150x). To the naked eye, the two planets may be difficult to “split”. Note: Magnification may be limited by atmospheric conditions as both planets will be very close to the horizon.
Figure 2 – A simulation of the view through a telescope on December 21st. Image created with Stellarium.

When and how can this be viewed?

  • Jupiter and Saturn will be closely spaced for several days before and after minimum conjunction of 12/21.
  • They will be low in the Southwest at dusk.
  • They will set within 90 minutes of sunset.
  • The conjunction can be viewed naked eye, using binoculars, or with a telescope.
  • Weather permitting, the BAA will be providing a “Looking Up!” live stream of the conjunction on at least one evening between 12/18 and 12/23. For more details, check out out our website and Facebook Page as the date approaches.

Clear Skies!