Friday, September 17, 2021

Great conjunction; it’s not usually this good


Stargazers from Earth could clearly see Jupiter and Saturn in the night sky Monday night closer than they have been in hundreds of years.

Tuesday’s night sky from Exeter, UK (Steven Rieder/Flickr Creative Commons)

Four moons around Jupiter are visible in the astrophotography, which also shows the planet’s Great Red Spot, a storm with crimson-colored clouds. You can also see Saturn’s shadow being cast on the planet’s rings.

A “great conjunction” occurs when Saturn, Jupiter, and Earth come close to being in a straight line, meaning Jupiter and Saturn come very close to each other in the Earth sky. Because of the orbits of the planets, a great conjunction occurs about every 20 years. For this one, Earth is actually on the other side of the sun from Jupiter and Saturn. That makes this one a little close to the sun to see perfectly or for a very long time just after sunset.

But Jupiter and Saturn haven’t been this close to each other in the night sky since July 7, 1623. During that great conjunction, it wasn’t very easy to see the planets. And Jupiter and Saturn, as seen from Earth, were both very close to the sun in that year.

The closest one before that, which was perfectly visible in the morning sky, was on March 4, 1226. The minimum separation this time was 0° 6.1′ (6.1 minutes) of arc, compared to only 2.1 minutes of arc in 1226.

Jupiter and Saturn will be separated by more than 1° of arc during the great conjunctions of November 4, 2040, and April 8, 2060.

For reference, the sun and moon span about 1° of arc in the sky, and there are 60 minutes in a degree. That means, the two planets were separated by a little more than one-tenth the apparent size of the moon Monday.

The two planets will come a little closer—to within 6.0 minutes of arc—on March 15, 2080. That’ll then be the only great conjunction where the planets are separated by under 10 minutes of arc until well after the year 2400.

  • Sortable tables of great conjunctions
Paul Katula
Paul Katula is the executive editor of the Voxitatis Research Foundation, which publishes this blog. For more information, see the About page.


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