Finder Chart for Jim’s Pick of the Month December 2020 Grand Junction of and

Image via Rice University

On December 21, 2020, the day of the December solstice is the great of Jupiter and Saturn. It will be the first Jupiter and Saturn conjunction since the year 2000 and the closest conjunction since 1623 - 14 years after Galileo made his first telescope. The closest observable Jupiter and Saturn conjunction before that was in the year 1226. At their closest in December, Jupiter and Saturn will be only 0.1 degree apart. The closest Jupiter-Saturn conjunction in 2020 will not be seen again until March 15, 2080.

From Portland, after sunset at 4:30 p.m., Jupiter and Saturn conjunction will be low in the evening twilight and will set quickly, so a good clear southwestern horizon is essential. Viewed from earth, Jupiter and Saturn will be 0.1 degree apart. Jupiter will appear brightest at magnitude of -1.97, while Saturn, to the upper right, will be half the brightest at +0.63 magnitude. Far above the southern horizon is the near first quarter and red planet Mars.

They appears to be close, yet separated 455,762,323 miles apart from each other. The planet Saturn, the sixth planet from the sun, is 1,006,711,393 miles from earth, while Jupiter, the fifth planet, is 550,949,070 miles away. Both Jupiter and Saturn will set at 6:52 p.m. towards the SW horizon.

Binoculars will separate them into two objects with Saturn, the fainter of the two, lying above the mighty Jupiter. For most telescope viewers, will see Saturn’s rings and Jupiter’s belts in the same view, along with the four Galilean of Jupiter and Saturn’s largest moon, .

Astronomers use the word conjunction to describe meetings of planets and other objects on our sky. They use the term great conjunction to describe meetings of the two biggest planets in our , Jupiter and Saturn.

These great Jupiter and Saturn conjunctions happen every 20 years. Each year, Saturn completes about 12 degrees of its around the sun, whereas Jupiter completes about 30 degrees. Therefore, in one year, Jupiter closes the gap between itself and Saturn by about 18 degrees (30 – 12 = 18 degrees). In a period of 20 years, then, Jupiter gains 360 degrees on Saturn (18 x 20 = 360 degrees), therefore lapping the ringed planet once every 20 years. According to Time and Date, over a period of one thousand years, from 1600 to 2599, there are only six great conjunctions where the minimum separation between Jupiter and Saturn is less than 0.2 degrees: 1623, 1683, 2020, 2080, 2417, and 2477. Over the next 10,000 years, Jupiter will transit or occult Saturn three times: February 16, 7541 (a transit); June 17, 7541 (an ); and February 25, 8674 (a transit).