Viewing the "Christmas Star"

December 21, 2020

Jupiter and Saturn in their closest conjunction since 1623AD, 397 years ago! The last time it happened at night like this was 1226AD, 794 years ago.

Brenda had been reading about today being the best day to see the "Christmas star", so we drove over to our best star-gazing point, the hill in the Mt. Harmony Cemetary. We got there about 5:30pm and were the first car there, and parked on the hilltop with a good view. By 6pm we were getting the pink sky above the church, and it got quickly darker after that. The view of Jupiter and Saturn at left was taken at 6:14pm.

The ecliptic plane shows that the Earth's rotation is tilted 23.5° with respect to the plane representing the celestial equator in which all the planets are revolving around the Sun. This day is the winter solstice, the shortest day of the year. That tilt is what gives us our seasons. If they were all in exactly the same plane, I'm thinking that Jupiter and Saturn should have appeared to be on the same arc containing the Moon, but we are looking at them over a vast distance, much greater than the distance from the Earth to the Sun, so if Saturn orbited the Sun is just a slightly different plane, it would account for it appearing slightly off the arc. I'll have to talk to one of my astronomer colleagues about that.

From Wikipedia I find the following: "Saturn's orbit plane is inclined 2.485 degrees relative to Earth's, and Jupiter's is 1.303 degrees.", so that may account for what I saw. This closest separation was 6.1 arcminutes compared to 2.1 arcminutes in 1226AD.

This is a NASA depiction of the kind of alignment that results in a conjunction like we saw. The weather was excellent for our viewing. I was surprised to get that sharp an image of the planets, using 400mm telephoto at about 1/20sec, ISO 800, f5.6

This was our view of the Atlanta skyline from Midtown down to the dome at 5:15pm. We were perched on the highest point in the cemetary.

This is the skyline view at 6pm. It was getting dark quickly.

Another 6pm view.

The skyline view at 6:04pm, just 10 minutes before the picture of the two planets above. From the camera settings, the light sensitivity had been increased by a factor of 17 in the 49 minutes between the first and last skyline photos above. By the time we left about 6:30 the road at the top was covered with cars and a number of family groups had parked at the church and walked out to that hilltop.