Starship Asterisk*

Saturn and Jupiter in Summer 2020

Explanation: During this northern summer Saturn and Jupiter were both near opposition, opposite the Sun in planet Earth's sky. Their paired retrograde motion, seen about every 20 years, is followed from 19 June through 28 August in this panoramic composite as they wander together between the stars in western Capricornus and eastern Sagittarius. But this December's skies find them drawing even closer together. Jupiter and Saturn are now close, bright celestial beacons in the west after sunset. On solstice day December 21 they will reach their magnificent 20 year Great Conjunction. Then the two largest worlds in the Solar System will appear in Earth's sky separated by only about 1/5 the apparent diameter of a Full Moon.

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sounds like this conjunction will occure during suppertime!

APOD Robot wrote: ↑Sat Dec 12, 2020 5:05 am Saturn and Jupiter in Summer 2020

Explanation: During this northern summer Saturn and Jupiter were both near opposition, opposite the Sun in planet Earth's sky. Their paired retrograde motion, seen about every 20 years, is followed from 19 June through 28 August in this panoramic composite as they wander together between the stars in western Capricornus and eastern Sagittarius. But this December's skies find them drawing even closer together. Jupiter and Saturn are now close, bright celestial beacons in the west after sunset. On solstice day December 21 they will reach their magnificent 20 year Great Conjunction. Then the two largest worlds in the Solar System will appear in Earth's sky separated by only about 1/5 the apparent diameter of a Full Moon.

So, assuming the left-most planet images are the oldest and the right-most the newest, it looks like Jupiter and Saturn ended up farther apart on Aug 28 than they were on June 19. Yet they will apparently get closer together from Aug 28 to Dec 21?

johnnydeep wrote: ↑Sat Dec 12, 2020 4:19 pm

APOD Robot wrote: ↑Sat Dec 12, 2020 5:05 am Saturn and Jupiter in Summer 2020

Explanation: During this northern summer Saturn and Jupiter were both near opposition, opposite the Sun in planet Earth's sky. Their paired retrograde motion, seen about every 20 years, is followed from 19 June through 28 August in this panoramic composite as they wander together between the stars in western Capricornus and eastern Sagittarius. But this December's skies find them drawing even closer together. Jupiter and Saturn are now close, bright celestial beacons in the west after sunset. On solstice day December 21 they will reach their magnificent 20 year Great Conjunction. Then the two largest worlds in the Solar System will appear in Earth's sky separated by only about 1/5 the apparent diameter of a Full Moon.

So, assuming the left-most planet images are the oldest and the right-most the newest, it looks like Jupiter and Saturn ended up farther apart on Aug 28 than they were on June 19. Yet they will apparently get closer together from Aug 28 to Dec 21?

• Retrograde motion doesn't last forever.

During this northern summer Saturn and Jupiter were both near opposition, opposite the Sun in planet Earth's sky.

Their paired retrograde motion, seen about every 20 years, is followed from 19 June through 28 August.

johnnydeep wrote: ↑Sat Dec 12, 2020 4:19 pm

APOD Robot wrote: ↑Sat Dec 12, 2020 5:05 am Saturn and Jupiter in Summer 2020

Explanation: During this northern summer Saturn and Jupiter were both near opposition, opposite the Sun in planet Earth's sky. Their paired retrograde motion, seen about every 20 years, is followed from 19 June through 28 August in this panoramic composite as they wander together between the stars in western Capricornus and eastern Sagittarius. But this December's skies find them drawing even closer together. Jupiter and Saturn are now close, bright celestial beacons in the west after sunset. On solstice day December 21 they will reach their magnificent 20 year Great Conjunction. Then the two largest worlds in the Solar System will appear in Earth's sky separated by only about 1/5 the apparent diameter of a Full Moon.

So, assuming the left-most planet images are the oldest and the right-most the newest, it looks like Jupiter and Saturn ended up farther apart on Aug 28 than they were on June 19. Yet they will apparently get closer together from Aug 28 to Dec 21?

This is a matter of motion and perspective. Think of the planets as racing cars around a track. You are viewing not as a spectator but as a passenger in the fastest car. Think of Earth as a racing car going around the track at 100 miles an hour. Nearby trees are quickly passed and left behind but trees far in the distance are passed much more slowly. Also Earth has the inside track. Jupiter is in the middle track but it is built much more larger so Jupiter has much more distance to travel. Say if Earth's 1 year orbit is equal to the race car going 100 mph, then Jupiter is moving at 10 mph. So you see Earth quickly catches up to Jupiter, passes and leaves Jupiter behind in the dust. Saturn even farther out than Jupiter is actually moving at 2.9 mph (Almost a 30 year orbit.) So Saturn will soon be passed by Jupiter as well on the race track. Now return back to the planet's regular orbit speed. If this close conjunction happened at the end of June, and we will see the Jupiter/ Saturn approach at the end of December, that means some 6 months has passed by. The planet Earth is now on the opposite side of the Sun in its orbit and is seeing Jupiter and Saturn from the back stretch, actually going in the opposite direction. Good opportunity to actually see Jupiter pass by Saturn.

johnnydeep wrote: ↑Sat Dec 12, 2020 4:19 pm
So, assuming the left-most planet images are the oldest and the right-most the newest, it looks like Jupiter and Saturn ended up farther apart on Aug 28 than they were on June 19. Yet they will apparently get closer together from Aug 28 to Dec 21?

Yes, this is correct. It didn't bother me all summer that they did not seem to be converging until I heard that they'd be in conjunction on Dec 21. At that point, probably in early August, I started watching for them to get closer, and it seemed to me that they were not making near enough progress to get there by Dec 21. Then it finally occurred to me why this is. (I'm elaborating on De58te's description with some visual aids.)


I have the following, which is very approximate, taken from solarsystemscope.com

In the first image, on June 21, 2020, you can see that Jupiter and Saturn are near opposition from the Sun, viewed from Earth. And it looks like they should be pretty close in the sky. Then, if you look at the positions each month following, you can see that Jupiter is slowly "catching up" to Saturn. Throughout the next 6 months, it slowly slides between Saturn and the Sun (Saturn comes into opposition as viewed from Jupiter). But the motion of the Earth during this time is much more dramatic and is having the most significant effect on the alignment of the two giant planets from our viewpoint. Indeed, it looks as though some time in early November, we reached our maximum elongation (from the view of the giant planets). This would have increased the angle between them that we saw, even as Jupiter's progress was trying to reduce it. But then, finally, as the Earth began to get to the far side of the Sun from the giant planets, our own position will finally help us to see them lined up. We're not fully around to the opposite side from them, but we reach a point very close to being on a line drawn through Saturn and Jupiter on Dec 21.

One unfortunate thing that this shows is that as they've come into conjunction, they're quite a bit farther away from us now than they were in the summer. So they're going to appear quite a bit smaller in a telescope on Dec 21 than they did last summer.

Thanks to MarkBour, De58te and neufer for the helpful explanations! Orbital geometry was always hard for me to visualize.