Since 2020 opened, we've discussed the "great conjunction" of Jupiter and Saturn in the What's Up? almanac. This is the year when the gas giant Jupiter overtakes Saturn on its orbital path. But what exactly does that mean? Jupiter and Saturn are outer planets because they orbit the Sun outside of the Earth's orbit as shown in the graphic,. Note that the planets are not to scale, and planet Earth is tiny compared to the gas giants in the solar system.

Jupiter is the largest planet in the solar system, containing about 99% of the mass outside of the Sun itself. Jupiter's orbit of the Sun takes 11.86 years to complete. The gas giant Saturn has a fabulous and famous ring. It takes 29.46 years for Saturn to make a complete circuit around the Sun. Jupiter will only catch up with Saturn twice in those 29 years. (Actually 2.48 times.) That is called, the great conjunction, and it is said to occur once in a generation. It means that Jupiter and Saturn will appear close together in the sky from Earth. It happens once every 20 years as all the planets keep making their rounds. Luckily for us,during the great conjunction of 2020, planet Earth is in a great position for observers to really enjoy the show. Jupiter and Saturn are over the horizon when it appears that Jupiter finally laps Saturn on December 21, 2020. Coincidentally, this is the day of Earth's Winter solstice this year, too!

What is a conjunction, anyway? Basically a conjunction is two astronomical bodies (Sun, Moon, planets, and even stars) that appear close to each other as viewed from a third body. For us, that third body is the Earth unless otherwise noted. It is important to realize that such conjunctions are merely "apparent" because light from astronomical objects is refracted by the Earth's atmosphere and this affects the position. There may also be more than two bodies in conjunction, in the case of Jupiter and Saturn during their great conjunction, a 3-day old Moon will pass 3 deg. S of Jupiter and Saturn while they are a mere .4 deg apart, making a triple conjunction!

They've been dazzling in the night sky for a few months now. Earth sped past Jupiter when the planets reached opposition on July 14, Jupiter was 180 deg. away from the Sun and shining brilliantly at -2.8. Telescopically, Jupiter is accompanied by 4 large moons first discovered by Galileo. Saturn was at opposition on July 20, shining at +0.1 mag., its rings tilted at 21.7 deg. toward Earth and a disk 18.4". Saturn's rings are made of ice crystals orbiting the planet at various distances. That is somewhat like the ridges on a vinyl record album. There is one large gap near the center of the rings, dividing them into two parts called "The Cassini Division." This gap can be seen with small telescopes and there are other gaps and variations in the crystals that can be seen with larger amateur instruments on a clear night. At opposition (when they are opposite the Sun in the sky as seen from Earth) both planets were in the constellation of Sagittarius.

The image above shows where Jupiter and Saturn were near the time of their oppositions - - 6 days apart. For the outer planets, opposition is the best time to observe them because they will be in the sky all night. They transit the meridian around midnight when they are highest in the sky. They rise as the Sun sets, shine all night and then set just before the Sun peaks over the horizon. From this point on, they will rise before the Sun sets, a bit earlier each day. Also, opposition is the time that the outer planets show their whole disk, that means the disk is 100% illuminated. Only outer planets appear with a fully illuminated disk because we can see them from Earth when the Sun is behind us.

Each opposition is known as an apparition, it begins when the planet first appears in the dawn sky. The planet moves in the sky until opposition is reached by Earth and the outer planet in their orbits. The apparition ends when the planet is last sighted on the western horizon at sunset. It should be said before closing that not all great conjunctions are visible from Earth. In 2020, during the last great conjunction, Jupiter crossed in front of Saturn when both planets were too close to the Sun to be observed. In 2040, Saturn and Jupiter will meet up again and perform a similar dance, this time at the western side of the constellation Virgo.

Future issues of the What's Up? almanac will continue this discussion of the Great Conjunction until the end of the year. Check it out each month as this once in a generation event flies past us in the night sky.

--See You Under the Stars!
Astra for Astra's Almanac