The Great Conjunction

Jupiter and Saturn have been dazzling in the evening sky for months now. As Jupiter passes Saturn in its orbit, they appear very close together in Earth's sky. Because Jupiter is closer to the Sun and moves faster than Saturn, it passes the ringed planet in its orbit once every 20 years. This is the Great Conjunction that happens once in a generation. The great conjunction occurs on December 21, 2020, the same date of Earth's winter solstice. We are extremely lucky that this year's conjunction will occur when Jupiter and Saturn are over the horizon. This was not the case for the 2000 great conjunction that was too close to the Sun to be observed. This is often the case for great conjunctions, but the dance of the planets can be observed for many months while they move about their orbits. After the great conjunction the outer planets will seem to move apart.

Although Jupiter and Saturn have spent most of 2020 in the eastern side of Sagittarius, in October both planets reached their stationary points and now are continuing in their usual prograde direction. On December 18, both Saturn and Jupiter will cross into the constellation of Capricornus, and so the actual Earth-based moment of conjunction will occur on the far western edge of that constellation. That moment occurs at 14UT and means that the actual moment will have passed that evening when the Sun sets as early as it ever does in the northern hemisphere. (In New York City, local time for sunset on that day is ~4:32 PM while the moment of closest conjunction is at 10am EST.) At that time, Jupiter will be 0.1 deg away from Saturn in Earth's sky!

It should go without saying that the Great conjunction is not merely those special few moments when the distance between two planets is smallest. The coming of the great conjunction was obvious in the Summer of 2019 when Jupiter was in Ophiuchus and Saturn was sitting in Sagittarius. They came to opposition about 1 month apart. Because Jupiter passes across ~30 deg. of the ecliptic every year, it seems to move 1 constellation per year. Saturn might have been ahead of Jupiter but the gas giant was certainly catching up. That being said, the great conjunction could be seen easily over the past 9 months and after the great conjunction, Earth observers can watch as the distance between the two giant planets increases. In the evening sky, observers can watch as Jupiter pulls away from Saturn. The show will be brief as Saturn will disappear in the west the first week or so in January, with Jupiter following shortly thereafter..

But Saturn and Jupiter have not been alone on their journey, for each month the Moon sped past the two outer giants. One might think that each month a fabulous conjunction could have been seen, but in North America, most of the time those conjunctions occurred during the day with the Sun shining. By the time Saturn and Jupiter were over the horizon, the close circumstances of those conjunction had past. But in December, the Moon appears below Saturn and Jupiter, when it is a mere 2.4 days old. This is sometimes called the "toenail" moon. Generally, the dark surface of the moon can be seen because that part of the moon is still facing the Earth. Earth is very bright, that is, it has high albedo. It can reflect up to 35% of the light that reaches it but it varies a bit due to cloud and snow cover. The dark surface of the moon reflects that light and sometimes even the surface features can be made out, especially with optical aides. For the December 16 image, the location of Miami, FL was used so that the planets were well above the horizon.

To illustrate the great conjunction, these three images were taken out of Stellarium, the faithful open-source program that has been used for many images at Astra's Stargate. These images were created using Miami, FL for the location. This is because it is at 24 deg. N latitude. This gives a dark sky for the early evening setting of the two planets. Jupiter sets around 7:45pm local time at this location on the night of the conjunction. Miami was also used to create the image of the December conjunction with the Moon. For the close up views, a FOV or field of view in Stellarium covered about 6.5 deg but only a small portion of that field was used. At this scale, Stellarium shows the presence of the brightest of Galilean moons.

The top image shows the location of Jupiter and Saturn at the moment of the great conjunction that is at 14UT or 10am EST. This can only be observed in locations where the Sun is not shining. This means that people fortunate enough to be located in Asia, India or Australia AND the planets are above the horizon at your location, you may be able to observe the great conjunction at the moment of closest approach. An astronomical conjunction means that two objects are located on the same RA or hour angle, basically seen as one object directly above or below another. Another astronomical definition is that both the objects are on the same ecliptic longitude. In the case of the great conjunction of 2020, Jupiter and Saturn are located at 300 deg. ecliptic longitude. But conjunction also has a "civil" meaning. Generally, if 2 objects in the sky are within 5 deg. of each other, it can be said that they are in conjunction.

So actually, the great conjunction is more than just that moment in time when the 2 planets seem to make their closest approach in Earth's sky. The second image shows the positions of Jupiter and Saturn at 7pm on December 21, 2020, when the sky over Miami will be dark enough to see the two planets--but look! Jupiter is now on the other side of Saturn. When Jupiter is directly in front of Saturn, inhabitants on Jupiter could say that Saturn was at opposition. What a gorgeous sight the ringed planet would make in the sky over Jupiter, shining all night long! The distant planet of Mars comes into opposition every 2 years plus 50 days, but at Jupiter, opposition of Saturn only comes once every 20 years. And at those great distances, it would be very hard for the giant planets to detect Earth because it is so close to the Sun looking back from the outer solar system.

The last image of the great conjunction comes about 11 days later at on the first day of 2021. Jupiter appears as though it has sped away from Saturn, and indeed, it has. But in this image, the 2 planets are separated by only 2 degs. This means that the images are showing only about a 2.5 deg square of the sky, large enough to fit 5 moon diameters across. The brightest star in the images is 5.25 mag star known as Sigma Capricorni. It would probably hard to see this star even at a dark site because of the glare from Jupiter. Jupiter will disappear in the sunset early in the month, it will be at conjunction with the Sun on January 29. Saturn will disappear even faster, not just because it is fainter, but its conjunction date is January 24, 2021. When Saturn first slips out of the early morning glare of the Sun in mid-February, Jupiter returning before the end of February. By the 28th of February, Jupiter and Saturn will be ~8 deg. apart and somehow, the fleet-footed planet Mercury will be poking up between them.

Now we must end the adventure of the great conjunction. We should not be sad, though, because the dance of the planets will continue as long as the solar system continues to exist in its present state. In 10 years or so, Jupiter will be 180 deg. away from Saturn but it will start to catch up again. Because the planets are bound to the Sun, that's where we will find them.

What's Up has been focused on the great conjunction this year. The first article in the series was presented in the August 2020 What's Up? issue. The second article appeared in the September 2020 What's Up? issue. Please keep reading the What's Up almanac for more information on the great conjunction and the latest astronomical news. Who knows where we will be for the next one?

Astrologers have long pointed at the great conjunctions of Jupiter and Saturn as bringing plague to Earth and so have associated the 2020 conjunction as a cause or portent of coronavirus. Astra's Stargate is an astronomy site and I only want to state that planetary line-ups are all Earth-based. Jupiter actually lapped Saturn on November 2, 2020 if the solar system is viewed from above, but from Earth's surface this event appears to occur 49 days later. Planets in our solar system, including Earth have stable orbits around the Sun. They resonate with each other because they have each made their orbits together for countless millennium. They are entirely predictable when the cycles they follow are understood--they don't rule human lives.

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