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'Great Conjunction' seen for first time in 800 years

Posted December 16, 2020 12:01 p.m. EST
Updated December 21, 2020 10:05 p.m. EST

On Dec. 21, after drawing visibly closer and closer in the night sky over the past few months, Jupiter and Saturn will appear the closest they have since March 4, 1226, nearly 800 years ago. They were even closer on July 16, 1623, but the pair were too close to the Sun for Renaissance era observers to see them.

Jupiter and Saturn will pass within 6 arc-minutes (0.1 degrees) or about 1/5th the diameter of the full Moon. The conjunction on February 16, 1961, was 14 arc-minutes or about 1/2 the Moon's diameter. Another sub-degree conjunction will not occur again until March 15, 2080.

How to see it

Look in the southwest around 6 p.m. They’ll be hard to miss. The pair will be above the horizon until 7:30 p.m. local. As they set, look for them to appear to move further apart a bit due to the Moon illusion.

Look each evening to watch them grow closer, then further apart after Dec. 21. You should be able to fit a dime, which covers about 1/10th of a degree when held on edge at arms length, between them. For comparison, your little finger covers about a degree. This works with kids or adults.

The event is exciting to astronomers because you will be able to see both planets, and their moons, in the same field of view with a small telescope or even a pair of binoculars.

The Morehead Planetarium and Science Center in Chapel Hill is also hosting a streaming event beginning at 6 p.m. with live views from Raleigh Astronomy Club telescopes. Free registration is required.

What are conjunctions?

The technical definition is when two planets (or a moon, spacecraft, etc.) have the same right ascension, the longitude part of astronomical coordinates. The practical definition is when two objects appear very close in the sky, within a few degrees of each other.

This is measured in angular separation. The objects are of course physically very distant. Jupiter is nearly 545 million miles away, and Saturn another 456 million miles beyond that.

Jupiter and Saturn appear close together but are separated by more than 450 million miles

Think of the planets like cars on a race track. Earth completes a lap in a year, Jupiter 11.9 years and Saturn 29.46 years. Conjunctions occur when a speedier planet like Jupiter passes a slower one in an outside lane. Jupiter catches up to and passes Saturn every 19.6 years, but not all conjunctions the same.

Jupiter "laps" Saturn in the great conjunction December 21, 2020

The lanes of the solar system speedway aren’t level, so how close the pair appear as Jupiter laps Saturn depends on what point on the track that occurs. Jupiter’s orbit is tilted 1.3º and Saturn’s 2.5º. which means they can be separated by nearly 4 degrees or a lot less. Passes within 1/2º

How rare are conjunctions?

Conjunctions of the planets are very common, occurring about once a week and most often involving the Moon and the speedier inner planets of Venus and Mercury. During 2020, there were 56 conjunctions of the visible planets plus the Moon (plus another 96 if you include those that a telescope: Uranus, Neptune and Pluto). There were also a dozen or so more that occurred during daylight hours.

Conjunctions involving the outer planets (like Jupiter and Saturn) are far more rare because these planets take so long to orbit the Sun.

The "great conjunction" of Jupiter and Saturn on Dec. 21 is the 18th closest in the 430 times this has been/will be visible between 3000 BC and 3000 AD. To find a closer Jupiter-Saturn conjunction you have to go back to July 16, 1623, but the pair were within 13 degrees of the Sun, lost in the twilight. The conjunction on March 4, 1226, would have been visible. They were separated by 2 arc-minutes or about 1/15th the Moon’s width.

Using the same data describing planetary positions that NASA/JPL uses in planning missions, across 3000 BC to 3000 AD, of the nearly 500 conjunctions of Jupiter and Saturn, about half are within a degree. Only 3.4% of conjunctions are closer. The next closer conjunction will occur on March 15, 2080.

On even rare occasions, the planets pass each other on a part of the track where the lanes are level. When this happens, Jupiter being larger completely block, or occult, Saturn. Retired NASA astrophysicist Fred Espenak and Italian astronomer Aldo Vitagliano ran their calculations over 80,000 years and found the last Jupiter-Saturn occultation on June 1, 6856 BC. Saturn's rings would have been visible behind Jupiter, but the telescope would not be invented for another 8 millennia. The next one will occur on February 16, 7541.

Christmas Star

You may have seen comparisons of next week's conjunction and the star of Bethlehem, but don't expect the pair to shine as brightly as some of the images you may have seen on social media.

Morehead Planetarium and Science Center featured a show during the holidays for many years which illustrated possible astronomical explanations offered by astronomers and biblical scholars. These include a comet, supernova, as well as a conjunction of bright planets such as Jupiter and Venus.

However, none of the close appearances of Venus and Jupiter during the 6 to 4 BC time period generally accepted by theologians as the birth year of Jesus were any closer than the width of the Moon (1/2 a degree), not likely to appear brightly at all.

Still, the great conjunction of Jupiter and Saturn provides a unique opportunity you can share with the whole family that provides a glimpse into the scale of the solar system.

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