The Christmas Star – 2020 Jupiter-Saturn Great Conjunction

Jupiter and Saturn will align in tonight’s (December 21st, 2020) skies in what astronomers are calling the “great conjunction.” Tonight’s great conjunction, nicknamed the “Christmas Star,” marks Jupiter and Saturn’s closest apparent encounter in nearly 400 years.

What is the Great Conjunction?

Saturn and Jupiter appear to close in on each other as the Great Conjunction 2020 approaches on 21 December (apparent distance given in degrees and arcminutes). Credit: Pete Lawrence
Saturn and Jupiter appear to close in on each other as the Great Conjunction 2020 approaches on 21 December (apparent distance given in degrees and arcminutes). Credit: Pete Lawrence

Astronomers use the word conjunction to describe meetings of planets and other objects on our sky’s dome. They use the term great conjunction to describe Jupiter and Saturn’s meetings, which are the two biggest worlds in our solar system. Though the two planets will appear spectacularly close together on the sky’s dome now, Jupiter and Saturn are actually 456 million miles (734,000 million km) apart. Saturn is nearly twice as far away as Jupiter.

Jupiter-Saturn conjunctions happen every 20 years; the last one was in the year 2000. But these conjunctions aren’t all created equal. The 2020 great conjunction of Jupiter and Saturn will be the closest since 1623 and the closest observable since 1226 — and nearly 800 years since skywatchers were able to witness the event at night! 2020’s extra-close Jupiter-Saturn conjunction won’t be matched again until the Jupiter-Saturn conjunction of March 15, 2080.

The planetary alignment has also been called a “Christmas star,” since it falls on the first day of winter in the Northern Hemisphere and only a few days before Christmas. 

Where (and when) to look?

In Trinidad and Tobago, the best time to see the great conjunction will be shortly after sunset (5:49 PM) and look towards the southwestern skies.

About 30-45 minutes after sunset observers should look 10º above the southwest horizon to see Jupiter and Saturn shining almost as one.

Both planets will dip below the horizon at 8:00 PM

The great conjunction will take place just within the boundaries of the constellation of Capricorn while a 50% illuminated First Quarter Moon shines high in the south.

You can read more about how to photograph the celestial event at Sky At Night Magazine.

What you’ll see during the Great Conjunction

Though to the naked eye, the planets will appear at first glance to shine as one, the close study will reveal otherwise. Through binoculars, you will easily see Jupiter and Saturn separately in the same field of view.

Using a small telescope, which will only need a low-power eyepiece (around 50x) to separate the two planets in the same field of view.

It should also be possible to see Saturn’s rings, its giant moon Titan, and Jupiter’s Galilean moons Ganymede, Io, Callisto and Europa all in the same field of view.

A 200mm lens will be required to capture a photograph of the planets and possibly Jupiter’s moons in the gathering darkness.

While Jupiter shines at a magnitude of -1.97 during the event, Saturn will be far less bright at magnitude +0.63.

How will our skies behave?

12:45 PM Visible satellite imagery showing a large expanse of cloudiness, showers and isolated thunderstorms east of T&T due to a surface to low-level trough. (NASA/CIRA/RAMMB)
12:45 PM Visible satellite imagery showing a large expanse of cloudiness, showers and isolated thunderstorms east of T&T due to a surface to low-level trough. (NASA/CIRA/RAMMB)

Not great news for local astronomers, astrophotographers and sky enthusiasts. A surface to low-level trough is forecast to continue affecting the Windward Islands, including T&T tonight.

This means partly to mostly cloudy skies, with models showing few areas with 30-50% cloud cover across parts of Central and Northern Trinidad with most areas experiencing 70-90% cloud cover and even periods of showers, briskly moving across both islands. Heavier rainfall is forecast to favor the southern and eastern areas of both islands.

Given that weather is not forecast to be friendly to our skygazers, you can still catch the great conjunction with a number of webcasts: the Virtual Telescope Project, the online observatory Slooh, the Lowell Observatory, the Vanderbilt Dyer Observatory, and the University of Exeter.

If you’re staying up late tonight, it’s not the only celestial event to look out for, check out the details on the Ursids Meteor Shower!

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