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Great meetings of the moon, Jupiter and Saturn

August 27, 2020

Once every 20 years, the planets Jupiter and Saturn pass each other in what's called a great conjunction. This year's is their closest meeting in 400 years.

Astronomy Photographer of the Year 2020 I Our Moon
Image: Andy Casely

This year has been a special one for hobby stargazers, particularly if you like Jupiter and Saturn. The two planets — the fifth and sixth in our solar system, out from the sun — came their closest to Earth in July.

They have been moving west over the northern hemisphere summer, making them visible to many watchers in the early evening. That's also become possible because the Earth is currently on a smaller and faster orbit around the sun.

It's been possible to see Jupiter's moons Ganymede, Callisto, Europa and Io, Saturn's rings and its largest moon, Titan.

Infographic illustrating our solar system and the relative positions of the planets (DW)

But that's not all. The two planets are heading for a once in a generation "great conjunction."

That only happens once every 20 years or so, when Jupiter and Saturn pass close to each other in the night sky. On December 21 of this year, Jupiter and Saturn will be their closest since 1623.

Meetings with the moon

As August draws to a close, however, it will be possible to witness the moon in conjunction with both Jupiter and Saturn on two nights, back to back.

First, on August 28, our moon will be in conjunction with Jupiter. The moon will appear just south of the planet.

Infographic — the moon in conjunction with Jupiter and Saturn, August 2020 EN

If, for instance, you're on the east coast of America, in the Washington D.C. area, the moon will appear in the south-southeast about 24 degrees above the horizon at roughly 8:44 p.m. EDT (12:44 a.m. GMT, August 29). Jupiter "will appear about 2 degrees above the moon and Saturn will appear about 9 degrees to the left," says the American space agency, NASA.

Then, the following night — on August 29 from about 8:42 p.m. EDT (Washington D.C. area) — the moon will effectively switch sides and appear near Saturn.

NASA says the Moon will appear in the south-southeast at about 21 degrees above the horizon, with "Saturn appearing to the upper right of the moon and Jupiter appearing farther to the right."

Planet Jupiter
Jupiter is having a year of meetings with Mars, Pluto and a "great conjunction" with SaturnImage: picture-alliance/dpa/D. Peach

A waxing gibbous moon

The moon will change its own appearance and grow as it meets Jupiter and Saturn. It is currently what's described as a "waxing gibbous" moon.

A waxing moon is one which is growing. And gibbous, as defined by NASA, means that the moon is less than a full moon, but larger than its shape in its third quarter.

On August 28, the moon will be about 80% illuminated by the sun and by August 29, it will be about 90% illuminated.

That means the next full moon will follow early morning on September 2.

We have a new moon whenever our moon is aligned directly between the sun and the Earth — as seen from Earth. That occurs about every 29.5 days. And it's also known as a conjunction.

NASA — graphic illustrating the phases of the moon
The phases of the moonImage: NASA/Bill Dunford

Great conjunctions 

Conjunctions occur frequently. The term is used to describe any two objects in the solar system that align, such as two planets, the moon and a planet, the sun and a planet, or spacecraft.

A conjunction happens when the two objects appear close to the ecliptic — that is, the plane of Earth's orbit around the sun.

But while the two objects appear to be close viewed from Earth, they are not actually close in space.

In March 2020, it was possible to see Mars in conjunction with Jupiter. And Jupiter and Pluto are having what's known as a "triple conjunction" this year. They were seen close in April, June and will come close again in November.

Planet Saturn
It takes Saturn about 30 years to orbit the sunImage: picture-alliance/AP/NASA/JPL/Space Science Institute

But great conjunctions are far less frequent and are specific to a close meeting between those two very bright planets, Jupiter and Saturn — as seen from Earth.

It takes Jupiter just under 12 years to orbit the sun. It takes Saturn just under 30 years to orbit the sun. So, as Jupiter orbits the sun faster than Saturn, Jupiter crosses paths, as it were, with Saturn about every 19.6 years — you could say Jupiter "laps" Saturn, as if on a race around the sun.

There is some evidence to suggest that the astronomer Johannes Kepler believed the biblical Star of Bethlehem was caused by a great conjunction.

Astrologers also consider these events to be spiritually significant.

DW Zulfikar Abbany
Zulfikar Abbany Senior editor fascinated by space, AI and the mind, and how science touches people