The planet Saturn, seen by the Cassini probe, in 2012, while the Sun is behind.

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NASA / SIPA

An unusual rendezvous is expected in the sky on Monday.

Jupiter and Saturn, the two largest planets in the solar system, will come together as close as possible, during a “Great conjunction” which will not occur again until 2080 in such proportions.

After sunset, at 6:22 p.m. GMT (7:22 p.m. KST), the two gas giants will appear in the same field of view of an observing instrument.

They will give the impression of brushing against each other when they are actually located several hundred million kilometers from each other.

An absolutely clear sky

To enjoy the show, you will have to bring a small observation instrument, find a very clear sky, and look towards the South-West, over a strip of territory encompassing Western Europe (Ireland, Greater -Brittany, France, Spain, Portugal) and a large part of Africa.

The apparent rapprochement between the two planets has already started for several months, and it will reach a minimum distance on the day of the winter solstice (a coincidence of the calendar), which will almost give the impression that the two stars in the sky do one.

"The Great Conjunction" corresponds "to the time it takes for the two planets to find similar relative positions with respect to the Earth", explains Florent Deleflie, of the Paris Observatory - PSL.

Jupiter, the largest, circles the Sun in 12 years, Saturn in 29 years.

And every twenty years or so, the two planets seem to come closer when we observe the celestial vault from Earth.

Go back to 1623

By a perspective effect, the two giants will then appear side by side, "with a gap of only 6 arc minutes between them, which corresponds approximately to 1 / 5th of the apparent diameter of the Moon", continues Florent Deleflie.

"With a small observation instrument, even a simple pair of binoculars, we can see in the same field the equatorial bands of Jupiter and its main satellites, as well as the rings of Saturn", rejoices the astronomer.

To the naked eye, the rapprochement will give the impression of a double planet, "Jupiter and Saturn being two very luminous stars", he explains.

The last Great Conjunction took place in 2000, but we have to go back to 1623 to find a gap as small as that of Monday.

And before finding such a close conjunction, it will be necessary to wait until March 15, 2080. This evening, the show will last several tens of minutes.

And no regrets going home at 8 p.m. for the curfew, because the two planets will already be in bed.

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