Jupiter and Saturn converge towards closest conjunction in 400 years

Giant planets Jupiter and Saturn are closing together for a rare spectacle that has not been seen since the Middle Ages. The two worlds have been shining together in the constellation of Sagittarius in the southern sky for many months.

They are gradually drawing together, and on 21 December, 2020, just days before Christmas, they will resemble a bright “double star” after sunset.

It will of course be a line-of-sight effect, as Saturn is far beyond Jupiter in the Solar System, and will actually be 733 million km from it.

But at their closest, the two planets will appear separated by just 6.1 arcminutes – that only a fifth the apparent diameter of the Moon in the sky.

Due to the timing, the event is being compared by some to the Star of Bethlehem, an event which some have attributed to a planetary conjunction, if it was not simply a legend.

The planets are nearing the end of their current apparitions, so both will only be visible in the early part of the evening before they set.

For those at northern latitudes, you will need to look low in the south west, as soon as it gets dark, to spot the pair. Both are bright, though Jupiter is noticeably the brighter of the two.

The conjunction will be higher in the sky for astronomers in the southern hemisphere.

The planets will be so close that it will be possible to see them both, plus their brighter moons, in the same field of view with a telescope and moderate-power eyepiece.

The conjunction is the closest the two planets have appeared in the sky for nearly 400 years – the last time was in July 1623.

If it is cloudy on the night of the closest approach, you will be able to see the planets just a little farther apart on evenings either side of the 21st.

They will be separated by less than the apparent diameter of the Moon between the dates of 16 and 25 December, 2020.

As well as being of visual interest, the conjunction offers great opportunities to capture a great photo. With the two planets low in the twilit sky, it should be possible to add an interesting foreground to your composition.

Related: The night sky this month

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