Jupiter and Saturn in a close embrace on December 21st

Jupiter and Saturn are in a major conjunction on December 21st. At this year’s winter solstice, there is another special celestial event to look at.

Astronomical conjunction occurs when two celestial bodies seem to pass each other when viewed from the earth.

For great conjunction to occur, the two largest planets in our solar system must meet: When the orbits of Jupiter and Saturn overlap, the two giants seem to merge in the sky. This event can be observed approximately every 20 years.

However, some major conjunctions are larger than others. How closely the planets come together in the sky depends on the slightly oval shape of the orbits of both planets and the inclination of the respective orbit to the equator of the sun. In some major conjunctions, Saturn and Jupiter come extremely close. In other cycles, however, they pass each other at some distance.

Of course, in reality, the two planets are not at all close. When they meet on December 21, they will be approximately 730 million kilometres apart.

At the last major conjunction on May 28, 2000, the apparent distance between Jupiter and Saturn in the sky was 68.9 arc minutes. That is rough twice the diameter of a full moon.

Now in 2020 the grand conjunction – which, by the way, coincides with the winter solstice, i.e. the shortest day of the year in the northern hemisphere and the longest in the southern hemisphere – will only be around 6.1 arc minutes. That is a little less than the diameter of a ten-cent euro coin that you stretch out an arm’s length from you and hold it against the sky.

What did antiquity see in the night sky?

When it comes to major conjunctions, researchers tend to relate them to important historical events. Johannes Kepler had investigated whether the so-called Star of Bethlehem, which in Matthew’s Gospel leads the three wise men from the Orient to the birth of Jesus, was actually great conjunction. Kepler calculated that such a thing must have occurred around the year 7 AD.

“Astronomers often try to explain events from ancient times in such a way that an astronomical phenomenon is to be looked for behind them, which once fascinated people,” says Faherty. According to Hartigan, however, the ancient Great Conjunction at the turn of the ages probably wasn’t particularly noticeable.

When that Grand Conjunction is over, star lovers won’t have to wait that long to catch the next one. On March 15, 2080, the two giants will again be only six angular minutes apart. “A young person who goes out this time could see the next one in 2080,” says Hartigan. “It’s actually a nice connection between the generations. You can’t help thinking that people saw them in the past – and that people will see them in the future. “