If there’s one thing amateur astronomers ask at this time of year, it’s the star of Bethlehem.

There have been many candidates over the years to explain the Christmas star from one eclipse to the various combinations of planets, and there is not a single order with majority support.

However, this year we are going to see one of the main suspects, the closest combination of the two largest planets, the closest in 400 years. As an added bonus, this happens on an astronomically important date: Winter Solitis, December 21st.

First of all, what is a combination? In astronomy, two (or more) objects appear in the sky, one on top of the other. Since all the planets in the Solar System lie on approximately the same plane, a planetary conjunction automatically means that they appear very close to the sky. When the planets involved are Saturn and Jupiter, it is called the “great conjunction.”

It takes Jupiter 12 years to orbit the Sun and Saturn 30 years. This gives a cycle in which the two planets are very close in the night sky from our point of view on Earth about once every 20 years. Usually “too close” here means that the planets will look as close as two bright stars, but this year it’s hard to tell them apart, but a pair of binoculars or a telescope of any size will allow you to see the disks of the two planets. It is the largest fusion since 1623, shortly after the invention of the telescope.

There is no written record of this combination being observed, as Saturn and Jupiter were very close to the Sun at this time, and would be below the horizon at the same time as the Sun set. The first record of a large combination of telescope observations is from 1683, and this year it is almost twice as close.

If you want to see this year’s combination, you have to be somewhere with a clear horizon on the south / southwest side. If you look in this direction just after sunset, say from 4:45 pm, you will see both planets just above the train line.

They have been very close for the past few weeks, but it will be almost impossible to separate them tonight. They won’t be around again until 2080, so for most of us this is a real opportunity that happened once in a lifetime.

Contact the Limerick Astronomy Club by email: limerickastronomyclub@gmail.com