If you had to go stargazing on the night of the 21st December, you’d be able to see something similar to what helped the Three Wise Men travel to Bethlehem. It’s not a shining star but actually two planets passing by really closely to each other when seen from Earth.
This is called a conjunction, and they happen quite often with different planets. There can be a conjunction between the Moon and a bright star for example or it can even happen with more than two objects. The one we’ll be seeing this year will be quite a special one, because it is a conjunction between the two biggest planets in our solar system: Jupiter and Saturn. When these two planets meet and appear the closest, we call it a Great Conjunction. Even though they will look as if they’re really close to each other, this doesn’t mean that they will actually be closer together, in fact, they will still be millions of kilometres apart. But from our vantage point on Earth, they’ll seem to be so close to each other that they almost touch; creating one very bright point of light in the sky. The date also makes it a touch more special, since it will be appearing on the Winter Solstice, which marks the beginning of the winter season. Because of its vicinity to Christmas Day, it is also being called the “Christmas Star” or the “Star of Bethlehem”. In fact, some astronomers have said that the actual Star of Bethlehem seen by the Three Wise Men might have actually been a conjunction similar to the one we’ll see this month, but it would have most likely been involving different planets.
Conjunctions of Saturn and Jupiter actually happen every 20 years, but none of them are as close to the spectacular one we will be seeing this year. These ‘meetings’ happen because each planet has a different orbital period around the sun. Saturn takes 29.4 years to orbit around the sun, while Jupiter takes 11.9 years. This means that Jupiter ‘catches up’ to Saturn every 20 years and overtakes it. From Earth, we see this phenomenon as if the two planets are growing closer and closer together before moving away from each other again. Even though they happen every 20 years, they don’t always occur at a time where we can actually observe them. For example, the last conjunction which happened in the year 2000 appeared too close to the sun, so it was hidden by the solar glare. At that time, they didn’t meet as closely as they will this year. The last time that happened was in July 1623, but even then, it was very hard to spot. That means that the last truly observable Great Conjunction of this kind happened almost 800 years ago, in 1226, the peak of the Middle Ages! It’s safe to say that you won’t want to miss this unique, and rare event.
Saturn and Jupiter have spent the last few months slowly approaching each other, getting closer and closer every day. This means that on a clear night, you can start observing them from now and see the distance between them getting smaller every day. They will be the closest on the night of the 21st of December, so close that Saturn will seem as close to Jupiter as some of Jupiter’s moons. After that, they’ll start to move apart once more. What can you do to catch this spectacular sight? Well, you certainly won’t be needing any binoculars or telescopes to admire the view, you can easily spot it with the naked eye. Without a telescope, they will look like one bright point of light, but if you do happen to have one at home, you’ll be able to see the two planets separated clearly, as well as some of the moons surrounding them. Jupiter is the brightest of the two while Saturn is a bit fainter and will be seen above it. If it will be a clear evening, you’ll be able to see them low in the southwestern horizon just after sunset. They’ll be so bright that you won’t need to be in total darkness to see them, but you do need a good unobstructed view towards the southwest. You can use a compass app on your smart phone to find out where this is. If you’d rather stay cosy inside and stargaze from your sofa, you can use the app Stellarium to look at simulations of the night sky or you can even watch it happen live through observatory telescopes in the US! Watch it on the day (in their timezone) here:
On this darkest day of the year, during quite a lousy year, it’s nice to have such a spectacular sight to look forward to. Make sure you soak it in while you can because it will only be around for a short while. If you do miss it, the two planets will be staging yet another wonderful show in just 60 years, so make sure to mark it in your calendars!
