Happily, there are no tiers in the sky so stargazing is one of the few things we can do with impunity. So make sure you turn skywards on December 21st when what astronomers call a ‘great conjunction’ will light up the sky. This is the moment that Jupiter and Saturn’s orbits will cross. Weather permitting, the celestial meeting of the biggest planets in the solar system will be visible to the naked eye from anywhere in the world. 

Conjunctions are the alignment of two planets (or other celestial objects) so that they appear to be in the same, or nearly the same, place in the sky. They don’t actually touch, but from the perspective of someone standing on Earth, they seem to. Conjunctions happen often, but great conjunctions are a rare occurrence as it takes decades for these giant planets to orbit the sun.

It takes Jupiter twelve years to do so, and Saturn thirty. A celestial rendezvous between the two takes place only once every twenty years, and on slightly different planes or orbital inclinations. What makes the upcoming great conjunction so rare will be the proximity of the planets passing each other: they’ll appear to be only 0.06° (or 6 arcminutes) apart. For scale, this is a tenth of how the moon looks to the naked eye. Jupiter and Saturn may appear to be overlapping, even though they’ll be 400 million miles apart.