If you only ever look at the night sky once a year, make it Tuesday night. Look towards the sunset on the evening of 23 February and, if it is clear, you will see an awe-inspiring sight.

Venus and Jupiter will come to within 0.2 degrees of each other, far closer than the apparent size of the full moon.

The spectacle will be visible from anywhere on Earth, except near the Poles. With the unaided eye, or better still through a pair of binoculars, it should be a beautiful sight - the two brightest planets in the sky almost touching. It will be best seen about 45 minutes after sunset.

They appear close together but in reality they are far apart. It is what astronomers call a conjunction. It has no great astronomical significance but it may have a historic one. There are some historians who believe that what we are seeing is a rerun of the Star of Bethlehem.

Venus and Jupiter came close together in August of 3BC and again in June of 2BC. Some believe it could be the unexplained Star of Bethlehem. Others say it is too late and that a more convincing explanation is a conjunction of Jupiter and Saturn in 4BC.

Acid rain

Venus is 134 million miles away and is the brighter of the two planets. It is bright because it is covered in an opaque layer of highly reflective clouds. The clouds are corrosive. Inside them it rains droplets of dilute sulphuric acid, though not a drop of this reaches the ground.

At 540 million miles distant, Jupiter is much further away. It is much larger than Venus, 11 times its diameter and it too is cloud covered.

Even lower in the sky is the elusive planet Mercury. It is quite tricky to see, requiring a clear horizon and good conditions. Count yourself fortunate if you see it.

Add a quarter moon to that part of the sky and it adds up to one of the best sky spectacles of the decade. The best of it will only last a day though.

By Wednesday, Venus and Jupiter will be two full moon widths apart and the close conjunction will be over.