Jupiter and Saturn aligned in the night sky on Monday, appearing closer than they have in centuries.
The last time the two gas giants looked this close together was during Galileo’s time in the 17th century.
While conjunctions – where objects appear very close to each other in the sky – are not rare, this was an “exceptionally close” one – the closest since 1623, Professor Michael Burton said.
Prof Burton said: “When that happened it was in the daytime skies so people would not have seen it because it would have been too light.
“The last one which actually would have been well placed to be seen was 1226, so we’re going back 800 years to the last one which would have been up in the dark sky to be seen.”
He said: “Certainly one of the possibilities might have been two of the planets coming together.
“The whole question of what might the Christmas star have been is a very interesting question in its own right.
“Whether it was a conjunction, whether it was a comet, whether it was a supernova. No-one actually knows what it was.”