The solar system's two biggest planets, Jupiter and Saturn, were set to come within planetary kissing range in yesterday's evening sky, an intimacy that will not occur again until 2080.

This "great conjunction", as it is known to astronomers, occurs fortuitously on the winter solstice for those in the Northern Hemisphere, and the beginning of summer in the global south.

The two planets will, in fact, be more than 730 million kilometres (400 million miles) apart.

But because of their alignment in relation to Earth, they will appear to be closer to each other than at any time in almost 400 years.

Optimal "conjunction" is set to take place at 1822 GMT.

Looking with a telescope or even a good pair of binoculars, the two gas giants will be separated by no more than a fifth of the diameter of a full moon.

With the naked eye, they will merge into a "highly luminous" double planet, said Florent Deleflie from the Paris Observatory.

The last time Jupiter and Saturn nuzzled up this close was in 1623, but weather conditions in regions where the reunion could be seen blocked the view.

Visibility was apparently better the time before that during the Middle Ages, on March 4, 1226 to be precise.