The great conjunction of Jupiter and Saturn

On December 21, just a few days before Christmas, the two biggest planets in our solar system, Jupiter and Saturn, are meeting in a great conjunction. The two planets are so close together in the night sky that – cloud cover permitting – they should be visible to the naked eye as almost a single bright dot.

This photo was taken in Antalya, Turkey, on December 17. Jupiter (in the sky on the right, below) and Saturn (above right) are easily recognizable. The two planets have been visible in the early evening for several months now, getting ever closer.

It's not unusual for Jupiter and Saturn to cross paths, because they orbit the sun at different speeds. Jupiter overtakes Saturn on the inside lane about every 20 years. The last time there was a big conjunction like this was on May 31, 2000, but that time the duo was too close to the Sun to be seen in the night sky.

Jupiter takes 12 years to circumnavigate the sun. It's the biggest planet in our solar system, with a diameter of 142,984 kilometers (88,846 miles) at the equator. Its mass is two-and-a-half times greater than that of all the other planets in our solar system combined.

Saturn isn't in as much of a hurry – it takes 30 years to complete a single orbit. It's the second-biggest planet in our solar system, and the sixth furthest from the Sun. Its particular characteristic is its sequence of rings, which consist of ice and rock. Jupiter also has rings, but they're not as conspicuous.

The sight, low in the south-western sky just after dusk, is not to be missed if you have clear visibility. The last time the planets were this close was in 1623; the next close conjunction will be on March 15, 2080. There won't be any closer great conjunctions in our lifetime.