Today's Jupiter-Saturn Conjunction

 

All fall Jupiter and Saturn have been very close together in the evening sky. They have also been gradually moving closer and closer. This process will culminate today, on December 21, the Winter Solstice. Astronomers call a close approach of two or occasionally more planets in the sky a conjunction. Jupiter and Saturn will have a very close conjunction tonight, on the evening of the December solstice. The conjunction will be visible just after sunset.

During this conjunction, Jupiter and Saturn will appear about 1/10 of a degree apart. For comparison, the full Moon is about a half a degree in width. Jupiter and Saturn have a conjunction roughly every 20 years, but the conjunctions are seldom this close. The last time a conjunction of Jupiter and Saturn was this close and far enough from the Sun to be easily visible was in 1226. The next time they are this close will be in 2080 and then in about 2400. Perhaps by that time there will be people living near Saturn who will be able to see a conjunction of Jupiter and Earth.

Occasionally Jupiter and Saturn appear so close in the sky that Jupiter passes in front of Saturn and blocks a little of Saturn. Astronomers call this an occultation. Such events between planets are rather rare. The last time Jupiter occulted Saturn was about 8,000 years ago.

As an added bonus, on the evenings of December 16 and 17, the thin waxing crescent moon was very close to Jupiter and Saturn.

From a location with a clear southwestern horizon, look low in the west to southwest as soon as it starts getting dark after sunset. The brightest star-like object you see will be Jupiter. The nearest reasonably bright star-like object will be Saturn. The show won't last long, for these planets will set fairly soon after it gets dark.

The two planets will be close enough that they will fit into the same field of view in a telescope or binoculars, but one can still observe these planets with the naked eye. Both should be visible to most people. Some people with less than perfect vision might see them as only one star-like object.

If it is cloudy on the evening of December 21, the planets will still be very close in the sky until they disappear behind the Sun into early January. They will then reappear in the predawn sky around March or so. Lots of opportunities to see it.

Because this conjunction is so close to Christmas, many in the media are calling it the Christmas star. It certainly qualifies as one possibility for the original Christmas star. In 7 BC there was a triple conjunction of Jupiter and Saturn, when three conjunctions occurred in rapid succession. There was also a triple conjunction of Jupiter and Venus (much brighter than Saturn) during the years 2 and 3 BC. Either of these triple conjunctions might have been the original star of Bethlehem. It is fascinating to speculate about it, whether it is religiously significant or not!