This only occurs once every 800 years… And tonight is it!

Tonight the ‘Christmas Star’ will be visible!

From Space.com

Time and again

Usually when Jupiter and Saturn line up they are separated by about one degree, or twice the apparent diameter of the moon.

But on Dec. 21, we see Jupiter and Saturn separated by just 6 arc minutes. That’s equal to 0.1 degrees or about one-fifth the apparent width of the moon. Contrary to what other websites suggest, this will probably not be close enough to make the planets appear to merge into a single bright star (although some might see that if they remove their eye glasses). But in any case, it will be something exceedingly rare to see.

How often do these two planets come as close as that? Some websites say it has been nearly 400 years, while others say it’s been almost 800 years. Indeed, the last time these two planets appeared so close was on July 16, 1623, when they were only 5 arc minutes apart — that’s actually 397 years ago. There is a caveat however, for those living in temperate latitudes, such as New York, Paris or Tokyo, where the two planets were not visible because of their close proximity to the glare of the Sun and low altitude above the horizon. 

This is not to say that the great conjunction of 1623 went completely unobserved. Visibility of this rare alignment would have only been visible from the tropics near the equatorial regions.  If anybody living in northern South America, central Africa or Indonesia cared to look, they would have had a brief view of Jupiter and Saturn low in the west-northwest twilight sky on the evening of July 16th.

But the last time most of the world’s population had a favorable view of these two planets coming so close to each other was on March 5, 1226, when they were even closer together (just 2 arc minutes) compared to what we will see on Dec. 21.