Planetary Encounter

I have always thought of Jupiter and Saturn as the charismatic megafauna of the solar system. Over the past few months, I’ve been enjoying these bright denizens of the night sky slowly approaching each other, like a giraffe and a lion coming together to acknowledge each other’s magnificence. Soon, they will slowly drift apart. 

You’ll see them at their closest approach low in the west after sunset on December 21. But, as a student studying in Iceland recently wrote to me, “I enjoyed the solstice sunset today because it might be cloudy every day through the 21st.” Take a look at Jupiter and Saturn any evening around the winter solstice. A pair of binoculars will provide a nice view. For an up-close look at each, I recommend Astronomy Picture of the Day (swat.ink/astro-picture-of-day). Saturn’s mane will look especially impressive.

You can also watch the moon move farther from the pair of planets over the next several nights. You can use the moon’s diameter (0.5°) as a handy ruler to estimate how far apart the two planets are at that point. 

Why are we talking about the moon’s diameter in degrees, rather than in more familiar units of measurement like miles? 

Since the true distance and size of objects in the sky isn’t apparent to the eye, astronomers often use angle as a unit expressing how big they appear to us. Imagine one line from your eye to one side of an object, and another line from your eye to the other side of the object. The angle between those two lines (with the vertex located right at your eye) tells you how much of your field of view it is covering: that is, how big it appears to you relative to other things you can see. 

In our night sky, Jupiter and Saturn are now closer together than the (apparent) width of the moon.

David Cohen is an astronomy professor at Swarthmore College. He is married to the editor of the Swarthmorean.