A Year-End Treat With Two Giant Planets

(Plain Text Version)

During the current year, Jupiter and Saturn have been moving closer together in the sky.  Now, they are close enough to be covered by one’s thumb held at arm’s length.   For the next few weeks, they will continue to close in.  Their closest position will occur on December 21. On this evening they can be covered by a toothpick! 

Of course, this is just an appearance (called a conjunction).  The two giant planets are always millions of miles apart.  Conjunctions occur when two or more objects appear close together.  Conjunctions are not unusual, but ones this close ones are very rare. 

This is a good time to talk about orbital inclinations.  Imagine you had a flat piece of paper bigger than the solar system.  You could arrange it so that the orbit of Jupiter stays on the paper.  To do the same thing for Saturn, you would have to move the paper, but you would lose the orbit of Jupiter.  If you add a second piece of paper, you could use one for Jupiter’s orbit and one for Saturn’s orbit.

These sheets of paper are called orbital planes.  Because two sheets are needed, we say that the orbital planes of Jupiter and Saturn are inclined at different angles.  Before we go, let’s mark the orbits on their sheets.

In the diagram above, the positions of the two planets are shown for four nights.  The orbital paths are from the two sheets of paper, as viewed from Earth.  Notice that the planets have similar, but not identical paths.  This year, Jupiter and Saturn will be close together in two ways.  First, on December 21, they come close in their orbital journey around the Sun.  Secondly, on this date, their orbital paths are close to each other.

Many centuries can pass between these events.  Usually, the planets meet when their orbital paths are further apart.  They will be seen in the same area of the sky, but they will be further apart.

Tonight would be a good time to start watching the dance of the giants.  No equipment is needed. 

If a telescope is available, a unique view will be possible: for a few days before and after December 21, both planets can be seen in the same view.

This link has an animated graphic which may help in visualizing orbital inclinations:

https://www.mapleprimes.com/questions/203062-How-Can-I-Visualise-Inclined-Circles