Jupiter and Saturn are the stars of September’s night sky

Saturday, Sep 12, 2020 6:03 AM

Greetings, Stargazers.

September is the month when daylight hours shorten faster than any other month. Even though autumn doesn’t officially begin until the equinox early on the morning of the Sept. 22, it feels like we went straight to winter, as the abrupt temperature drop this week forced us to quickly harvest or cover everything in the garden.

Astronomically speaking, autumn is usually given the short straw. The summer and winter seasons both offer brighter stars and more easily identifiable constellations, but this September, the two most interesting planets to see through a telescope are prominently displayed in the southern evening sky.

Jupiter and Saturn were at their closest approach to Earth in mid-July. That means the theoretical best time to view them would have been July 14 and July 20 respectively, when they were crossing the meridian at our local midnight. The meridian is the north-south line running overhead, where they would reach their highest point in the sky. However, because of the misfortune called daylight saving time, this event would be “sprung forward” to 1 a.m. Furthermore, Durango’s longitude makes this meridian-crossing an extra few minutes after that.

I don’t know about you, but I prefer to look at planets when their highest position in the sky is right after dark, rather than well after 1 in the morning. This week, both planets are at their highest point, about 30 degrees above the southern horizon, in the early evening. Jupiter, at magnitude -2.5, is by far the brightest thing in the evening sky, and Saturn, at magnitude 0.35, is brighter than all but a couple of stars.

Through standard 10x50 binoculars, the four Galilean moons of Jupiter are easily visible. Io, Europa, Ganymede and Callisto would be visible with the naked eye if they weren’t so close to Jupiter. With a bit of magnification from a small telescope, you can see the scale of the Jupiter system, as these moons are many Jupiter-diameters away from their host planet, and much farther from Jupiter than our own moon is from Earth.

To give you a sense of scale, if you used a basketball going through the hoop to represent the Earth, and a tennis ball for the moon, you would need to put the tennis ball at the 3-point line to get a proper Earth-moon scale. (September feels like a really odd time of year for basketball references, but with the playoffs in full-swing in this odd kind of year, I’ll use it.) On this same basketball scale, Jupiter would be one-and-a-half times the size of the backboard, and Callisto, the most distant of the Galilean moons, would be well up in the stands beyond the far basket. Because the moons move so quickly, every night will show you a new pattern.

If you have to choose one single object to look at through a telescope, you wouldn’t go wrong with picking Saturn. If star parties ever get back on the schedule, I would expect the most “oohs” and “aahs” or other verbal exclamations from people getting their first glimpse of Saturn. Even small telescopes will give a good view of the rings, and they are bright enough that the image will look closer to what you see in photographs than almost any other object seen through a telescope.

The main attraction of Saturn is the giant ring system, made mostly of water ice. The rings are tilted at 27 degrees from the plane of the solar system, and our view of them varies between this maximum tilt to an edge-on view with a roughly 15-year period. The tilt this year is about three-fourths of its maximum, and the next edge-on view will be in 2025. With a 6-inch or 8-inch telescope, you can see the dark Cassini gap in the rings.

Over the next three months, Jupiter and Saturn will be easy to see in the evening sky, and they will be getting closer and closer to each other. In December, during a rare, extra-close conjunction they will only have a fraction of a degree of separation.

Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory. Email him at hakes_c@fortlewis.edu.