If you bundle up and head outside on a clear night on these cool November evenings, you’re sure to notice brilliant red Mars high in the southeast. While it has started to fade somewhat as Earth pulls away from it after its closest approach last month, the Red Planet is still the brightest thing in the evening sky right now, and will continue to be an obvious red beacon throughout the winter. Because Mars orbits the Sun only about 40 percent further from the Sun than us, it goes through the biggest change in apparent brightness of all the planets. When on the same side of the Sun as the Earth (as it is now) it is only about 38 million miles away; when on the opposite side of the Sun, its distance is about 225 million miles, rendering the planet about 35 times dimmer.
But the most interesting evening show to watch over the coming month will be the one happening in the southwest. Looking in that direction, you will see two additional bright and obvious objects: brilliant Jupiter (currently almost as bright as Mars, but white in color) – and to its upper left is yellow-tinted Saturn. The two planets in mid-November appear about 4 degrees apart in the sky; and therein lies the interest: over the next month and a half, Jupiter will move eastward against the stars, approaching Saturn and closing the apparent gap between them. Meanwhile the stars themselves will appear to shift slightly westward from night to night, due to the Earth’s orbital motion around the Sun.
This means that the planets will continue to draw closer together while sinking towards the western horizon when observed at the same time each evening. By December 1st the two planets will be only two degrees apart, and will be separated by a mere one degree on December 11th. And on December 21st, the two will likely appear to have merged into one as seen with the naked eye… a mere 1/10th of a degree will separate the two! (Of course, the appearance of closeness is an illusion due to our line of sight… Saturn in actuality will be about a half a billion miles beyond Jupiter.) The two haven’t appeared this close together since the year 1623… and won’t do so again until 2080! (See photo at bottom of article, December 20, 2020)
The obvious shifting of these planets makes it easy to understand why the Greeks referred to them as planetes asteres, literally “wandering stars.” You too can keep track of and measure their changing separations by using the “handy” tool that you carry with you at all times: your hand… when held at arm’s length.
And while we are talking about things to look for in November, you might want to go outside and check on the Geminids (December 13th). One doesn’t have to stay up past midnight to see it. That shower radiates from near the starry head of Castor in the constellation of Gemini the Twins, and it clears the northeastern horizon only a couple of hours after sunset. Viewing conditions this year are favorable, since moonlight will not interfere. (See my article last spring “International Dark Sky Week” for comments on the appearance of Gemini).