Newsletter Archive

That Brilliant Red Star in the Sky Tonight...

October 1, 2020

If you go outdoors after sunset tonight, and face towards the place where the sun just went down, you will still be facing in the direction of the sun. You would see it there, a little below the western horizon, if you could only see through the earth. Now if you turn yourself around 180° and look a little above the eastern horizon, you will be looking at the point in the sky that is opposite to the sun. You should see a remarkably bright red star there.

There are a handful of "stars" that do not belong to a proper constellation, but wander around, trespassing in one constellation after another. The ancient Greeks noticed five of these migrating stars-without-a-home, and called them πλανήτης, or planetes, meaning "wanderers". The bright red star in tonight's evening sky is one of these wandering planets. You can probably guess which one from the color.

Two of the wanderers are trapped near the sun, and only appear to us over sunrises and sunsets. This limits our opportunities to see them, but makes for a very beautiful scene when we do. The other planets can wander anywhere in the sky — or at least anywhere the sun and the moon can appear, meaning never the extreme northern or southern sky—and sometimes these planets will appear on the opposite side of the sky from the sun. When they are here, planets do interesting things. They grow brighter, and they appear all night long on a schedule opposite to that of the sun. Like a full moon, they rise in the east as the sun sets in the west, they climb high into the sky at midnight, and they set in the west as the sun rises in the east. A planet "at opposition" behaves like the full moon, as the opposite of the sun.

As you have probably realized by now, that bright red star in the sky tonight is Mars, and Mars is currently in the "full moon position", opposite to the sun.

In the month of October, 2020, Mars will be near opposition.

Mars, Jupiter, and Saturn all pass through opposition from time to time. Jupiter and Saturn do this about once a year, and they grow a tiny bit brighter when they do. Mars takes much longer between oppositions —over two years —but when it does oppose the sun, the change is much more dramatic. Most of the time, Mars looks like a fairly dim and uninteresting red star in the sky, but when it is at opposition, it becomes so bright that it outshines all other stars, and rivals even Venus and Jupiter. The date of exact opposition in 2020 is October 13, but Mars will be bright and near opposition for much of the month.

If you imagine a model of the solar system, you may be able to see why the planets grow especially bright at opposition. Picture Earth and Mars lined up in their orbits, so that the Sun, Earth, and Mars all lie along a straight line, with Earth in the middle, and the Sun and Mars on opposite sides. If Earth and Mars are like two racers in parallel tracks of a raceway, this is the point at which Earth passes or "laps" Mars on the inside lane. Opposition happens when Earth passes another planet in its orbit, thus placing the outer planet and the sun on opposite sides of the Earth. This is also the moment when the Earth comes the closest to the outer planet. The outer planets appear brighter in the sky at opposition, because the Earth is closer to them than usual. (This is also the moment when the planet is lit up like a full moon, and we are looking directly at the sunlit side, instead of seeing it from a sideways angle. The difference in angle is not significant for distant Jupiter and Saturn, so we always see them as "full". But there is a significant difference for Mars, which is another reason for its dramatic change in brightness. Mars is not only closest at opposition, it is also unusually "full".)

So go outdoors after sunset tonight, and look for Mars on the opposite side of us from the sun!