Planetary Oppositions 2020-2040
Interesting things happen when planets are on the opposite side of the sky from the sun. Here are tables of dates when Mars, Jupiter, and Saturn reach opposition, for the next decade or two.
If you've studied the phases of the moon, you may remember that the full moon is in a way the “opposite phase.” It is on the opposite side of the sky from the sun, and its behavior is the opposite of the sun's. It rises in the east just as the sun sets in the west, it it high in the sky in the middle of the night, and it sets in the west just as the sun rises in the east. Other phases behave differently, but the full moon and the sun are always on opposite sides of the sky. (And that must also mean that they are on opposite sides of us in space, which is a helpful clue to what is going on.)
Planets can be in different places in the sky, and sometimes they can be opposite to the sun, just like the full moon. (At least some of them can be. Mercury and Venus only appear over sunrises and sunsets, and we never see them far from the sun in the sky. It is as if they are trapped close to the sun for some reason.) The official name for this place in the sky opposite to the sun is “opposition.” And whenever a planet is at opposition, it can behave a little strangely. Like the full moon, planets tend to be a little brighter when they are at opposition. Brilliant Jupiter and sluggish Saturn don't change enough that you would ever notice the difference unless you were paying very close attention. But reddish Mars, which is normally fairly dim, grows much brighter whenever it is at opposition, like a god of war flaring up in anger. Mars can become almost as bright as Jupiter when it is at opposition. If you like studying planets through a telescope, you may also notice that you get a better view of the planets when they are at opposition. They appear a bit larger … especially Mars. Also, if you pay attention to where the planets are on the map of stars, you find that they normally travel along a circular “planet highway” from west to east. But whenever a planet is at opposition, it changes directions and goes through the constellations backwards! (We call the “planet highway” across the map of stars the “zodiac,” and the “ecliptic” is the exact path of the sun on this map, like the centerline of the highway. The official name for going backwards is “retrograde motion.”)
If you want to watch a planet while it is at opposition, the tables below give the dates for all planetary oppositions during the years 2020-2040. The planets don't move very fast, and they tend to spend some time near opposition, so you have several weeks around each date to look for them at opposition. Just use each date as a ballpark figure. Do you notice any patterns in the dates?
(If you are wondering about Uranus and Neptune, you can only see those through a telescope, and they are very uninteresting when you do. They behave in all the same ways as Mars, Jupiter, and Saturn when they are at opposition, but their brightness and their size in a telescope change so little that it isn't really worth worrying about. Also, for more detailed planetary schedules, you might also be interested in the planet calendars.)