Sciency Words: Opposition and Conjunction

Today’s post is part of a special series that first appeared on Planet Pailly. Every Thursday, we take a look at a new and interesting scientific term to help us all expand our scientific vocabularies together. Today’s sciency words are:

Opposition and Conjunction

Admit it: you want to go to Mars. Despite all the radiation and sandstorms and saltwater, you still kind of want to do it. But which way is it to Mars? Bonus credit if you can point in the correct direction right now without checking a smartphone app.

Mars, like pretty much everything in space, is a moving target. Sometimes, it’s fairly close to Earth. Other times, it’s all the way on the far side of the Sun. To make life slightly easier, astronomers have special terms to describe the positions of other planets relative to Earth.

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Opposition: Earth and Mars, as pictured above, are on the same side of the Sun, almost perfectly lined up. In this situation, Mars is said to be “in opposition.”

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Conjunction: Mars is now on the far side of the Sun, basically as far from Earth as it can get. Mars is now said to be “in conjunction.”

In my mind, these terms would make more sense the other way around. Mars should be in opposition when it’s on the opposite side of the Sun, don’t you think? But I’m guessing this all originates from a more geocentric view of the Solar System. Opposition, therefore, gets its name because the Sun and Mars are on opposite sides of the Earth.

What about Mercury and Venus? Since neither can be on the opposite side of Earth from the Sun, they’re never in opposition. Instead, astronomers use slightly different terms.

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Superior Conjunction: Venus, as pictured above, is on the opposite side of the Sun as viewed from Earth. This is called a “superior conjunction.”

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Inferior Conjunction: Venus is now on the same side of the Sun as Earth. This is an “inferior conjunction.”

Of course, all this terminology can be shifted around if you want to take the perspective of a planet other than Earth. From a Venusian point of view, Earth could be in opposition or conjunction, and Martians could observe Earth to be in superior or inferior conjunction.

Knowing where planets are in relation to each other is critical for interplanetary voyages. Next week, we’ll start planning a Martian vacation, keeping an important question in mind: would you rather travel to Mars when Mars is in opposition or conjunction?


Written by James Pailly.

To add more Sciency Words to your vocabulary, visit the Planet Pailly blog.

  • Leonardo Faria

    Natasha Henstridge looked smoking hot in Ghosts of Mars, and she didn’t even wear a space suit. I wonder if Mars’ bad repute is really warranted.

  • Kirov

    I would have gotten bonus points, but I just arrived in Texas, don’t know which way the cardinal directions are cause I’m all turned around, and can’t see the stars from my hotel window. But the answer is the direction of Earth’s orbit around Sol, so you can extend your apoapsis into Mars’ orbit. Unless of course you’re using an Alcubierre drive. Then you can go straight there, even during conjunction.