Sciency Words: Opposition and Conjunction

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Sciency Words is a special series here on Planet Pailly celebrating the rich and colorful world of science and science-related terminology. Today, we’re looking at two related terms:


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.

Jn06 OppositionOpposition: 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.”

Jn06 ConjunctionConjunction: 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.

Jn06 Superior ConjunctionSuperior Conjunction: Venus, as pictured above, is on the opposite side of the Sun as viewed from Earth. This is called a “superior conjunction.”

Jn06 Inferior ConjunctionInferior 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?

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Today’s post is part of Mars month for the 2015 Mission to the Solar System. Click here for more about this series.

4 thoughts on “Sciency Words: Opposition and Conjunction

  1. It’s interesting how much astronomical terminology is pre-Copernican. But it makes sense if you think about it. Copernicus was 500 years ago, but astronomy is far more ancient, going back ten times older to the Sumerians, at least.


    1. I’ve found the terms very useful, so long as I keep in mind which planet’s perspective I’m using. Hopping around the Solar System can get really confusing if that’s not specified.


      1. That reminds me of an issue NASA once had between the astronauts and the engineers on the Apollo project. The engineers wanted to use one navigation coordinate system for the entire journey, but the astronauts, all with backgrounds as test pilots, insisted that when over the Earth or the Moon, that aircraft like navigation be used.

        It meant that Apollo ended up using three coordinate systems. One when they were over Earth, another when they were in transit between Earth and the Moon, and a third over the Moon.

        And *that* reminds me that the width of modern cars is ultimately based on the width of Roman carts, which were themselves wide enough for two horses to draw them.

        Funny how terms and paradigms become embedded, lasting long after their pragmatic cause.

        Liked by 1 person

      2. I did not know that about the Apollo missions, although based on my experiences playing Kerbal Space Program, I definitely see the advantages of switching frames of reference during different stages of a mission.

        Liked by 1 person

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