The scene: The Astra family kitchen, just after dinner. Mother, Father, and Daughter are discussing science education and the way scientists think. An anecdote about thirteen-year-olds and Saturn leads to the following exchange betwixt Father and Daughter:
Me: So when Mars is in opposition from the Earth–
Dad: That’s when the Earth is outside of Mars.
Me: *blank stare*
Dad: Oh, wait, Mars is the fourth planet from the Sun.
Me: (with exaggerated patience) Yes, Daddy, it is.
Dad: Well it’s not like I go there often! I forget these things!
It’s funny, the things we start to take for granted. (Although I don’t think knowing that Mars is the fourth planet from the Sun is such an unreasonable thing to ask…unless, apparently, one is asking it of one’s exhausted father, whose mind is still stuck in his office, fighting with a clumsy programming language from the 1970s). But I tend to assume everybody knows what it means when Mars is in opposition, forgetting that just a few years ago, I didn’t know myself.
So, for those who are interested, here is a quick-and-dirty guide to planetary alignments.
Disclaimer: Anyone who uses this knowledge to attempt to give an air of validity to their astrological mutterings will be sacrificed to whichever god(dess) of knowledge, wisdom, and/or common sense happens to be crankiest at the time of said infraction.
Planetary alignments are almost always discussed from the vantage point of a particular planet (usually Earth, because duh) and with reference to the Sun. So it is incorrect to say “Earth and Mars are in opposition to each other.” In fact, it’s actually physically impossible for both to be in opposition with respect to the other, but we’ll get to that.
When discussing alignments, the planets are divided into two categories: superior and inferior. Superior planets are planets whose orbital paths lie outside that of the particular planet; inferior planets are those whose orbital paths lie inside that of the particular planet. Since the particular planet is almost always Earth, Mercury and Venus are usually the inferior planets and Mars, Jupiter, Saturn, Neptune, and Uranus are usually the superior planets.
If we’re feeling adventurous and want to talk about alignments as seen from, say, Jupiter, then Mercury, Venus, Earth, and Mars would be inferior planets and Saturn, Neptune, and Uranus would be superior planets.
Okay. So now that we’ve got our planets classified, it’s time to talk about the various kinds of alignments.
Opposition: A planet is in opposition when it is on the opposite side of the particular planet from the Sun. This means that only superior planets may be in opposition.
Conjunction: A planet is in conjunction when it is on the same side of the particular planet from the Sun. There are two kinds of conjunctions.
Inferior Conjunction: A planet is in inferior conjunction when it is between the particular planet and the Sun. This means that only inferior planets may be in inferior conjunction.
Superior Conjunction: A planet is in superior conjunction when it is on the opposite side of the Sun from the particular planet. Put another way, a planet is in superior conjunction when the Sun is between it and the particular planet. This one is a bit harder to remember than the other two; I tend to remember it by process of elimination (I can remember opposition and inferior conjunction, so superior conjunction must be the Other One). Just to make things more confusing, both inferior planets and superior planets can be in superior conjunction.
I’m not going to discuss Greatest Elongation or the Quadratures since you hear about them much less frequently and I’m too lazy to make more of my spectacular slap-dash explanatory diagrams. Greatest Elongation is kind of cool, so head over to Wikipedia to find out about it. I’ve never known an astronomer who actually cared about quadratures; they don’t have any practical astronomical applications that I’m aware of, so I’m just going to pretend they don’t exist. Because I can.
This is what happens when you leave a science educator unemployed. She starts composing diatribes, complete with explanatory doodles, for the purposes of Public Education Whether They Like It or Not. In this case, I suspect it’s something more along the lines of Beat My Few Regular Readers over the Head with Astronomy Even Though They’re Really Only Here for the Knitting and/or Writing.
Oh, and go read The War of the Worlds. Or listen to the radio broadcast, which I actually like better. Just, whatever you do, don’t watch any of the film adaptations! Having seen (part of) all four, I call them The One with the Glasses and Churches, The One Where the Martians Are Really Adorable, The One Where It’s Night and Day at the Same Time with Really Really Bad CGI, and The One Which Mysteriously Starts with a Gratuitous Sex Scene. I call all of them Bad, and really want to get my hands on a copy of the rock opera.