Word of the Week: Dichotomy!

Hey, Sky Fans! What? Word of the week… on Monday? As the country’s leading purveyor of luxury astronomy text products since 2015, we here at Sky Watch are not above bucking tradition. Actually, there’s a couple of interesting things going on in the sky this week. I didn’t want to do what we’ve all seen in badly edited text books: “See Chapter Seven: The Shortstop,” so you flip ahead 200 pages only to get to some other part that says “See Chapter Four: Hot Dogs,” which puts right back where you started.

Instead, Word of the Week today, and not Word of the Week on Wednesday. It’ll be okay.

Yesterday afternoon was gorgeous, a wonderful early spring day. I’d cleaned up after fixing the car, eaten the food, and was out chasing a ball around when I stopped for a second to have some water and watch the clouds roll by overhead. Ever since I was a kid, I’ve loved the days when it’s calm down here on the ground, but the are screaming by like fluffy cars on an invisible highway. As a big cloud moved out of the way, just about overhead was a gorgeous still-crescent moon. I’ve said it many, many times. I love the moon in a dark night, sure, but there’s something a little more special about seeing it in the blue. The subtlety and surprise of it always grabs me.

It was still a crescent, barely,  and it reminded me of one of my favorite words in astronomy, and in all of English. Away we go.


What a great word. Outside astronomy, it means two things that are in conflict with each other, or something that’s seemingly in conflict with itself. Even the word has that feel to it. It sort of feels bumpy, but bouncy at the same time. I like the dichotomy of its sharp consonants with its rhythm.

In astronomy, dichotomy is a simple one. It’s just the word for the point or phase when an object in space is half lit by the sun, as seen from a given viewpoint. The moon is in dichotomy when it’s in its first quarter phase, as it will be tonight, or has waned away to third quarter. We can see Venus and Mercury in dichotomy, in their half phases, too. Anywhere, any point of view, where something can appear half light and half dark, that thing is in dichotomy. Seen from Saturn, for example, Titan, Enceladus, and even Earth can be.

Word of the Week: Dichotomy!
Word of the Week: Dichotomy!

First quarter, the waxing dichotomy, is one of my favorite phases; it might actually be my favorite of them all. I love the symmetry and balance of it, half lit, half not. The sharp terminator running, from pole to pole, straight as a school kid’s ruler. The impossible shape that it is, hanging overheard in the night, testing you, testing your ability to figure out where its nighttime side ends and your own night begins. Through binoculars, the shadows of the mountains, and valleys and crater rims are long and dramatic.

I love seeing it rising at midday and setting at midnight, cutting the day in half. This means it’s finally left the early, young phases behind. Like an older child, it can stay up a little later now, and we have more time with it as evening goes on.

If you can visualize it, try to imagine where the moon is in its orbit during the first and third quarter phases: off to the sides, just past the Earth’s outstretched fingers. I like to close my eyes and imagine the scene from far overhead, far over the north pole, with the moon not quite ahead of us, and not quite behind in our eternal dance.

The Dichotomies of the Moon
The Dichotomies of the Moon

The days right around first quarter are the ones when Mare Tranquliatites, the Sea of Tranquility, where first astronauts stopped for a bag of cheese-flavored pretzel bits, first sees daybreak, and lets it shine, shine, shine all around the world. We’ve been there.

It looks like a black and white cookie.

Third quarter’s terrific, too, backward, and disorienting, but since it rises at midnight and sets and midday, it takes a little extra work to see.

If you have a minute tonight, maybe grab the trash, and head out and see what you can see. Clear skies, everyone!



7 thoughts on “Word of the Week: Dichotomy!

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