“Oh. I have one other eclipse question, Dr O’Brien.”

“What’s that, Teena?”

“Well, I found a list of solar eclipses—”

“An interesting place to start, especially for a 10‑year‑old.”

“And it had three kinds of eclipse — total, partial and annual. ‘Total‘ must be when the Moon covers up the whole Sun like when I wink my eye tight. ‘Partial‘ sounds like when I only squinch up my eye like this. I guess that happens when we’re just on the edge of an eclipse track so we still see part of the Sun like we see just part of the Moon most of the time. But my eye wide open is like there’s no eclipse at all. There’s no fourth way to hold my eye left over for ‘annual.’ Besides, ‘annual‘ means ‘every year.’ Is there some special kind of eclipse that comes every year but we don’t see it?”

Cathleen doesn’t quite hide a smile. “Sorry, dear, I think you’ve misread a word. It’s not ‘annual,’ it’s ‘annular.’ They’re very similar and they both came from Latin but they came from different Latin words and have different meanings today. ‘Annual‘ means ‘yearly,’ just as you said. ‘Annular‘ means ‘ring‑shaped‘, like a circle with a hole in the middle.”

“A ring‑shaped eclipse? Is there a big hole in the Moon we only see sometimes?”

“Quite the reverse. The Moon and its shadow are compact, no holes even in an annular eclipse. What we see in those eclipses is a ring of the Sun’s light around the outside of a black disk of Moon‑shadow. The bright ring is called an ‘annulus‘ and you must be very careful to use the special dark glasses to look at it.”

“But … Uncle Sy said the reason we’re so lucky we can see eclipses is that the Moon is just the right size to match the size of the Sun. Does the Moon get smaller for an annular eclipse?”

“Hold up your thumb. Now move your arm out until your thumb just covers my head. Can’t see me at all, can you? Now move your arm out just a little farther until you can see my hair but not my face. Got it? Your thumb didn’t change size, did it?”

“No, it just looked smaller and I could see more of you past it.”

“Right. That’s how an annular eclipse works.”

<drawing Old Reliable from its holster> “Excuse me, Cathleen, I think this might help.”

“What are all those circles, Uncle Sy, and why does it blink?”

“It’s like a map of space. The blue disk represents Earth and the gray disk represents the Moon. If the Moon were always the same distance from Earth it’d follow the black circle, but it doesn’t. It follows the red line which isn’t a true circle. It’s a special shape of squashed circle, called an ellipse. Very few moons or planets follow a truly circular orbit — their track is almost always elliptical to some degree. Now you tell me what the blinky things are about and don’t say it’s when the Moon stops in its orbit because it doesn’t. The animation motion pauses to call attention to when the eclipses happen.”

“Okayyy… Oh! They’re what we’d see in an eclipse, right? The red … ellipse?… brings the Moon closer to us or farther away. When it’s close like over there it’s like my thumb covering Dr O’Brien’s whole head and we don’t see any of the Sun and that’s a total eclipse, right? When we have an eclipse if the Moon’s outside of that circle like on the side, it’s like my thumb farther away and that’s why your picture has the orange ring, it’s an annulus, right?”

“You broke the code, Teena, Well done!”

“I think it’s silly to have two words like eclipse and ellipse that sound so much alike but they’re so different. Like annual and annulus.”

“Sorry about that, sweetie, but we pretty much have to take the language as we find it. English has a long and complicated history. Sometimes I’m surprised it works at all. Sometimes it doesn’t and that makes problems.”

~~ Rich Olcott

“C’mon in, Sy.”

“Morning, Cathleen. You know my niece Teena.”

“Hi, Teena. What brings you here to my office?”

“I’m working on a school project about eclipses, Dr O’Meara, and I noticed something weird. Uncle Sy said you could explain it to me. You know how an eclipse isn’t in just one place, the Moon writes its shadow along a track?”

“Of course, dear, I do teach Astronomy.”

“Sorry, I was just giving context.” <Cathleen and I give each other a look.> “Anyhow, I found this picture of lots of eclipse tracks and see how they weave together almost like cloth?”

“Oh, it’s better than that, Teena. Look at the dates. Is there a pattern there, too?”

“Oooh, the Springtime ones go northeast and the Fall ones go southeast. Hey, I don’t see any in the Summer or Winter! Why is that?”

“It’s complicated, because it’s the result of several kinds of motion all going on at once. Have you ever played with a gyroscope?”

“Uh-huh, Uncle Sy gave me one for my birthday last year. He said that 10 years was old enough I could make it spin without hitting someone’s eye with the string. He was mostly right and I promise I really wasn’t aiming at Brian.”

<another look> “Well … okay. What’s a gyroscope’s special thing?”

“Once you start it spinning it tries to stay pointing in the same direction, except mine acts dizzy a little. Uncle Sy says the really good ones they put in satellites don’t get hardly get dizzy at all.”

“Good, you know gyroscope behavior. Planets spin, too, though a lot slower than your gyroscope. Do you know about planets?”

“Oh yes, when I was small and we looked at the eclipse my Mom and Uncle Sy explained about how we live on a planet that goes round the Sun and sometimes the Moon gets in the way and makes a shadow on us but when the Earth turns so we’re facing away from the Sun we’re in Earth’s shadow.”

“Nice. Well, here’s a diagram about how eclipses happen. It shows four Earth‑images at special points in its orbit. Each Earth has Moon‑images at two special points in the Moon’s orbit. There’s also an arrow coming out of each Earth’s North Pole to show the axis that the Earth spins on. We’ve got three circular motions and each one acts like your gyroscope.”

“Does the Moon spin, too?”

“We talked about this a couple years ago, sweetie. The Moon always keeps one face towards the Earth so it spins once each month as it orbits around the Earth. Dr O’Meara’s just using a single circle to cover both, okay?”

“Okay. So there’s three gyroscopes, four really but one’s hiding. The picture says that all three point in different directions, right, and they stay that way?”

“Perfect.”

“Excuse me, but those angles don’t look right. The Earth axis is pointed too close or something.”

“Sharp, Sy. You’re partially correct. Actually, that axis is at a proper 23° angle from the perpendicular to Earth’s orbital plane. It’s the lunar orbital plane and its axis that are off. They’re supposed to be at a 5° angle to Earth’s plane but they’re drawn at 15° to highlight that important line where the two planes meet. The gyroscopes keep that line steady all year.”

“What’s so important about the line?”

“If the Moon is too far above or below Earth’s plane, its shadow is too far above or below Earth to make an eclipse. Eclipses only happen when the line runs through the Sun AND when the Moon is close to the line. The line only runs through the Sun in the Spring and Fall, in this century anyway, so those are our eclipse seasons.”

“Why not every century?”

“A century ago, the eclipses came a few months earlier. The gyroscopes slowly drag the line around Earth’s solar orbit, shifting when the eclipse seasons arrive. If you want a New Year eclipse you’ll have to wait a long, long time.”

~~ Rich Olcott

• Thanks to Naomi Pequette, Peak Nova Solutions, whose “Eclipses” presentation inspired this post.