Word of the Week: Angular Diameter!

Hey, sky fans! Welcome to March! It’s Women’s History Month, so I’d like to take a minute to thank all of the women in my life, my family, and all of the women who make the world go. Thank you, deeply.

Today’s a pretty nifty anniversary. On March 1, 1966, 51 years ago (doing my math freestyle!), the Soviet Venera 3 spacecraft crash-landed on Venus, which made it the first human-made space probe ever to land on another planet. Most of the instruments failed before it got there, but either way, it’s a milestone.

Let’s see… last weekend there was a solar eclipse, which I didn’t write much about. Come on, there was pizza and Skee-Ball. The other day, though, someone asked me what was going on. Why was it an annular eclipse, rather than a total eclipse, which is what the US will see this summer?

Here’s where there’s some more science to talk about.

It’s Wednesday. You know what that means. Start the Word of the Week music! I’ll get the coffee started.

Angular Diameter!

Things that are near look bigger than things that are far, right? This smallish cupcake outside my window (don’t ask) looks bigger than that much bigger house down the street does. We happen to be alive at a terrific time, when the small-but-nearby moon is very close in apparent size to the big-but-far-away sun. This apparent size is called angular diameter or angular size.

Angular diameter is the apparent size of objects (measured in degrees, minutes, and seconds, just like a circle), often compared to each other, as seen from the some vantage point. A lot of astronomy is about lookin’ at stuff, isn’t it?

Remember, though, it’s the size as they appear in our sky, not their actual size.

The sun is about 93 million miles away from Earth, which is about 400 times farther than the moon’s 239,000 miles. Also, its 800,000-mile diameter is about 400 times bigger than the moon’s 2100-mile diameter. Have you ever noticed that that bright full moon in the springtime sky looks to be about the the same size as an afternoon sun? That’s why. Their average angular diameter is the same; about a half of a degree.

Word of the Week: Angular Diameter!
Word of the Week: Angular Diameter!

Since the Moon’s orbit around the Earth isn’t a perfect circle, it’s nearer to us at some points than it is at others. Remember a few weeks ago, when our word of the week was apsis? This in that neighborhood of astronomy. The nearest point, the near apsis, of the moon’s orbit is called perigee, and the farthest apogee. Near things look bigger than far things. When an eclipse happens when the moon is near perigee, it appears bigger than at other times; its angular diameter is a little bigger than average; a little bigger relative to the sun’s. So, its easier for the moon to cover the whole face of the sun. Right, this gives us a total solar eclipse, with the whole face of the sun blocked by what is, from our point of view, a bigger moon.

On the other side of this, the farther from perigee the moon is, the less of the Sun it’s able to cover, even when the moon crosses straight through the middle. In these, the sun’s angular diameter is bigger than the moon’s. Exactly, the bigger sun leaves behind a ring around the smaller moon; an annular eclipse. Sunday’s eclipse happened far enough from perigee (though still quite close) that that gorgeous and glowing ring of fire was visible 93 millions miles behind the darkened, face of the moon. Annular means “ring-shaped,” after that fiery ring.

This gets a little more complicated when you add in Earth’s orbital distance from the sun which affects the the sun’s angular diameter. For example, if there were to be a solar eclipse when the moon is at apogee (farthest from Earth), and Earth is at perihelion (nearest the sun), the difference in their angular diameters would be at the greatest; the moon would look small, the sun would look big. This would make the biggest possible solar ring around the moon in an annular eclipse. Let’s leave this here, though.

I hope this helps. Was anyone there and able to see the eclipse? Do you have any photos to share?

Remember, tonight, if the clouds cooperate, the Moon will be right next to Mars in the sky. It’d be a terrific time for you to dust of those binoculars you got for your birthday a few years ago. Uranus will be in the neighborhood, too.

Clear skies, everyone!


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