Occultations, Eclipses and Transits!

Hey, everybody! Did you have a chance to get look at the Moon and Aldebaran doing their thing the other night? Around here, the skies were iffy for a good bunch of the day, and I figured it’d be another interesting sky thing, lost to clouds. After sunset, though, I was able to point them out to my daughter and to a couple of other people in the neighborhood.

Hours later, after checking in a few times, and seeing the Moon creep closer and closer, I went out to look straight up at the sky. This was the first time I’d seen something occult something else in a while, and really enjoyed watching it. It was much more gradual than I expected. Aldebaran is a very bright, very noticeable star. It’s in a neighborhood in our skies that’s crowded with bright stars, but even among those it stands out.

The Moon, though, is the brightest thing in the night sky, even on its dimmest, crescent  days. Other than the Sun itself, nothing even comes close to a nearly-full Moon like the one that was messing with Aldebaran.

Over time, more and more of Aldebaran’s light got overwhelmed and washed out by the Moon’s. So, what usually looks like a distinctly orange light in the distance was slowly reduced to a tiny point of light, before it eventually disappeared to my naked eye entirely into the Moon’s glare. The fact that it was visible at all against the Moon says a lot about how bright it appears from Earth. From there, it was time to take out the binoculars to watch it disappear behind the Moon once and for all.

While I stood watching, I got wondering about something. This was the Moon moving into the line of sight between Aldebaran and the Earth, and blocked it out. This is called an occultation. The verb form is occult. If the Moon moves into the line of sight between the Sun and the Earth, we call it an eclipse. What’s the difference?

Maybe this is a good time to start a new feature here at Sky Watch! It’s time for the Sky Watch Word of the Week! And, it’s a two-fer.

What’s the difference between an eclipse and an occultation? Do you know? I didn’t. I had to look it up. I had to read through a bunch of books to figure this out, and my copy of Webster’s New Collegiate Dictionary let me down.  So, I hope I have it right. That’d be embarassing, huh?

When something passes across the face of a more distant object and blocks all or part of it out as seen from a given viewpoint, we say that the nearer object occulted the more distant one. The verb to occult simply means to block out. When, from our perspective on Earth, the Moon passes between the Sun and the Earth, the Moon occults the Sun. Similarly (and to show how deftly I can switch between verbs and nouns), earlier this week, when the Moon passed between the Earth and Aldebaran, we had ourselves an occultation of Aldebaran. The two events are basically the same thing. The only real difference is Aldebaran is something like four million times farther than the Sun.

This is where things got a little foggy for me.

Occult (verb): To Block Out
Occult (verb): To Block Out

An eclipse is when the viewpoint that’s seeing the occultation passes into the shadow of the occultation. In truth, though, it’s is also used to mean an occultation; they’re essentially the same thing, synonymous. Sheesh… Let’s blink and take a breath. I guess another way of looking at it is the eclipse is sort of the result of the occultation, as seen by a given viewer. So, when the Moon blots out the Sun, the Earth, by falling into the Moon’s shadow, is eclipsed. In a lunar eclipse, the Moon is eclipsed when it falls into the Earth’s shadow and turns that beautiful ruddy red color. What little I know about linguistics makes me think the original definition was the “fall into shadow” one, and it became a synonym for occult later on.

So, if you’re walking along, singing a song, and walk into one of the long, late-fall shadows of a tree in your neighborhood, it’s you who’s been eclipsed, not the Sun. The tree has occulted the Sun (from your perspective). I guess if you happen to be putting out a truly spectacular amount of light and are able to team up with the tree to cast a shadow on the Sun, then the Sun will be eclipsed, too. You also might have a new career path to look into.

The third word in our inaccurately named two-fer is transit. A transit is when something that appears much smaller than a bigger object crosses the face of that bigger object as seen from a given viewpoint. Mercury, which is, in nearly every sense of the word, smaller than the Sun, transited ol’ Sol as seen from Earth in May 2016. It only blocked out a tiny part of the Sun when it did. When the Moon, which appears much bigger relative to the Sun, transits the Sun, it’s called an occultation. The difference between an occultation and a transit is just the size of the piece being blocked out. The word transit also has other meanings in astronomy, which we’ll talk about another time.

You’re right. They’re all really similar words, but there are some small differences. I hope this makes sense and clears things up. Thanks for stopping by, and clear skies, everyone!


7 thoughts on “Occultations, Eclipses and Transits!

  1. On occultation day it was clear all day. I set up my camera and tripod in the office to be ready later that evening. Well, clouds rolled in. I could barely make out the moon was even up there the clouds were so thick. Maybe next time.

    Liked by 1 person

  2. I’ve found a lot of astronomy terms–and science terms in general–have annoyingly similar definitions with only very subtle distinctions between them. I’ve never really looked into the distinction between an eclipse and an occultation, but the idea that eclipses are just occultations plus cast shadows makes a lot of sense to me.


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s