Posted by: SteveInCO | 30 Apr 2012

Annular Solar Eclipse, 20 May 2012

It’s an annular solar eclipse, and you can see it in large parts of the western US.

I suppose I should clarify:  “annular” is not some weird redneck mangling of the word “annual.”  These things don’t happen every year.  It comes from “annulus” which is Latin for ring.

Here is the Wikipedia page for this particular eclipse.

We here on earth are the beneficiaries of an extremely fortunate coincidence.  The sun is four hundred times (or so) larger than the moon.  And it is four hundred times further away as well.  So the moon and the sun appear to be almost exactly the same size.  They cover about half a degree of sky, each.  On average the moon is just a tiny bit larger.

But the earth is an elliptical orbit about the sun, so sometimes the sun appears a little larger than average, sometimes a bit smaller than average.  The moon, in turn is in an elliptical orbit about the earth, and its apparent size also varies.  At any given time the moon is likely to look larger than the sun, but sometimes, the moon looks smaller than the sun.

When it just so happens that the moon travels directly between you and the sun, you will see either a total (if the moon is looking bigger than the sun) or annular eclipse (when it is looking smaller).

You will note that I said “between you and the sun,” not “between the earth and the sun.”  That’s because people more than about 100 miles away from you are far enough off axis that they won’t see the moon completely cover the sun.  Instead, they’ll see a partial eclipse.

Total and annular eclipse geometries (exaggerated). 
A–total solar eclipse.
 B–Annular eclipse 
  C–Partial eclipse.

So what we will see this time (a relatively few lucky people, that is) will be an annular eclipse.

Eclipse Anular

Eclipse Anular (Photo credit: Wikipedia)

People on the Chinese coast and in Southern Japan will see it shortly after sunrise.  The path of the annular phase will then cut across the Pacific Ocean, barely missing the Aleutian Islands (that’s the chain of islands running southwest from mainland Alaska), then crossing into the “lower 48” just south of the California-Oregon state line, then sweeping southeast.  By the time it reaches Albuquerque it will be almost sunset there; the sun will set before the moon is completely done uncovering the sun after the annular phase.

Here is a more detailed map of the path through the United States.

OK so it’s going to spend most of its time crossing over the Pacific Ocean, so relatively few people will see it.  There’s more bad news though.  Guess what?  Annular eclipses basically suck by comparison to total solar eclipses.

The beauty–the absolutely stunning beauty–the beauty that turns people into fanatic eclipse chasers who will travel to the other side of the world for the five minutes of totality (and imagine their frustration when it’s cloudy!)–of a solar eclipse comes from being able to see the solar corona, the sun’s atmosphere.  I’ve never seen it.  I am assured no photo, no description can do it justice.  But the point is, you just cannot see it when any part of the solar photosphere is exposed.  (The photosphere is the searingly bright disk of the sun that you see when you ignore the advice of anyone with more intelligence than blue green algae, and you look at it.  Which you shouldn’t.  Oh, and I should add:  just don’t do this.)

And of course with an annular eclipse, no matter where you are, you will see some photosphere; that’s why it’s called an annular eclipse.  So this is boring.  This is not the sort of eclipse the fanatic solar eclipse chasers chase.

But I am going to go see it anyway, because it’s there, and I live near Colorado Springs and Albuquerque isn’t that far away (and it’s not that much further to other places on the line of annularity further west).  I figure I might as well do this once in my life for grins if I can, and this is an easy trip; if I don’t mind staying up late I can even skip getting a hotel.

If you are thinking that you will just wait until we have a total eclipse (and I wouldn’t blame you really), you will have to wait only another five years, because if you are in North America, the next total eclipse is on 21 August 2017.

That day we will have a beaut of a total eclipse, and it will draw a line from Portland OR to Savannah GA, and crosses through southern Wyoming, not too far from where I live in Colorado.  You can bet I will chase that one!  It’s on my “bucket list.”

How to watch this (or any) eclipse

Even if you are not able to travel to anywhere in the swath of land that will see the annular phase, you can watch the eclipse, but all you will see will be a crescent sun of varying thicknesses.  Anywhere in the western US (except parts of Texas) you will be able to see at least something.  (I saw an 85% eclipse once in junior high school.)

Again do not look at the sun.  Well, you can with eye protection.   Here’s one possibility:

Eclipse viewing glasses can be used to observe...

Eclipse viewing glasses can be used to observe the sun. (Photo credit: Wikipedia)

You can use a telescope if it has a proper filter on it (over the “big end” or objective of the telescope, not over the eyepiece!  An eyepiece filter will heat up and most likely crack; you don’t, you just don’t, want to be looking through the scope when that happens.  Instant and permanent blindness could be the result!)

You can also project the image of the sun onto something.  That could be as easy as putting a pinhole through a large piece of cardboard and seeing the image of the sun that falls on the ground in the shadow of the cardboard.

Whatever you decide to do, observing a partial or even an annular eclipse can be fairly cheap and fun.  And it can be interesting.  It won’t be spectacular, though.

Addendum (1 May 2012)

I just read a news story regarding the full moon this Saturday (the 5th) being the largest one we will see all year.  That is because it is happening with the moon near its perigee, the point in its orbit where it is closest to earth.   If you think about it this makes perfect sense.  If this eclipse is going to be annular, it is  because the moon looks smaller than usual, because it is further from the earth than average.   Surely the full moons before or after that eclipse will be big–because the moon will be on the opposite end of its elliptical orbit, and be closer to the earth.

The moon’s orbit in May 2012 looks something like this, with the apogee happening near new moon, and the perigee happening near full moon.  The eccentricity is exaggerated for clarity.

It should be noted that the moon’s orbit is not always in this configuration!  If it were, all eclipses would be annular, and all full moons would be large.   The axis of the ellipse–from perigee to apogee–could point in almost any direction with respect to the sun.  I could go further into it but that involves a lot more diagrams and would be a huge digression even by my standards.  Perhaps some other time.

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