Annular Eclipse, 20 May 2012, Cedar City UT

The centerline of the annular eclipse of 20 May 2012 passed through Southern Utah, so we made an expedition down to see the event. It was awesome! This page documents a little about the eclipse, some of the physics, and our pictures (what you're probably here for --- scroll down to the bottom for the physics).

I recorded an interview for Access Utah during the eclipse, which they broadcast on UPR. You can listen to it online.

First the good stuff (images), then some explanations of what is happening!

Images from the Eclipse

Click on images for a larger version!
Kanarraville, UT was right on the centerline, and they stepped up to host several thousand people for the eclipse. This is one of the signs guiding people in. thumb
We were heading home after the eclipse, so to avoid the exit traffic we set up in Cedar City (10.3 miles north of the centerline, but still in the path of full annularity). I had a white light filtered scope (foreground) and a hydrogen alpha scope (Coronado PST, behind me). thumb
[Hydrogen alpha] Before the eclipse began, I practiced taking pictures to make sure I knew what I was doing when the moment came! thumb
[Hydrogen alpha] First contact is the moment the Moon first begins moving in front of the Sun. thumb
[Hydrogen alpha] There was about 1h 10m between the moment of first contact and the start of annularity. We could chat and watch the eclipse progress slowly. thumb
[Hydrogen alpha] Notice the detail you can see on the surface of the Sun -- granulation (the orange peel texture), filaments (the dark lines), and prominences on the limb. This is the advantage of viewing in hydrogen alpha. thumb
[White Light] This is a view through the white light telescope; it is very similar to the view through the ubiquitous eclipse glasses that people were using. The telescope magnifies the image enough that you can see sunspots on the surface. thumb
We had a crowd of about 30 people sharing the views through our telescopes. thumb
Eclipse glasses cut out most of the Sun's light so you can watch the event progress without using a telescope. thumb
[Hydrogen alpha] By the time we had gotten deep into the eclipse, I think it was easy to tell that the full eclipse would be annular; once enough of the Moon was covering the Sun, you could tell the Moon was smaller than the Sun. thumb
[Hydrogen alpha] This was taken just a few minutes before annularity began. thumb
[Hydrogen alpha] This was taken as we entered annularity. near the edge of the Moon, the Sun is peeking through the lunar topography, making small areas visible and other areas not visible. This gives all the impressive structure seen near the thin edge. This is known as "Baily's beads". thumb
[Hydrogen alpha] In all, the Moon spent about 5 minutes completely within the disk of the Sun. Here, it is nearer one edge than the other, giving a lopsided appearance to the ring. thumb
[Hydrogen alpha] This is what we came for --- the Ring of Fire! We were still close enough to the centerline that it looks symmetric. thumb
[iPhone] The eclipse ended just after sunset. As the Sun was going down, I captured this sunset image. Notice the lens flare on the lower right? It shows the eclipsed Sun! thumb
[Hydrogen Alpha] Here is an attempted composite to show the progress of the eclipse from before first contact to annularity. The one labeled "Small" will link to a 1024x780 (300 kB) image; the one labeled "Big" will ilnk to a 7190x5760 (2.9 MB)images that will show lots of detail. thumb


Eclipses 101

Shadow Games: Solar vs. Lunar Eclipses

Eclipses are occassioned by objects passing into shadows. In the case of solar eclipses the shadow of the Moon is crossing the Earth; in the case of lunar eclipses the Moon is passing through the Earth's shadow. The two cases are shown below.

A solar eclipse occurs when the shadow of the Moon falls on the Earth; the Moon is between the Earth and the Sun. The geometry of the Moon's shadow is such that the shadow is very small by the time it reaches the Earth, so only people under the shadow see a total solar eclipse.

A lunar eclipse occurs when the shadow of the Earth falls on the Moon; the Earth is between the Moon and the Sun. The geometry of the Earth's shadow is such that the entire Moon fits in the shadow; everyone who can see the Moon at the time of the eclipse can see a lunar eclipse at the same time because..

The Moon's Orbit: Total vs. Annular Eclipse

We live in a very special place in the solar system: the Moon's size and the size of its orbit mean that from our perspective here on the surface, the Moon is just about the same size as the Sun in the sky. That makes solar eclipses particularly interesting because we can still see the Sun's light during an eclipse! During an annular solar eclipse the Moon does not cover the entire Sun. Why? The answer has to do with the Moon's orbit.

The Moon's orbit is slighly elliptical; sometimes the Moon is closer to the Earth and sometimes the Moon is farther from the Earth. Consider the image below (with the elliptical shape greatly exaggerated):


At perigee (the closest the Moon gets to the Earth) the diameter of the Moon in the sky is about 0.56 degrees. At apogee (the farthest the Moon gets from the Earth) the diameter of the Moon in the sky is about 0.48 degrees (smaller).

The consequence of this is that the Moon covers the Sun by a different amount depending on where it is in its orbit during an eclipse. The diameter of the Sun on the sky is only 0.53 degrees, so if the Moon is near apogee (as it was during this eclipse) then the Sun is not entirely covered, and we see a ring. This is shown in the image below.


When the Moon is close to you, it appears bigger (we say "it subtends a larger angle") and can completely cover the Sun (total solar eclipse). When the Moon is farther away, it appears smaller ("it subtends a smaller angle") and cannot completely cover the Sun (annular solar eclipse).

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