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.
| 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).
| [Hydrogen alpha] Before the eclipse began, I practiced taking
pictures to make sure I knew what I was doing when the moment came!
| [Hydrogen alpha] First contact is the moment the Moon first
begins moving in front of the Sun.
| [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.
| [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.
| [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.
| We had a crowd of about 30 people sharing the views through
| Eclipse glasses cut out most of the Sun's light so you can
watch the event progress without using a telescope.
| [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. ||
| [Hydrogen alpha] This was taken just a few minutes before
| [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".
| [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.
| [Hydrogen alpha] This is what we came for --- the Ring of
Fire! We were still close enough to the centerline that it looks
| [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!
| [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.
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
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
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