Monday, April 03, 2006

So I figure that's enough ranting. Now for a reminder of why I love astronomy:


(Click to enlarge)

This image is of the solar eclipse earlier this week. Solar eclipses occur when the moon comes between the Earth and Sun. However, there's more to it than just that, otherwise we'd have a solar eclipse every ~28 days (one full lunar cycle).

When viewed edge on, the plane in which the moon orbits is slightly tilted in relation to the plane the Earth and Sun lie on (hence the reason the shadow moves along a different line in the sky than the sun, intersecting only at the one point). Because of this, most of the time, when the moon is on the line between the Warth and the Sun, it is simply too high or too low to cause an eclipse.

Sometimes it's between the point where it's too high or low and the point where it will completely come in front of the Sun. In this case, the moon will only cover part of the Sun and the result will be a partial ecplipse, such as this one I photographed in Spring 2005.

Additionally, the moon's orbit around the Earth is not perfectly circular. It is slightly elliptical. This means that at some points in its orbit, it further than other points. As common experience should tell you, ther further away an object is, the smaller it will look (which is why the sun appears the same size in the sky as the moon dispite being millions of times bigger). Therefore, since the moon is further away, it will be smaller, and may not cover the sun entirely. This is known as an annular eclipse in which the moon will be silhouetted on the sun leaving a ring (such as in this picture).

Thus this image is an extremely rare "total solar eclipse" in which the moon completely covers the full disk of the sun. But what's all that fuzzy stuff around it in the center one? That's called the corona and is essentially the sun's extended atmosphere which is shaped by the Sun's immense magnetic field. It's actually always there, but it's extremely faint in comparison to the sun, so we can't see it unless the sun is somehow blocked out, as in the case of a total solar eclipse. It is primarily composed of the nuclei of ionized hydrogen atoms.

You may also be wondering why you didn't happen to catch this eclipse given that it only happened a few days ago. The reason is that this one only happened to be visible from regions of northern Africa and the Middle East. You should not be asking yourself, "why only such a small location given that half the Earth can see the sun at any time?" The reason for this is something called parallax. In the scenario of a total eclipse, only the locations directly below the center of the moon will see the eclipse. Locations slightly further away will be viewing the event from a slightly different angle.

While this wouldn't seem like it would play much of a difference, try a quick experiment. Imagine your left eye is someone standing in southern Africa and that your right is someone standing in England. Close one eye and hold your fist out in front of you and cause it to eclipse something on the other side of the room (or outside if possible, the further away the better). Make sure the object you choose is just barely covered by your fist. Now without moving your arm, change eyes. You'll notice that your fist is no longer covering the object at all.

This effect that you have just observed is precisely what happens in the case of an eclipse for different observers and is what astronomers call parallax (Parallax also has many other applications in astronomy such as directly measuring the distance to a great number of stars to extremely high precision thanks to the HIPPARCOS satellite). This quick experiment is also reasonably close to actual scale in terms of angular sizes and relation between sizes for the earth and moon. The distances between objects and true sizes aren't even close, but those don't matter in this case.

So you're probably wondering why there's the strange disjointed path. After all, we never see that. There's only one sun in the sky. This image is actually a compilation of 18 images taken ~3 minutes apart (and presumably one more to use as the beautiful background). I can say that these were taken ~3 minutes apart because of the spacing of the suns.

In 24 hours, the sun makes a full 360º path around the sky. Thus, converting hours to minutes and dividing, we find that the sun moves 1º every 4 minutes. Although it doesn't seem that there's any scale marked on this image to permit me to figure out how many degress there is between each image from which to figure out the time between images, there actually is a very easy one: the sun itself.

Both the sun and the moon have an angular size of 1/2º. That means that if the little suns were butted right up against one another, it would have traveled 1/2º between images, which in turn implies that it would have been 2 minutes (4/2) between each image. Since there's a little more space, roughly 1/2 of a sun width (ie, 1/4º), I can estimate there was approximately another minute between pictures. Thus 2 + 1 = 3.

So ultimately 18 images of the sun were taken and then reassembled to produce this dramatic image. While in and of itself it is quite stunning, a closer look reveals more information than meets the eye. This concept is one I feel is important to keep in mind in the sciences. Things are not always what they seem to be at a first glance. If this wasn't the driving concept behind science, we would still hold with many ridiculous ideas such as the Earth being flat, or alchemy, or perhaps more relavant today, intelligent design.


Image copyright: Stefan Seip
Found via: NASA Astronomy Picture of the Day


Update: The original version of this post contained erronious math which was noted by reader, Benjamin Franz, in the comments. I have corrected my math here, but wanted to make sure he was given due credit.

11 comments:

Steve said...

I'm looking forward to the next total solar eclipse, visible from the southeastern US. Those are too few and far between.

Mike said...

Hey, just discovered your blog. I am a fellow Kansas who is pro-science and anti-religious right. I look forward to your posts.

One favor...can you provide titles for your blog postings, that way those of us who use RSS readers can see the titles of your posts as we browse your blog...without titles, everything just say s"untitled" over and over again! thanks

Jon Voisey said...

If I recall correctly, that will be in 2008?

Sadly, I doubt I'll be in the southeast. Thus, for me, it will most likely only be a partial eclipse. But those are exciting in their own way.

Jon Voisey said...

Mike: I was wanting to add titles, but I've just started using this service and can't seem to find the place to add them. I'll have to poke around some more tonight.

Steve said...

It happens on August 21, 2017. I'm about a hundred miles, or so, from the path of totality, so I'll have to do some travelling, if I want to experience full coverage.

www.hermit.org/Eclipse/2017-08-21

Jon Voisey said...

It seems that there is going to be a total solar eclipse in 2008 but totality for North America will only be visible in Canada.

Figures. One goes north of me, one goes south.

Don Sheffler said...

Jon - when you're logged onto blogger, click the SETTINGS tab.

Your subtabs below that in blue are "Basic", "Publishing", "Formatting", etc. - Choose "Formatting".

Scroll down and see where it says "Show Title Field" make sure your choice is YES. It's probably NO right now and you can't see anywhere to title your posts.

Welcome to blogdom. That's a great time-lapse photo of the eclipse, thanks for posting it.

Hey, all that mumbo about why total eclipses are so rare, and how they only appear in small swaths of the earth when they do actually occur... y'know, that doesn't seem very "fine tuned" to me.

Hmmmm.

Benjamin Franz said...

Your math is off. If the Sun appears to move 360 degrees in 24 hours, that is 15 degrees per hour or one degree every 4 minutes. This means it moves its own width every 2 minutes - not every 7.5 minutes.

Jon Voisey said...

bejaman: Thanks for correcting my math. That's what I get for trying to skip steps and do things from memory instead of working things out from scratch.

I'm going to ammend my original post so I don't have the false information floating on the surface, but I happily let it stand that I make mistakes too.

don: Thanks for that tip. I'll go poke at things now.

Benjamin Franz said...

Your math is off. If the Sun appears to move 360 degrees in 24 hours, that is 15 degrees per hour or one degree every 4 minutes. This means it moves its own width every 2 minutes - not every 7.5 minutes.

Mike said...

Hey, just discovered your blog. I am a fellow Kansas who is pro-science and anti-religious right. I look forward to your posts.

One favor...can you provide titles for your blog postings, that way those of us who use RSS readers can see the titles of your posts as we browse your blog...without titles, everything just say s"untitled" over and over again! thanks