Venus Transit

A couple of weeks ago, I wrote about the solar eclipse that was visible in the Western US. Unfortunately, the weather did not cooperate with my eclipse viewing. I think if I had really tried to see it, I would have been able to see something through the clouds, but there was also a tired and fussy baby involved in the equation.

For those of you who also missed it, a few of my friends posted some pictures of the eclipse on Facebook and gave me permission to share them here:

Partial eclipse in Chicago, IL. (Ivy Fitzgerald)

Annular eclipse through eclipse viewing glasses (left) and through the clouds (right) in Tokyo, Japan. (Amy Lovell)

Or if you’d like to see a time lapse of the entire eclipse, here is one of many YouTube videos.

While I missed the eclipse, there is another exciting solar event coming up: the Venus transit. This is very much like the solar eclipse, only this time, Venus will be blocking the sun instead of the moon. Of course, Venus appears much smaller than the moon to us on Earth, so instead of blocking the entire sun, Venus will just appear as a black dot moving across the sun.

Why does Venus appear so much smaller even though it is actually much larger than the moon? The apparent size of objects in the sky depends on the ratio of the object’s size to the object’s distance from us.

For example, the sun and moon appear to be about the same size in the sky from our viewpoint on Earth. This is why we can have a total solar eclipse where the moon completely covers the Earth. Looking on Wikipedia for estimates of sizes and distances, I find that:

This ratio is almost the same using some average, approximate values for distances. This is why the sun and moon appear to be the same size in the sky when we look up.

So how big will Venus be compared to the sun (or moon)? The Earth orbits the sun at a distance of ~150 million km, while Venus orbits at a distance of ~108 million km. So, when Venus is directly between the Earth and Sun (when we see this Venus transit), it must be ~32 million km away. Venus is approximately the same size as the Earth and so

So Venus will appear much smaller than the Sun. These numbers do not give me a good idea of what this will look like, but this picture from the Wikimedia Commons might help:

This is a fairly small dot, but definitely visible and I am excited about the chance to see it!

We have all probably been told from a young age that we should not look directly at the sun, so how are we supposed to view this transit? The sun is so bright it will actually damage our eyes. There are several different types of methods of looking at the Venus transit (or a solar eclipse) safely. A number of online stores sell “eclipse viewing glasses” which are really just filters that block the sun. The standard set of glasses appears to be made of Optical Density 5 filters. This means that the light that passes through the glasses is 105 (or 100,000) times less than the light that enters the glasses. You can still see the sun through the glasses, but it appears 100,000 times dimmer so it will not damage your eyes. Another good method is to use binoculars, or a telescope or other lens to project an image onto a piece of paper other surface. Do not look at the sun through the lens!!! This would certainly cause you harm! But, if you point the binoculars at the sun and put a surface behind the viewing lens, you will see an image of the sun that you can look at.

The Venus transit will be visible over much of the world, including all of the United States. NASA has a map of visibility if you would like to know if you will be able to see it and when. Click on Local Transit times in the bottom right to find out when you can see the transit at your location. We should be able to see it in the late afternoon/evening of June 5. I am hoping the weather cooperates this time so that we can go out and look! I even got some eclipse viewing glasses for the occasion.

This will not happen again in our lifetimes, so you should try to go out and see it. If you live in a part of the world where the transit is not visible, or the weather does not cooperate, NASA is also broadcasting the transit live from Hawaii so you can watch it on your computers at home.

UPDATE after the Venus Transit:

We were much luckier this past week in our solar viewing. The afternoon was partly cloudy again, but the Venus Transit started around 4 PM and lasted through sunset where we live, so there was a much bigger window during which we could try to find a sunny moment.

As mentioned last week, Venus looks much smaller than the Sun as viewed from Earth and so it was difficult to see the Venus transit just using eclipse glasses and our eyes. We may have been able to project an image using binnoculars or our little telescope, but do not have a fancy telescope and as always, have a little girl who had little patience with this watching the sky nonsense. However, we were able to see the Venus transit using our digital camera with the eclipse glasses in front of the lens. Here are some pictures my husband took:

These both show the image of the sun through the glasses, which filters out most of the light. The black dot is Venus moving between the Earth and sun. The image on the left was taken at 5:58 PM and the image on the right was taken at 7:12 PM. You can see on the right that the clouds have come in, partially blocking the light from the sun.

It was exciting to get a chance to see the transit since it won’t happen again until 2117. If my daughter lives for a very long time, she may get to see the next one, but I am certainly going to miss it!