THE SUN IS DOING WHAT?

(http://galileo.rice.edu/sci/observations/sunspot_drawings.html)

Hiya, Sean Lally Physics Guy here. Today on “How do we know that?” we are looking at the Sun. You’ve heard of it, yes? It keeps us warm, it makes plants grow, it gave us the Beach Boys…

But did you know that it also rotates? It’s true, I tell you! It takes about 25-35 days to complete one rotation. Now I know what you’re thinking – big deal, the Earth rotates once per day. Well, the Sun has a diameter about 100 times greater than that of the Earth. At the Earth’s equator, the average speed is close to 1000 miles per hour. At the Sun’s equator, it’s nearly 3 times that speed.

So, how do we know that?

In Galileo’s heyday, the early 1600s (that’s around 400 years ago), it was ridiculous to imagine that the Sun was moving. It was dangerous enough to imagine that the Earth was moving. Galileo was tried and convicted for writing and teaching that very notion. Galileo may have not been the first person to use a telescope, but he’s the first person to write detailed observations through his telescope about the night sky. In a few short weeks, Galileo discovered that there were many more stars in the sky than previously thought, that the Moon was heavily cratered, that Venus went through phases (strongly suggesting that it rotated around the Sun), that Jupiter had satellites of its own, that Saturn had “ears” (later shown to be rings) and that the Sun had little spots on it!

Yikes! The Sun has spots? These were noted by projecting the image of the Sun onto a piece of paper. They had also been noted by Christoph Scheiner, Thomas Harriot, and Johannes and David Fabricius.

Can one see these spots easily? Well, yes and no. If you try to look at the Sun with the naked eye, then no – in fact, it’s a bad idea, in general. If you look through a designated solar telescope, a pretty costly device, then yes, it’s usually pretty easy. (Alternately, a scope fitted with an appropriate solar filter is also a good way to view the sun – and much cheaper!)

We will not assume that you have easy access to a solar telescope, or telescope fitted with a solar filter, so let’s first have a look at Galileo’s images.

http://galileo.rice.edu/sci/observations/sunspot_drawings.html

Now comes the impressive part. Galileo created the drawings in a series over the course of several days. Imagine them as a “flip book.” Or better yet, imagine that someone had created a film of the images.

Actually, you don’t have to imagine this at all. Look at the video clips (same page).

http://galileo.rice.edu/sci/observations/sunspot_drawings.html

What are the possible explanations of what was seen? Are there alternate explanations (other than the Sun rotating)?

So, why do you think so many of Galileo’s contemporaries were skeptical? What possible harm could come from imagining that the Sun rotated?

So, it seems as though the Sun rotates. Why might this be?

For that matter, why does the Earth rotate?

That’s it for today – have fun, see you in the Sun!

http://galileo.rice.edu/index.html
The Galileo Project, a great source for all things Galileo

http://galileo.rice.edu/sci/observations/sunspots.html

Galileo – Discoveries and Opinions of Galileo


Personal Solar Telescopes

http://www.coronadofilters.com/
http://www.solartelescope.net/
http://www.cloudynights.com/item.php?item_id=104
(Sunspotter – a modern classic)

ALL TEXT AND IMAGES COPYRIGHT SEAN LALLY 2009 (except where noted)