Tuesday, June 19, 2012

A Transit of Venus DIY parallax kit

I've been wondering what the parallax effect in a Transit of Venus actually looks like. I know the 18th century astronomers used it to measure the solar system, because it says so in books. (See my post of June 6th The Transit of Venus in history which includes some good links). But just how pronounced is this parallax effect? Can anyone see it or is it miniscule?  Well, see for yourself ...

Here's a pair of computer generated images of the transit that happened a fortnight ago. 

Transit of Venus, 6 June 2012, 02:00 UT from Capetown
Transit of Venus, 6 June 2012, 02:00 UT from Cork
They show the transit at the same instant, 2am Universal Time.  One from Cork and the other from Capetown.  The pronounced black dot is Venus. The grey dots are sunspots. There is a small but quite definite difference in where Venus appears on the Sun’s disc. The effect is much too small to detect when you view these two images together on the screen. But you can see it very clearly if you flick between them.

Try it! It's a bit a of do-it-yourself job I'm afraid, and here's my suggestion how to go about it.

Create a new folder on your computer’s desktop and save each image into it.[1] Then click on one of the images to view it …  this will open your computer’s viewer software … now click the right arrow key to view the other image ... by repeatedly clicking the right arrow, you will see Venus shifting up and down. That’s parallax. 

Venus seen from Capetown appears higher up (further north) on the Sun’s disc than Venus seen from Cork.  
In the diagramme, A is the apparent position of Venus viewed from Capetown, and B is the apparent position of Venus viewed from Cork. (The diagramme hugely exaggerates the AB distance. Strictly speaking the Earth should be a small dot like Venus.)

Seeing Venus wobble as you flick between the two images, helped me to appreciate what Edmond Halley was banging on about when in 1716, he instructed the next generation of astronomers: go to the ends of the Earth to detect the parallax in the 1761 Transit of Venus, and use this to measure the solar system.  I won't go into how, as it involved trigonometry.

That, and some additional notes and links, are in this pdf file.

Thanks to Tony Jackson and Terry Moseley for help putting this together.

[1] In case you need instructions for saving the two images, here goes for Windows users. I assume you’ve already created a new folder on your computer’s desktop called Venus. Now hover over one of the above images, right click, and choose the menu option Save Image As. Save it into the Venus folder. Do the same for the other image. Your Venus folder now contains two images and you can flick between them.