Of Particular Significance

Transit Day

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 06/05/2012

I hope you’re all ready for today’s transit of Venus across the face of the Sun.  You certainly can’t have missed that it’s happening, given the media hype.  (Look at the website http://transitofvenus.nl/wp/, their post from yesterday in particular, for all sorts of useful resources, including timing for the event in various locations, and on-line resources for you to watch in case clouds interfere at your location. UPDATE: that website is overwhelmed. In the continental U.S. transit begins just after 6 pm Eastern, 3 pm Pacific, differing by a few minutes from place to place; and sunset will occur before the transit is over. One online location to watch the transit is http://events.slooh.com/)   But — let me be the first to warn you — this is going to be very cool, but it isn’t going to be a spectacular event like a big meteor shower or a total solar eclipse or even a total lunar eclipse.  It’s going to be subtle, slow, and potentially very boring, unless you have the right mindset (or a truly excellent telescope, properly filtered for sun viewing).  So here are some suggestions:

Binoculars are a very good idea — DON’T LOOK THROUGH THEM!!!  From the side and well away from anyone’s eyes, project the image of the sun through the binoculars onto a white sheet of paper, which you want to have perpendicular to the line between the sun and the binoculars so that the sun’s image is nice and round.  If you move the binoculars away from the paper the image will get larger; you can focus the image the same way you always do.  (A small telescope would be even better but not everyone’s got one handy.)  Proper solar eclipse glasses (NOT SUNGLASSES!) will let you look at the sun safely (don’t do it for too long even with the glasses) but expect Venus to be pretty small.  If you don’t already have the glasses, forget looking at the sun directly.

Don’t miss the onset of the transit!!!!  which lasts about 15 minutes.  Or if you miss that, catch the end if you can.  This is going to be the most dynamic and evocative part of the transit, (except perhaps sunset in the US, Canada, Mexico etc. or sunrise in Europe and Africa – see below).  It will be during this period that it is easiest to detect the actual motion of Venus — to really see Venus traveling in its stately procession around the sun at the speed of 35 kilometers (about 20 miles) per second. It’s very rare that you can actually sense the motions of the planets with your own eyes — this may be your best chance!

During the transit, you have a chance to grasp how huge stars are.  Right now Venus is only a third as far away as the Sun, so it will appear three times larger in radius and about ten times larger in area than it would appear if it were placed right next to the sun.  So look at Venus, imagine it three times smaller in radius or ten times smaller in area, and that will tell you how large is the sun compared to Venus.  And then remember that Venus is just about the same size as the Earth — so you will see, with your own eyes and a little imagination, just how small the Earth is compared to an average star. 

Consider and enjoy the historical significance of these rare transits, how with the use of parallax they could be employed, in the mid 18th century, to obtain the best measurements yet (accurate to about 2%) of the great distances between the planets and the sun.   You can read my new article on that subject, just finished today.

Finally, sometimes at sunset the glare is reduced enough that it is safe briefly to glance at the sun.  Don’t do it for more than a few seconds, but it might give you a naked-eye opportunity to glimpse Venus, just as sometimes one can see sunspots at sunset.  And if you’re a photographer with a powerful telephoto lens, or better a proper telescope, you’ll have an opportunity for a great shot.


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