The 2012 Transit of Venus (TOV) attracted
quite a bit of media attention because it was the last such transit we will see
until 2117, and, thus, the last we will experience in our lifetimes. For those who missed the media coverage, a TOV occurs when Venus passes between the Earth and the Sun. During such events the planet appears as a
dark circular spot on the face of the sun.
You can think of a transit as a sort of a mini-eclipse of the Sun by a
planet or asteroid. A TOV is a relatively
rare event - the 2012 TOV was only the eighth
one since the invention of the telescope around the year 1600. This year's TOV lasted for six hours. In New
Jersey the transit began at 6:05 PM, but the Sun set
(in NJ) before the transit was complete.
I wanted to see the TOV from my home which has a nice view of
the western sky. But first, I needed to invent
something so that I could safely observe and record the event. As it happens my telescope nicely projects an image of the
Sun so I simply needed to provide a projection "screen". My $0 invention (not counting the cost of the
telescope) is shown here, at right. 
It
consists of a box (obtained from a liquor store) with the top and one long side
cut away. For the "screen" a glossy
white piece of paper was taped to the inside bottom of the box. The box was then turned upside down and taped
to a thin 
piece of scrap plywood. A
small section of the plywood was cut away so that the unit could sit snugly on
top of the telescope, as shown. On the
morning of the TOV (6/5) the sky was relatively clear so I collected an
image of the sun (at left as viewed on the "screen" from the floor). A blowup of the silky cloud covered solar
image obtained that morning is also shown at left.
Note that a number of sunspots 
were visible. To give some perspective of scale, the
bottom-most sunspot was some 27,000 miles long, more than three times the Earth's
8,000 mile diameter. The Sun's photograph had
an egg-like appearance because I was unable to get under the telescope tripod
enough to be perpendicular to the projection.
In spite of this limitation, I concluded that the apparatus was working
sufficiently well. All I needed now was
cooperation from the weather. That
cooperation was not forthcoming for most of the day - thick rain clouds covered
the sky into the evening. I was not able
to personally observe the beginning of the TOV but, fortunately, 
NASA provided a live feed
from Mauna Kea in Hawaii. At right is the screen shot of the NASA feed
just after Venus had fully "ingressed", i.e., its shadow was
completely within the visible disk of the sun.
The NASA image was exciting, verifying that the TOV was indeed in
progress. It was also gratifying that NASA
was seeing the same pattern of sunspots that I had observed in the
morning. But, I wanted to personally see
the TOV on my home-made apparatus. I spent
the day going up and down the stairs about a thousand times, checking and
rechecking weather maps, grousing and complaining, and testing mightily a
saintly spousal patience. Finally, at
7:37 PM, just after sitting down to supper, the sun broke through the
clouds. I immediately raced upstairs to record many
images, all of them of a partially cloud covered Sun. At left is one showing Venus having moved
northwest (since the NASA image at 6:30 PM) across the Sun's face.
It
consists of a box (obtained from a liquor store) with the top and one long side
cut away. For the "screen" a glossy
white piece of paper was taped to the inside bottom of the box. The box was then turned upside down and taped
to a thin 
piece of scrap plywood. A
small section of the plywood was cut away so that the unit could sit snugly on
top of the telescope, as shown. On the
morning of the TOV (6/5) the sky was relatively clear so I collected an
image of the sun (at left as viewed on the "screen" from the floor). A blowup of the silky cloud covered solar
image obtained that morning is also shown at left.
Note that a number of sunspots 
were visible. To give some perspective of scale, the
bottom-most sunspot was some 27,000 miles long, more than three times the Earth's
8,000 mile diameter. The Sun's photograph had
an egg-like appearance because I was unable to get under the telescope tripod
enough to be perpendicular to the projection.
In spite of this limitation, I concluded that the apparatus was working
sufficiently well. All I needed now was
cooperation from the weather. That
cooperation was not forthcoming for most of the day - thick rain clouds covered
the sky into the evening. I was not able
to personally observe the beginning of the TOV but, fortunately, 
NASA provided a live feed
from Mauna Kea in Hawaii. At right is the screen shot of the NASA feed
just after Venus had fully "ingressed", i.e., its shadow was
completely within the visible disk of the sun.
The NASA image was exciting, verifying that the TOV was indeed in
progress. It was also gratifying that NASA
was seeing the same pattern of sunspots that I had observed in the
morning. But, I wanted to personally see
the TOV on my home-made apparatus. I spent
the day going up and down the stairs about a thousand times, checking and
rechecking weather maps, grousing and complaining, and testing mightily a
saintly spousal patience. Finally, at
7:37 PM, just after sitting down to supper, the sun broke through the
clouds. I immediately raced upstairs to record many
images, all of them of a partially cloud covered Sun. At left is one showing Venus having moved
northwest (since the NASA image at 6:30 PM) across the Sun's face.
So, why do we care about all this? Historically, transits of Venus were very
important because they were used to determine the actual distance between
the Earth and the Sun. This distance is
called the "astronomical unit (AU)".
We had long known that Venus was 0.72 AU from the sun, but we didn't
know the value of the AU. During a TOV
the planet Venus is at its closest approach to Earth. This distance is 0.28 AU (1.00-0.72),
so if we are able to measure that distance we would know the value for the AU and, therefore, the size of the Solar System. This determination can be accomplished during a TOV by accurately plotting the paths of Venus across the Sun's face observed from two locations north and south of each other and calculating the distance to Venus by parallax. For details of this calculation using simulated data from Anchorage and Honolulu see the
excellent article by Dr. Sten Odenwald at http://sunearthday.nasa.gov/2012/articles/ttt_75.php. I used Dr. Odenwald's "measurements" and determined a value of the AU of 100 million miles, less than 6% different than the currently accepted value. Once we know the AU what else do we know? Well, for one thing we can immediately calculate the diameter of the sun from it's angular diameter (0.525 degrees - there are 180 degrees from horizon to horizon). Simple trigonometry using the AU value of 100 million miles gives a solar diameter of 900 thousand miles (cf. tabulated value 865 thousand miles). We can also easily measure the angular diameter of Venus (0.016 degrees corresponding to 7900 miles, accepted value 7500 miles) from the photo just above . All this leads to the distances to and diameters of all the planets as well as the distances from a planet and all its moons - only Mercury and Venus don't have at least one moon. So, that's why we care - and it's why you should go ahead and put the date of the next TOV, 12/10/2117, on your Outlook calendar. That way, the big event won't sneak up on you.
