Concerning Bill Gottesman's proposal of a method to measure the solar diameter:
Well, it's basically a very good idea, but there are a number of traps to watch for. First, the slits need to be parallel. They also need to be aligned closely to North-South (or rather, perpendicular to the sun's motion at the time the measurement is made). Finally, what you are measuring is the time it takes the sun to change its longitude by one solar diameter, so there is a correction for the declination of the sun and another for the motion of the Earth around the sun. If all you are interested in is the ratio of the solar diameter at the two solstices, then the first correction cancels out, but not the second one. Also, it may be harder to determine a point in time accurately than a point in space. I would favor two parallel "pin-slits" casting images on a plane normal to the suns rays. The angular diameter (in the direction perpendicular to the slits, which should be oriented close to North-South and used near noon to eliminate any residual effects of atmospheric refraction) is inversely proportional to the distance between the slits and the plane when the images just touch. That is, the measurement consists of moving the plane until the images on it just touch and then measuring the distance from there to the slits. --Art Carlson
