Apparent Diameter of the Sun
Most people know that the Earth moves in an orbit around the Sun once each year, and that the distance between the Earth and Sun varies around the orbit. Some people conclude that the reason that the Earth experiences seasons (hot summers and cold winters) is this varying distance between Earth and Sun.
Actually the seasons are caused by the angle at which the Sun shines onto the Earth. The plane of the Earth's equator is tilted by 23 degrees from the plane of the Earth's orbit. It is summer when the Earth is in the part of it's orbit where the Sun is shining more directly on one pole or the other. When it is winter in the Northern Hemisphere, it is summer in the Southern Hemisphere.
To show that the varying distance between Earth and Sun is not cause of the seasons, you can compare the distance with the time of year. When the distance between Earth and Sun is small, the Sun appears larger, and we should be hotter. When the distance between Earth and Sun is large, the Sun appears smaller and we should be colder.
The Mees White Light Telescope takes pictures of the whole Sun each observing day. We measure the diameter of the Sun on the pictures. The following graph shows the measurements for four years. The Sun appears largest, and is therefore closest, in December and January, during the winter here in the Northern Hemisphere. (The vertical scale is the angular size of the Sun in arc seconds. One arc second is 1/3600 of 1 degree. A dime one mile in the distance appears to be about 2 arc seconds in size.)
To show this change in the appearance of the Sun through the year, the following image compares the pictures taken when the Earth is near it's closest and farthest locations.
Last modified: Fri Jan 21 14:40:57 HST 2000