Solar disc phenomena: The eccentricity of the Earth’s orbit
Inspection of the AA shows that the apparent solar diameter changes from
a maximum of 16' 15''·9 on January 4th to
a minimum of 15' 43''·1 on July 4th
this being caused by the variation in the distance of the Earth from the Sun according to the eccentricity of its orbit.
If care is taken in making solar drawings with the same equipment throughout the year, with the image board holding the paper kept at a constant distance from the telescope, the variation in the size of the solar disc can be recorded and a value obtained for the eccentricity of the Earth’s orbit. Investigation of the figures just given shows that the variation in the apparent size of the solar disc is about 3·5%. In order to detect this and measure it reasonably well over the year, the diameter of the projected image must be of the order of 100 mm giving a variation of about 3·5 mm.