New measurements of the solar diameter

According to standard solar models, the energy radiated from the surface of the Sun is equal to that generated by purely nuclear processes in the core. In the absence of perturbations, structural changes take place only on the nuclear evolutionary time scale; for example, during the last 108Yr, the Sun's semidiameter would be expected to have increased by approx. 4 arc s. Claims based on meridian transit measurements that the Sun has been shrinking on a much shorter time scale, approx. 1 arc s per century, and thereby changing its potential energy, have been disputed and upper limits on any secular decrease have been shown to be consistent with a constant diameter. Measurements made during the 31 July 1981 total eclipse, reported here, together with a re-analysis of previous eclipse and Mercury transit measurements, confirm that there is no evidence for any secular change in the solar diameter, with a reduced upper limit. However, there is increased support for an approx. 80-yr cyclic variation. (author)