The constancy of the solar diameter over the past 250 years

Meridian circle observations which indicated a contraction of the solar diameter are reconsidered. It is shown that grave difficulties in these observations render the method unsuitable for investigating possible changes in the solar diameter. Observations of the time taken for the planet Mercury to transit across the face of the Sun and also observed durations of total solar eclipses both present more accurate methods of detecting long-term change in solar diameter. The combined data of these observations have been analysed and shown to demonstrate that there has been no detectable secular change in the solar diameter over the past 250 yr or so. (U.K.)