Visual and photoelectric measurements of the solar diameter (1972‐2002): Methods and results

Measurements of the solar diameter using both visual and photoelectric drift scan techniques have been made since 1972 using two almost identical 45‐cm Gregory‐Coudé telescopes at Locarno/Switzerland and Izaña/Tenerife. The method, in which a time measurement substitutes an angular measurement, is especially suited to obtain about 30 measurements of the absolute solar semidiameter per day. During the years 1972–2002 a total of 10996 visual timing measurements have been made on 320 observing days, an additional 1373 photoelectric recordings have been obtained on 117 observing days. The data were used to study the long‐term behaviour of the solar semidiameter R at unit distance and its possible variations. No fluctuations dR in excess of about ±0.05″ have been found, neither long‐term nor short‐term. The photoelectric semidiameter, which refers to the continuum at λ ≈ 585 nm, is Rphot = (959.89 ± 0.12)″. The visual semidiameter, which refers to the footpoint of the limb intensity profile at λ ≈ 550 nm, is Rvis = (960.62 ± 0.02)″.