Accurate solar ‘constant’ determinations by cavity pyrheliometers

Total solar irradiance was observed outside the earth's atmosphere by three types of absolute cavity pyrheliometer in a June 1976 sounding rocket experiment. The 1367 W/m 2 average solar ‘constant’ result is uncertain by less than ±0.5%, the most accurate determination to date. Nearly simultaneous observations by the Nimbus 6 earth radiation budget total irradiance detector of 1389 W/m 2 exceeded the rocket result by 1.6%. These recent results are discussed in the context of a summary of solar constant determinations made above the troposphere by cavity pyrheliometers.