Photometric Observations of the Sun

Abstract

Ground-based calorimetry and photometry of the Sun have been carried out for many years. Following the early years, ground-based photometry has largely replaced ground-based calorimetry, in part due to the advent of airborne and spaceborne detector systems for the broad-band measurement of the solar irradiance and the realization of the difficulty of correcting calorimetry measurements for the effects of the terrestrial atmosphere. Even from spacecraft, recent measurements of the total solar irradiance range from about 1367 to 1374 W/m2. Most of this difference can be ascribed to differences in instrumental scales, while a variation of about 1 to 2 W/m2 appears to be due to solar variability. The quiet Sun may also change, globally, over longer time scales. Using disparate data to understand solar variability will require cooperation between a number of current groups, supported by various governments, covering several zones of longitude.