Using the most recent composite time series of total solar irradiance spaceborne measurements, a solar constant value of 1366.1 W m(-2) is confirmed, and simple quadratic expressions are proposed to predict its daily value from the Zurich sunspot number, the MgII index, or the 10.7 cm radio flux index. Whenever these three indices are available on a daily basis (since 1978), it is possible to predict the sun's irradiance within 0.1% on average, as accurately as current measurements. Based on this value of the solar constant, an improved approximation of the extraterrestrial solar spectrum from 0 to 1000 pm is proposed. It is obtained by dividing the spectrum into nine bands and selecting representative (and recent) spectra, as well as appropriate scaling coefficients for each band. Comparisons with frequently used spectra are discussed, confirming previous findings of the literature. This synthetic and composite spectrum is proposed at 0.5-nm intervals in the UV (280-400 nm), 1-nm intervals between 0-280 and 400-1705 nm, 5-nm intervals between 1705 and 4000 nm, and progressively larger intervals beyond 4 mum, for a total of 2460 wavelengths. (C) 2003 Published by Elsevier Ltd.