Twenty years of ground and space measurements haven't yielded a consistent model for how and why the solar luminosity varies. New observational tools are essential to understanding how the sun's magnetic cycle modulates the irradiance and luminosity. We must probe the sun's deep interior by observing tiny changes in the solar surface asphericity. SPHERIS achieves these goals by measuring the solar brightness, shape and radius properties with unprecedented precision: surface temperature variations of 0.1K, shape oscillations which affect the limb at the level of 1 microarcsecond, and radius changes of smaller than a milliarcsecond. Over a three year mission lifetime SPHERIS will uncover the causes of the solar irradiance changes. With a physical understanding of these effects we will finally be able to answer the question, "can the sun's luminosity change by enough to affect the Earth's climate over the next few years, or over historical timescales?"


  • Broadband and line irradiance variations at few ppm
  • Solar Radius changes at less than 1 milliarcsec
  • Solar Shape at less than 0.1 milliarcsec
  • Solar Temperature changes at less than 0.1K
  • Low Frequency Oscillations at 1 microarcsecond
  • Astrometric and Photometric Telescope (APT) mass and power: 33 kg, 39 W
  • VIRGO Flight Spare mass and power: 15.8 kg, 13.3 W

University of Hawaii
Institute for Astronomy
Naval Research Lab
World Radiation Center

For more information contact
J. R. Kuhn

SPHERIS uses a novel astrometric and photometric telescope (APT) and a VIRGO flight spare integrated irradiance instrument. APT is a monolithic fused quartz telescope (following the heritage of the Gravity Probe B astrometric star tracker).