RADIAL-VELOCITY OBSERVATIONS OF THE SUN AT NIGHT

We have measured the Doppler shift of the solar spectrum for 5 yr to help demonstrate our ability to detect planets orbiting other stars. The sunlit surface of the Moon provides nighttime access to the solar spectrum integrated over the solar disk, as if the Sun were being observed at a stellar distance. The standard deviation of our 486 lunar observations spanning 4.9 yr is +/- 7.8 m s-1, consistent with our short-term internal precision and external calibration accuracy. Long-term periodic errors in the instrument are +/-4 m s-1. An artificial signal with a period of 2.5 yr and semiamplitude 8 m s-1 stands well above the noise periodogram of the instrument. Since calibration error and internal precision account for the scatter of the reduced data, there is little room left for long-period variations intrinsic to the Sun. The data series runs from 1987 April through 1992 February, during which daily sunspot counts increased an order of magnitude and are now well past the maximum of activity of Cycle 22. Nevertheless, our observations of the Moon indicate the Doppler shift of the integrated disk of the solar photosphere in violet absorption lines has varied less than +/-4 m s-1 over this interval. This suggests that the spectra of solar-type stars can be stable enough to reveal the Doppler shifts induced by planets with Jupiter's mass and orbital period.