Improving the Visual Magnitudes of the Planets in The Astronomical Almanac . I. Mercury and Venus

Abstract

Estimates for the apparent V magnitudes of the planets currently published in The Astronomical Almanac are based on phase coefficients, Delta m( i), presented by Harris along with values for V( 1,0) from de Vaucouleurs. Work is currently underway to update these values. The apparent V magnitudes of Mercury and Venus are examined here. This analysis provides new values for V( 1,0) and Delta m( i) derived from a variety of V photometric data sets for both Mercury and Venus. New data show that the previous value of V( 1,0) for Venus was approximately 0.10 mag too faint, because the small aperture used with photoelectric tubes not did not capture all of the light from Venus' relatively large disk. The Venus photometry also shows an abrupt and distinct ``tail'' beginning at a phase angle of about 160 degrees; that is, the curve abruptly changes direction somewhere between a phase angle of 160 degrees and 165 degrees and begins ascending. Circumstantial evidence suggests that this tail is caused by sunlight forward scattered through Venus' atmosphere. The rms scatter in the calculated magnitudes was found to be 0.10 mag for Mercury and 0.07 mag for Venus.