A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2014 Nov 19, 09:11 -0800
The only additional correction for Mars is parallax. Short answer for observations this month: add 0.1' to the observed altitude.
Long answer: for Mars, the horizon parallax can be estimated with very good accuracy by comparing the planet's brightness to other stars since the brightness of Mars is related to the distance from the Earth (unlike Venus, phase factors for Mars are small). If Mars is about first magnitude or fainter (as it is now), the parallax correction is 0.1 minutes of arc. If Mars is about as bright as Vega or Arcturus (as it was a couple of months ago), then the parallax is 0.2'. If Mars is about as bright as Sirius, then the correction is 0.3'. If it's as bright as Jupiter, then it's 0.4'. And on those rare occasions when Mars rivals Venus in brightness, its parallax correction can be 0.5'. The advantage of these rules based on brightness is that they are permanent, and you can determine the correction by direct observation without reference to any ephemeris data. If you don't like estimating the parallax from the brightness, the values are also tabulated in current editions of the Nautical Almanac. If you don't have a current Nautical Almanac, you can also get the distance to Mars in astronomical units or "AUs" from many different sources and divide 0.15' by that number. For example, current distance to Mars is 1.77 AU so the parallax correction is 0.08' or, nearly enough, a tenth of a minute of arc.
For current observations of Mars, based on any of the methods above, the horizon parallax of the planet is a mere 0.1'. The actual parallax in altitude is determined by multiplying the horizon value by the cosine of the altitude. It's important to remember that the navigational parallax for any body is opposite the refraction. Refraction lifts the stars and planets and other bodies, so it has to be subtracted from any altitude. Parallax lowers the Moon's observed altitude by as much as a degree, the Sun's altitude by as much 0.15', and the planets Venus and Mars by varying amounts from 0.1' to 0.6', and the correction for it has to be added onto the observed altitudes. For current evening observations, the altitude of Mars is low enough so that the cosine factor is effectively 1.0. The parallax correction is +0.1'.