# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: Planet rising**

**From:**R.H. van Gent

**Date:**2002 Feb 2, 00:25 +0100

"daveweilacher---.net" wrote: > So. How do you compute the time a planet rises. The same way as you do for the Sun (note that this depends both on your latitude and on the declination of the Sun or planet). The correction for the semi-diameter you can neglect, only the correction for refraction matters. This is about 34 minutes of arc, so you determine when its zenith distance is 90 degrees and 34 minutes. > I've got the meridian passage of the planet from the daily pages of the nautical almanac. Good. > > It's true rise should occur at 90 degrees before then. Arc to time has this convert to 6 hours earlier. Only when the planet's declination is near to 0 degrees (i.e. is near the celestial equator. Depending on the planet's declination and your latitude this interval could be significantly smaller or larger than 6 hours. The time difference between planet rise to meridian passage (H) is given by the relation: cos H = (cos 90� 34' - sin decl * sin glat)/(cos decl * cos glat) Convert the angular measure H into a time interval (hours) by dividing it by 15. When you neglect the correction for refraction, the equation simplifies to: cos H = - tan decl * tan glat > Its apparent rise should have some factor for refraction. (by way of example, the sun has 14 minutes in its 50' adjustment). See above. The 50 arc minute correction for the Sun is the sum of 16 arc minutes for the semi-diameter and 34 arc minutes for the refraction. Regards, ======================================================= * Robert H. van Gent * * E-mail: r.h.vangent{at}astro.uu.nl * * Homepage: http://www.phys.uu.nl/~vgent/homepage.htm * =======================================================