# Why does the latitude of an observer affect the hour angle of a body in Stellarium?

I am using Stellarium for astronomical data to help in studying celestial navigation. While trying to track down a source of errors in my sextant reading or sight reductions I noticed that the local hour angle (shown HA on Stellarium's ephemera) of Venus varied if I changed the latitude of my location whilst keeping time constant.

Can anyone explain this. I thought the HA would only be a function of time, longitude, and the position of Venus (RA, Dec). Why would my moving north along my meridian affect the HA of a body? Aren't both my meridian and Venus' fixed (time is stopped), so the angle between them wouldn't change?

I noticed that it changed systematically. For the date and meridian in question (16/5/2020; 21:30:00 0° Long.) it varied from 6h 47m 41.86s at both the N and S poles, to 6h 47m 43.78s at the Equator. delta=1.92s

I know it's only a tiny amount, but I would like to understand what the principle behind it is. Everything I found on Google seemed to suggest it shouldn't change. I wondered if it was something to do with parallax changing as I changed position.

Thanks for any help.

• If you literally tried 90 degrees latitude, the hour angle is undefined in reality, but perhaps it can be calculated mathematically. To avoid a possible issue, what are the results at 80 latitude? – JohnHoltz May 17 at 15:33
• Hi, thanks for answering. At 80° both North and South HA = 6h 47m 42.19s – richTourist May 17 at 15:41
• You may be correct that the calculation is using the topocentric position (parallax) instead of the geocentric position. However, Venus is currently near 27 degrees declination, so I would expect a slight difference between N and S pole. I would need to pull out a book to confirm that. Someone may provide the "real" answer before I get to that. – JohnHoltz May 17 at 15:52
• I can't see why it should make a difference, though. I thought the hour angle was measured between two meridians, not from the observer's point of view. wikipedia: The hour angle of a point is the angle between two planes: one containing Earth's axis and the zenith (the meridian plane), and the other containing Earth's axis and the given point (the hour circle passing through the point). – richTourist May 17 at 16:09
• The hour angle is the difference between two right ascension, and the right ascension of Venus changes based on your location due to parallax. The difference would be small, but 2 seconds HA given by Stellarium is also small! – JohnHoltz May 17 at 17:11

Date__(UT)__HR:MN R.A.__(a-apparent)__DEC 2020-May-17 01:00 05 22 07.26 +27 00 24.9 Equator 2020-May-17 01:00 05 22 09.23 +27 00 01.6 North Pole 2020-May-17 01:00 05 22 09.23 +27 00 48.4 South Pole