Calculate direction to align with an astronomical body on the horizon

[d3x0r]

Given a day of year, a latitude and longitude, I'd like to know the direction on the ground to align with a astronomical body with a given declination and right ascension that will appear either on the morning horizon or evening horizon (or both). I don't really care what time of day the alignment happens, so probably the day of the year doesn't matter.

This is probably something simple I should be able to compute for myself, but I'm drawing blanks.

Something like the above image - at some point on the earth (lat/long), I'd like to know the direction to align to a specific constellation (for example Cetus at +10Dec, 0:30RA , or Virgo at -4 Dec, 13 RA ), I'd like to know the compass direction to align either the red or green line shown from the point. Basically those stellar points need to be a tangent to the earth's sphere at the point....

[gzotti]

You can use the ArchaeoLines plugin to visualize an arbitrary declination line that intersects with the horizon. Or set a marker at your declination and find rise or set times, set this time and read azimuth. Else, outside Stellarium, you need to work out your problem in spherical trigonometry.

[d3x0r]

I'm not sure how to set a marker at a specific position - or add a selectable point.
Adding the extra lines from ArchaeoLines gave me a better idea - looks like it ends up being a simple calculation in this case - since the objects are near the equinox, the angles to them on the horizon don't change very much for time of year or position on the earth.

[gzotti]

Methinks thou shalt read ye olde User Guide... at least the introduction chapter. With Section 3.6.