What times of year you can see Canis Major in Galloway?
Winter. Galloway is about 55° north, so can see Canis Major which is about 25° south when it’s on the southern meridian. Best time is winter when the sun is opposite. CM has Right Ascension of about 7h, so the sun should ideally be about 19h. This is the case in early January, and visibility of the constellation should be a couple of months either side of this. Edit: Late Spring – Sirius should still be visible in the early evening until mid-April, but the rest will be below the horizon. You could try using Stellarium (free!) to check it out.
