Don't forget that the surface of the sun is not perfectly uniform: if you have a recent picture of the surface of the sun and sensitive enough instruments, you

Don't forget that the surface of the sun is not perfectly uniform: if you have a recent picture of the surface of the sun and sensitive enough instruments, you can effectively treat it as a "cluster of known celestial objects" (e.g. sunspots, solar flares) half a degree apart from each other. Also the moon is sometimes visible during the day, and since the topography of the moon stays constant over time that trick will work with the moon (with craters etc) even without a recent picture.