I live at about 53 degrees north. If I stand facing north the circumpolar stars trace an arc around the Celestial north pole over the course of the night. There are lots of long-exposure images online that purport to show the stars tracing complete concentric circles over a 24-hour period.
If I turn around I see the more southerly stars (such as Sirius) rising in the east and reaching a zenith in the southern sky before descending to set in the west. Their apparent motion is “convex” relative to the north pole. Is this difference real or is it an illusion? If the long-exposure image had a wide enough angle to include Sirius, would it also trace a circle around the north pole?
If the difference is real, where is the boundary? Is it the equator? Do all stars with a positive declination appear to orbit the Pole Star?
It’s an optical illusion. I suspect it looks that way because the sky looks “flatter” than it is. That is, you don’t think of the sky as being a hemisphere on which the stars are moving. (Of course they aren’t, but that is the simplest way to describe their movement.)
And yes in the Northern Hemisphere all stars in the northern celestial hemisphere trace circles around the north pole.
And all the stars in the southern celestial hemisphere circle around the South Pole, which is below your horizon. In a long exposure, star-streak photo, stars on the celestial equator will appear as a straight line.
You can equally well say that they all circle the north pole, or all circle the south pole, or all circle around the axis that passes through both poles. If you look at, say, Sirius at any given moment, it’s at an angle of 107º away from the North Celestial Pole. Look at any other moment, it’s still at 107º away from the NCP. It’s always the same distance from the pole, and the set of all points an equal distance from a single point is, by definition, a circle.
For comparison, take a globe, and pick any point on it. Spin the globe. It’s making a circle. Is it a circle around the north pole, or the south? Either: It’s the same circle no matter how you describe it.
Yeah - this I think is the same reason that the night terminator shadow on a half moon does not appear to be perpendicular to a line between the moon and the setting sun - it doesn’t work if you draw a straight line, but that’s because the sky isn’t straight (that is to say, if there was a long object, pulled perfectly taut and laser-straight, in the sky, we would probably not perceive it as straight because of perspective)
Or, as a (literally) more down-to-earth example - if you stand on a train track (make sure no train is arriving) and look one way down the track, you see the parallel tracks converging in the far distance. If you turn 180 degrees, you see them converging in the opposite distance - so they are diverging as they approach you in both directions - that should mean you can see them curve from divergence to convergence, around you - but you can’t perceive them as anything other than straight lines.
Thanks everyone. I think OldGuy’s explanation is correct. It’s a very powerful illusion - it’s very difficult to look at the path of the southern stars through the sky and believe they are circling a point behind you. I agree that it’s probably related to the moon terminator illusion but I don’t have a good intuitive understanding of that either.
beowulff, that’s a great photo that clearly illustrates the phenomenon, thank you.
