Two weeks ago we described a set of coordinates we use to map the position of the objects we see on the sky; at the end of that article, I mentioned that the fact that the sky is not static (as seen from the Earth) affects the way we map positions in those coordinates to positions in the visible sky. In today's article we'll start to see exactly how the objects on the sky move and why they do so.
To a first approximation, most of the objects on the sky appear to move as if they were all fixed to the inside of a gigantic sphere with the Earth at its centre (the aptly named "celestial sphere"); the notable exceptions are the planets, the Sun, the Moon and a few other sporadic visitors such as comets and asteroids. For today we'll ignore those, though, and focus first on the simpler cases.
That (apparent) movement is the result of the (very real) movement of the Earth in relation to the rest of the universe; one can, of course, take the " geocentric" view and talk about celestial objects moving, if that makes things easier to understand — as long as it remains clear that the movement we see directly is apparent, not real.
First, some introduction. From any given point on the surface of the Earth, it's possible to see the "projection" of the Earth's pole onto the celestial sphere (as described in the last article); the point where that projection falls is the celestial pole (north or south, depending on which hemisphere you happen to be on). If you measure the angle between the celestial pole and the horizon, you'll find out that it's identical to your latitude. In other words: if you're in Melbourne, at a latitude of approximately 37 degrees south, the south celestial pole will be approximately 37 degrees above the southern horizon.
Similarly, from any given point on Earth you can see the projection of the equator onto the celestial sphere: that's, obviously, the celestial equator. It will describe a great circle across your local sky, tilted with relation to your horizon by an angle that equals 90° minus your latitude (because the equator is perpendicular to the poles); from Melbourne, the celestial equator is tilted by 53° with relation to the horizon, and the highest point in the line of the equator is 53° above the northern horizon.
As we all know, the Earth rotates around its axis, towards the east, once every approximately 24 hours. The result of this movement is that the objects we see on the sky will seem to rise from the eastern horizon, move across the sky and set on the western horizon some time later. This apparent movement is actually an "orbit" around the celestial poles, parallel to the celestial equator, and this has some interesting effects.
The first is that the actual path of an object across the sky will depend on your local latitude. If you're standing exactly on the equator, objects will rise straight up from the eastern horizon and set straight down on the west. If you happen to be on one of the poles, on the other hand, objects will not rise or set: whatever objects are visible will remain visible, circling the horizon always at the same distance from it. At any other latitude, objects will describe a path that is tilted with relation to the horizon by 90° minus your latitude — that is, tilted in the same way as the equator (and this is useful in navigation: measuring the angle of the path of stars equates to measuring your latitude). The picture above shows this movement from a mid-latitude, with the celestial equator running very visibly through the middle of the image (I'll leave as an exercise to the reader to determine where the picture was taken from the angle between the equator and the horizon).
The second effect is that, if the declination of a star is larger than 90° minus your local latitude, that star will never set. Why is that? Well, if the declination of the star is larger than 90° minus your local latitude, the distance in degrees between that star and the nearest pole is less than your local latitude, and thus less than the distance between the pole and the horizon, and that means that the whole path of the star across the sky is visible. The star is then said to be a circumpolar star from your location (being on the pole, at latitude 90°, is a special case of this — every star visible from the pole is circumpolar). The picture on the left, a long exposure image showing the movement of the stars over several hours, shows this and the location of the southern celestial pole very clearly (photo taken in Brunswick by Michael Efford - click for larger version and more info).
There is, of course, the opposite case: from any location but the equator, some stars will be permanently below the horizon. They are, say, anti-circumpolar: they circle the opposite pole to the one that is visible. Going again to our observer in Melbourne, the constellation Crux, at a declination around 60° south, will be always visible on the sky, at every hour of every day, while Ursa Major, at around 55° north, will never be visible.
And that's it for today. Next week, we'll start looking into the slightly more complex movements that result from the Earth orbiting the Sun.