Last week we saw how it makes sense to use angles and arcs to identify the position of places on the spherical surface of the Earth. We arrived at a grid of great circles, and the intersections between north-south and east-west circles define two coordinates for each point: a longitude (east or west) and a latitude (north or south). We also saw that two special circles mark the origin of each of the coordinates: the equator marks the 0° line for latitudes, and the prime meridian marks the 0° line for longitudes. The equator is physically defined as being equidistant from both poles, while the prime meridian is arbitrarily selected.
Well, as seen from Earth, the sky looks like a very big sphere with the Earth at its centre. Because of this, it makes sense to use a similar system of coordinates to identify the positions of celestial objects. Note that I'm talking strictly about the apparent position of the objects on the 2D "internal surface" of the sky as seen from Earth — the universe is a three-dimensional place, and you won't be able to use positions represented in this way to, say, chart a navigation course between any of these objects. Of course, if you are in a position in which you may be asked to chart such a course, I expect you'd already know this.
We start by extending some of the terrestrial features onto the sky. For example, the celestial poles are simply "above" the Earth poles: you extend the rotational axis of the Earth upwards until it "touches" the celestial sphere, and that's the celestial pole. And it turns out that these are important points in the sky as well as on the Earth: if you stay out one night and look at the movement of the stars as the night progresses, they all seem to rotate around the celestial poles.
Similarly, the celestial equator is simply a projection of our own familiar line of latitude zero onto the sky; it is a great circle on the sky that is equidistant from both poles. And, having poles and an equator, we can easily get the an equivalent to latitudes; celestial latitudes are called "declinations" and are represented in the same way as latitudes on Earth (as degrees north or south of the equator, from 0° at the equator to 90° at the poles).
Now, the celestial equivalent to longitudes is a bit more complicated. You see, the sky "moves"; that is, as seen from the ground, the sky seems to rotate around the Earth, from the east to the west, at approximately one revolution per day. This means that we can't simply project the terrestrial prime meridian onto the sky to get the celestial prime meridian: the portion of the sky onto which the projection would happen would change by the minute and the coordinates would be useless. We need to do something different.
Let's start talking about the idea of "local sky". From any given point on Earth, at any given moment an observer can see half of the sky (assuming a clear night and an open horizon); which half is something that will change as the Earth rotates. On this view of the sky, we need to define two important imaginary points. One is "straight up", or the point directly overhead (from the point of view of the observer); this we call "zenith". The other is in the opposite direction, "straight down", or the point directly below the observer's feet; this we call "nadir" (if you're standing at the South Pole, the zenith is the south celestial pole and the nadir is the north celestial pole).
Now, the portion of the sky you can see (that is, your local sky) will change continuously, and so will the celestial positions of your zenith and nadir (unless you happen to be at the poles). More exactly, the declination (celestial latitude) of your zenith and nadir will remain static (and will be identical to your latitude on Earth), while the "celestial longitude" will change. But, importantly, this means that what is in your local sky will depend not only on where you are, but also on when you are there - and not just on the time of day, but also on the time of year.
How do we define the celestial longitudes, then? They are, indeed, similar to the terrestrial ones, but we need to define an origin based on celestial features. The origin that was chosen for this coordinate is the line where the sun is at the time of the vernal equinox (spring equinox in the north hemisphere, autumn in the south); this is a fixed location on the sky.
There is one more difference, though: we don't use degrees for this coordinate, we use time. The sky was divided into 24 north-south bands, each one of which takes approximately one hour to move a distance equal to its width across the sky; these bands are then the second basis for the grid system defining celestial coordinates, and the coordinate they define is called "right ascension". Each band is numbered as 1 hour and, predictably, divided into 60 minutes of 60 seconds each; it is similar to degrees, but the notation uses "h", "m" and "s" rather than the symbols for degrees, arcminutes and arcseconds. Also, the hours were numbered from the origin increasing continuously towards the east. There is no east or west right ascension, but a single coordinate that ranges from 0h0m0s to 23h59m59s.
Together, declination and right ascension uniquely define a position in the sky, and can be used to locate any object visible from Earth. They don't tell you directly where to look at, though: they need to be translated according to your local position and to the time of day and year into an actual direction towards which you can look at to find the object being pointed to. But this is something that we'll look into a bit later, as we need more background to work that out.
Next week, we'll start looking into the way the Earth moves and what they mean for our view of the sky.
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