Mapping the Sky
When you look up at the night sky it appears as though the stars are studded on a giant invisible celestial sphere, a clear rounded canopy sitting overhead. Indeed for most of human history this is what most people believed. There were a few dissenters who suggested that the stars were strewn amongst a three dimensional cosmos, but not until after Copernicus and Galileo was our model of a celestial sphere overturned.
However this model is often still a useful one, especially when drawing maps of the heavens.
Just as locations on the Earth’s surface have two co-ordinates (longitude and latitude) so too do stars in the sky. When refering to a star’s location in this way we still imagine the sky to be a celestial sphere, a useful illusion which makes our mapping easier.
Altitude and Azimuth (Alt-Az)
One way of defining the position of astronomical objects is with respect to the horizon: how high above the horizon is the object, and in which direction (i.e. N, S, E or W). This co-ordinate system is called Alt-Az, standing from Altitude and Azimuth.
This is perhaps the most intuitive way of defining a star’s position in the sky but because it is relative to a “fixed” Earth, as the sky appears to move over the course of a night a star’s alt-az co-ordinates change, as the star rises and traverses the sky from east to west. This is obviously a problem, as we want a star atlas to contain two fixed co-ordinates for an object, not an ever-changing set of them.
Right Ascension and Declination (RA & Dec)
Enter RA and Dec. This co-ordinate system fixes the changing positions of stars in the sky relative to the changing sky itself. Imagine the celestial sphere surrounding the Earth, with stars pricked as dots on its inner surface. Image also on this sphere a grid of lines drawn, much like the longitude and latitude grid on a globe of the Earth.
In fact, let’s align part our celestial grid with the grid lines on the globe.
Imagine the extension of the Earth’s equator out into space. Where this cirlce intersects the celestial sphere, we have the celestial equator. This imaginary line in space is fixed relative to the stars, just in the same way that the north pole of the sky is fixed at Polaris, the North Star. The celestial equator passes through Orion the Hunter, bisecting him through his belt.
We now have one fixed line on our celestial sphere and can define a star’s distance above or below that line, analogous to the latitude co-ordinate on Earth. This celestial co-ordinate we call Declination (Dec). A star’s dec will not change as the Earth spins and the stars appear to rise and set, because the celestial equator is moving with the stars*.
That’s only half the story though: what about the other co-ordinate, the analogy to longitude on our globe? The compliment to Dec is Right Ascension (RA).
Longitude on Earth is defined relative to an arbitrary line, the Greenwhich meridian. A location on the Earth is defined as being east or west of that line. Because the Earth spins once a day, and there are 24 hours in a day, for every 15 degrees you travel west of the Greenwhich meridian the Sun rises 1 hour later, giving us our time zones.
So we can arbitrarily designate a zero meridian for RA too. Indeed astronomers have chosen the zero line of RA to pass through the rather delightfully named First Point of Ares, the point at which the celestial equator crosses the ecliptic. All star positions are measured east of this line, and remain fixed as this line too moves with the stars*.
RA and Dec are the co-ordinates of the Equatorial Co-ordinate System, so called since the positions are relative to the celestial equator.
Much as with our own longitude and latitude system, in the Equatorial Co-ordinate System RA is measured in hours (where 1 hour = 15 degrees) and dec in degrees.
Example: Sirius, the Dog Star
Sirius is the brightest star in the night sky, with a magnitide of -1.46, and makes up part of the constellation of Canis Major, the big dog.
Its Equatorial Co-ordinates are RA: 06h 45m 09s and Dec: -16° 42′ 58″
What does this tell you? Let’s look at Dec first. It is only 16 or 17 degrees south (hence the negative sign) of the celestial equator. In fact this should be pretty obvious if you remember that the celestial equator runs through Orion’s Belt, and Sirius is just below and to the left of Orion.
And what does the RA tell you? Sirius is 6 and a quarter hours, or 100° (a bit more than a quarter of the way round the sky) from the First Point of Ares where the Sun sits on the Vernal, or Spring, Equinox (20 or 21 March). The Sun appears to move along the ecliptic once a year (as the Earth orbits the Sun), and so around a quarter of a year after the Spring Equinox the Sun with have an RA of 06h and so will be very near Sirius in the sky. This means that Sirius is only visible in winter months.
If you are observing Sirius due south at midnight, then you know that the star Regulus in Leo, with an RA of around 10h will be due south at 3am (or more exactly 3:23am, as Regulus’ RA is 10h08m, 3h23m greater than Sirius’). Regulus’ Dec is just under +12°, so it’ll be as high above Orion’s belt as Sirius is beneath it, give or take.
* the stars of course don’t move, it’s the Earth that’s spinning, creating the illusion of a moving sky, but it’s a useful illusion in this case.