Latitude, Longitude

Two Minute Astronomy, 4

In a previous episode, we came across the fact that the celestial sphere has not only poles and an equator, but a full-fledged coordinate system. Let’s explore this further and find out more about it!

A star chart marked with constellations as well as the celestial coordinate system. Image by DarkmoonArt_de from Pixabay

Before we begin, we need to learn some terminology that’ll come handy in future. The first two are zenith and nadir. Zenith refers to the point on the celestial sphere that you see when you look straight above, at any given time and place. Nadir is the diametrically opposite point; if somehow you could see straight down past the Earth below your feet, you’ll see the nadir. Another term is horizon, which is the ‘boundary’ between the ground and the sky. We can only see celestial bodies above the horizon (if there are no trees or buildings to obscure the view, that is).

The next two terms we’re going to see are very important if you want to describe a celestial object’s position in our sky, and they are called altitude and azimuth. Altitude is pretty straightforward: it is the ‘height’ above the horizon at which an object is present. Since usual measures of length are not very relevant while using the celestial sphere model, we instead use angular distance — degrees — here. The horizon is at 0°, while the zenith corresponds to 90° altitude. Azimuth is the direction in which the object is present, with due north being 0° and subsequent directions measured in clockwise (east: 90°, south: 180°, west: 270° and so on).

A pictorial representation of what we learnt so far (Image: Wikipedia Commons)

So coming back to the topic at hand, we do know that the Earth has its set of latitudes (parallel to the equator) and longitudes (passing through the poles). Imagine that the Earth somehow ‘projected’ them onto the celestial sphere, and voila, you now have the celestial coordinate system! Instead of the words ‘latitude’ and ‘longitude’ though, we use declination and right ascension respectively. So the celestial equator’s declination is 0°, and its two Poles are at +90° (plus because it is north of 0°) and -90° (minus because south). Right ascension is slightly trickier: since the celestial sphere completes one rotation in roughly 24 hours (of course it is the Earth which rotates and creates this illusion), the 360° rotation is divided into 24 equal parts of 15° each, and the zero line is designated 0h. It continues on to 1h, 2h, up until 23h (and 0h again). While rotating, the Earth crosses 15° (or one division) each hour. The diagram below will help in understanding this better.

This is a sky map generated via Stellarium for 10.8° N +5.30 GMT, with minimal stars and the horizon shown in green. The right ascensions are marked at the edges of the image, and you can see that since the local time is around 8 PM, the ‘topmost’ (the one that passes closest to the zenith at the shown point of time) right ascension is 20h. The concentric circles from smallest outward denote +85°, +80°, +75° and so on, with the Celestial North Pole being +90°.

Remember: altitude and azimuth depend on the time and place of observation, whereas right ascension and declination are fixed attributes of the celestial sphere!

We now have an idea of the celestial coordinate system. But how does the Earth’s rotation fit in with all this? What parts of the celestial sphere are visible at what times of the day, from a given place? We’ll explore the answers for such questions next week!