If I were to pick a subject of astronomy that confused the heck out of me, it would be coordinates.

There are so many systems to use, and then there’s the whole degrees to seconds thing. I mean come on!

So now that I have studied and learnt how different coordinate systems work, I thought I’d make a few blog posts explaining them in a way that would have considerably helped me when I was younger.

Let’s learn a few terms before we delve in!

We can create circles on the surface of a sphere. The circle where the centre is also the centre of the sphere is called the great circle. This is the biggest circle possible, and a good example is the equator.

If this circle does not pass through the centre, we call it a small circle. This could be any other line of latitude that’s not the equator.

The angle between 2 great circles (on the surface) is called the spherical angle.

Latitude And Longitude Lines

Latitude and Longitude are one of the more widely used coordinate systems we have.

Latitude tells you how far North and South a star is on the celestial sphere. At exactly 0° we have the celestial equator, a great circle that cuts the sphere in half. As we move above the celestial equator the lines of latitude become smaller and we start to move into positive values of latitude. For example, The Polaris (The North Star) is 90° N.

Rather than writing our latitude as °S, some organisations or programs will just write the latitude as negative, for example the Sigma Octans (The South Star) is -90° N or 90°S.

Longitude is a bit more difficult, as there is no real Westmost or Eastmost point on a sphere. So astronomers decided on making lines of longitude, which are identical great semi circles that pass through North and South. And to solve the issue of there being no West or Eastmost point, they made one of these lines 0° E and named it the Prime Meridian. This passes through Greenwich in England.

East is usually written as the positive, or you could write longitude as positive West eg. Satan’s Kingdom in Massachusetts is either 72°W or -72°E.

Parisians made their own Paris Meridian, due to some anger over Bonaparte’s exile I believe. Personally I think Paris is way cooler than Greenwich and would prefer the Prime Meridian going through there, but I can’t change the coordinate system (yet).

How To Write Latitude and Longitude

There are two ways to write latitude and three for longitude: degree decimal or sexagesimal notation or in terms of time.

Degree decimal is the nicest in my opinion. Both angles are written in degrees with decimals such as 52.623° W. It’s so simple!

Sexagesimal, requires a bit more work. Instead, we convert the decimals to arcminutes and arcseconds. To do this, we leave the integer part of the degree alone eg. for 52.623° we leave the 52 alone.

Multiply the decimal by 60, so in our example we would multiply 0.623 by 60 to get 37.38. The integer part of this value is the arcminute (written with a ‘ symbol). In our case this is 37’

Now take the decimal from the arcminute calculation and multiply it by 60. This gives us arcseconds which can keep the decimals and is written with a ” symbol. In this example we have 0.38 (from the 37.38) multiplied by 60 to get 22.8″

So out angle is now 52° 37′ 22.8″ W.

Longitude can also be written in terms of time. Since in 24 hours the Earth rotates 360°, then in 1 hour it sweeps 15°. So to convert to time, we take our degree decimal and divide it by 15, and then use the sexagesimal process above to to get hours, minutes and seconds.

With out example, we divide 52.623 by 15 to get 3.5082. Leaving the 3 alone, we multiply 0.5082 by 60 to get minutes (30.492) and then multiply 0.492 by 60 to get seconds (29.52). This gives us 3h 30m 29.52 s.

Hopefully I didn’t scare you from coordinates. They’re not something to worry about, but useful to understand on a basic level. Next time we’ll talk about Altazimuth and a few other systems!