Latitude and Longitude

We can say where a place on Earth is by giving its latitude and its longitude. With these two numbers we can pinpoint any location on the surface of the Earth.

( Because the Earth is rotating, however, the position of stars relative to Earth changes with time. We use a different system to locate stars as we shall see later. )

Latitude

The latitude of a location is how many degrees above (N) or below (S) of the equator it is.

The equator is at latitude 0, the North pole at latitude 90⁰ and places between are at a latitude between 0 and 90 N.

Imagine you were at the North pole. The pole star would be directly above your head (at the zenith). If you then walked south it would be lower in the sky the further south you went. In fact by measuring its declination (the angle it makes with the horizon) this will give you your own latitude.

In Middlesbrough the pole star is 55⁰ above the horizon. One way that sailors could figure out how far north they are is by measuring the position of the pole star.

       This is an instrument called an astrolabe. Christopher Columbus may have used one of these.

Describe how you would go about using the instrument above to find your latitude.

Where would the pole star be if you were at a point on the equator?

How do people in the southern hemisphere find which way is south?

Longitude

The longitude of a location is how many degrees east or west of the prime meridian it is.

Imagine lots of vertical circles on the surface of the Earth. A circle like this which goes through both poles is called a meridian.

The equator is a natural zero for latitude but the zero for longitude could be anywhere. Greenwich in London was internationally agreed. The line that goes through Greenwich is called the prime meridian. Other places on Earth have longitudes between 0 and 180 E or W of this.

Why was Greenwich chosen as the international meridian?

pic   Ian Britton - Freefoto.com

You cannot get your longitude just by observing the stars. We shall see later that you can work out your longitude if you know the difference between what time it is where you are and what time it is in Greenwich.

For a sailor to know accurately what time it is in Greenwich he will need an accurate clock. Various astronomical methods were suggested (including observing Jupiter's moons which orbit Jupiter in a predictable way) but the problem was eventually solved by John Harrison who invented a very reliable watch.

Does the sun rise sooner or later in France than in England? Why?

Why would it be difficult for sailors to observe the moons of Jupiter?

Find a picture of John Harrison and his watch H4.

There is an excellent film called Longitude which tells the story of his work. Get it. There are a few good clips on YouTube.

What capital cities can be found at the following? (you need a map of the world or you could use Google Earth)