Using The Stars For Direction, Latitude, And Time

Using The Stars For Direction, Latitude, And Time
Image Copyright: Fort Worth Astronomical Society at www.fortworthastro.com

In the Northern Hemisphere, Polaris (The North Star) is used to determine the direction of north, as well as our position on the Earth’s surface measured along a line running north to south called latitude.

Locating Polaris

Polaris is located in the constellation of Ursa Minor which lies just opposite the The Plough (Big Dipper) in the constellation of Ursa Major. A line drawn from the pointer stars (Dubhe and Merak) in the Big Dipper and five times their distance from each other will then lead to Polaris.

Determining North

Polaris sits almost exactly over the North Pole and is readily located as unlike the other stars it does not appear to move in the night sky. Facing it will mean you are looking in a northward direction and by extending your arms to the side, your left arm will point west, your right arm east, while your back will point south.

Latitude

Polaris appears directly overhead at the North Pole (90 degrees) but only slightly above the horizon at the Equator (0 degree). Therefore by measuring Polaris’ height above the horizon we are able to determine our Latitude. For example, if you lived in New York Polaris would appear due north and 40 degrees above the horizon.

Measuring Latitude

A rough method to measure this angle is by extending your hand and spreading your fingers which should then cover about 20 degrees of the sky between the tip of your little finger and thumb. Alternatively, you could use your outstretched fist which would cover around 10 degrees of sky.

Using The Stars For Direction, Latitude, And TimeTime (Sky-Clock)

Interestingly, Ursa Major along with neighboring Cassiopeia never set below the horizon and completes a whole counterclockwise (east to west) rotation around Polaris every 24 hours. Therefore, pointer star Merak for example, would revolve all the way around Polaris and return to its original place in a 24 hour period.

Now imagine Polaris as the centre of that 24 hour clock (1hr to 24hrs) and a line drawn from Polaris to Merak as an hour hand with each 15 degree rotation by Merak equaling 1 hour of time passing (360 degrees/24hrs).

To tell when say four hours have elapsed draw a new hour line running 60 degrees counterclockwise from your original hour hand and note where it would intercept the horizon. When the line eventually reaches that point four hours have elapsed.

Note: this method is useful for simple purposes as many more complex factors would be required for long term accurate calculations. For a fuller explanation read here.