How to Find Your Latitude at Night

With some simple math is it possible to measure your latitude directly at night time.

Full instructions are available at the EAAE Astronomy On-Line final event 1996

During 24 hours, the sky above us performs one full rotation.

If you record (by drawing or photographing) the northern sky several times during one night, you will see something like this :

[GIF, 29k]

The constellations Cassiopeia (that looks like an "M" or a "W"), the Big Dipper, and the Polar Star "P".

Note that nearly all stars appear to move due to the rotation of the Earth. Only one star seems to be fixed - the Polar Star "P" ; it is also known as the "North Star".

The Polar Star - the only northern star which stays fixed in the sky during all night hours - has a constant altitude above the horizon.

By a bit of simple math, it is possible to show that this altitude is very nearly equal to your own geographical latitude. For instance, if you live at latitude 50° North, then the North Star will be placed at an altitude of 50° above your horizon.

Exercises:

1. Imagine that you stand on the geographical north pole. Where in the sky would you see the Polar Star?

2. Measure your own geographical latitude by means of the Polar Star. This may be done by means of simple homemade equipment, for instance a Jacobean Cross Staff (a quadrant) or an oriental Kamal as shown on this Portuguese stamp.

3. Repeat the old Bishop Nicholas "Right Hand Latitude method" that was described in EAAE Astronomy On-Line final event 1996.

4. Is there any polar star in the southern sky? (Check a star map).