The Sun Shadow Board

The Vikings used a sun shadow board to find latitude. It was nothing more than a small wooden disc with a number of concentric circles representing different latitudes and a gnomon whose height could be adjusted for the time of year.

At noon the sun shadow board would be placed in a tub of water with the gnomon set to the correct date. If the shadow it cast at noon was beyond the desired latitude they were too far north, and if it did not reach it, then they were too far south.

To make your shadow astrolabe:

SUSPENSION POINT Fix in centre so astrolabe hangs level.

To make your shadow astrolabe:

1. Photocopy, laminate and stick shadow astrolabe card onto a piece of plywood. Alternatively draw a circle on a piece of plywood and mark it out in degrees. Note 00 is at 3 o'clock, on the horizon.

2. Drill three holes:

a One immediately above 12 o'clock to take the loop from which to suspend the astrolabe. b One in the centre for the shadow pin. c One immediately below 6 o'clock to take the weight that will hold it vertical with the 0-180° line horizontal and steady.

3. Attach the loop and the weight.

To use your shadow astrolabe:

1. Suspend it by the loop from some convenient and steady support. On a small boat at sea such a place is not easily found. The idea is that when you take the reading the astrolabe is horizontal. It might help to attach to the astrolabe a spirit level bubble of the type found in any hardware store, giving you a means of knowing when it is horizontal. If pushed, you could then hold it horizontal for an istant while you take a reading.

2. Turn the astrolabe so that the edge faces the sun.

3. Read the altitude where the shadow peg cuts the circle.

180 i

LEVEL Take one from rake one from A \ a ^

GNOMOI Make as thin as possible

You can add a weight here to help hold the astrolabe steady

13.2 Making and Using a Shadow Astrolabe

13.2b Template for Sun Shadow Board

Sun Shadow Board Navigation

A sun shadow board is difficult to make and there are other, easier, more accurate ways of checking latitude.

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