Shadow Sticks and Sundials

interpret simple shadow stick data to determine local noon and observerís longitude
describe how a sundial can be used to determine time


A Simple Sundial

Put a stick upright in the ground. Congratulations, you have made a very crude sundial. The shadow cast by the stick will move throughout the course of the day just as the hour hand of a clock moves. Every hour make a mark where the shadow is and you have a sun clock. This will give you a rough idea of what time of day it is but probably not a very accurate one. Why not?

What you can find with a shadow stick is the the time at which you local noon occurs on a particular day. This is the time when the sun is at its highest in the sky, i.e. it culminates. When this happens the shadow made by the stick will be at its shortest.

There are two reasons why there will be a difference between GMT, what your clock says, and your local noon.

1. We must take into account the Equation of Time. This will be explained in depth on the next page.

2. Your longitude will affect what the sundial reads and so needs to be corrected for.

Longitude Correction

Because the sun rises in the east and sets in the west people who live east of Greenwich get the sun earlier and people west of Greenwich get it later. In fact for every degree east you are your local noon will be 4 minutes sooner and for every day west it will be 4 minutes later. (If you divide the number of minutes in a day by 360 you get ...?) You should see then that by measuring the difference you can calculate your longitude. E.g. local noon in Truro happens 20 minutes after it does in Greenwich. This is because its longitude is 5 degrees West.

Remember - for every degree East of Greenwich you are your local noon happens 4 minutes sooner.


Why aren't my hour markers evenly spaced?

The position of the shadow not only depends on your longitude but also on your latitude as you should see from the diagrams below.

If you want your hour markers to be evenly spaced then the stick which makes the shadow, the gnomon, needs to be parallel to the Earth's axis. The solution is to mount the stick at an angle. It should lean to the north at an angle equal to the latitude of the location where the sundial is. The gnomon should then be parallel to the Earth's axis.

The sundial above is called an equatorial sundial where the plate on which the shadow falls is at right angles to the gnomon and therefore is on a plane parallel to the equator.. There are several other types.

The lines on the base of a horizontal sundial, e.g. the one above, are worked out using a complicated mathematical formula. There are plenty of places on the internet where you can download a template.

Have a look at www.sundials.co.uk for lots of different designs.

    
Images with kind permission from www.sundials.co.uk

Bear in mind that your sundial is telling you your local solar time. Unless you live on the same longitude as Greenwich this will be slightly different to what your watch says. If you were to set up a sundial in your garden you should take your longitude into account.

Important - to find North (the direction the gnomon points) we do not use a compass. This would tell us magnetic North which is different to true North. The best way to find true North accurately is by using Polaris.

Also bear in mind that to get an accurate measurement of what time it is we need to take into account the equation of time.