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The Sun Dial The sundial is known as an analemmatic Analemmatic sundials consist of hour points, (rather than lines) laid around an ellipse. They have a movable gnomon (This is the name given to the object that casts the shadow, which in this case is yourself!) perpendicular to the dial plane (the flat surface on which you stand). Remember that in summer the sun is high in the sky, so your shadow may not cut the ellipse. Imagine the point where it would cut the ellipse if it were extended. In winter the sun is low in the sky, so the shadow will be long. If you come at different times see how the length of your shadow changes during the day. We do not know the name of the inventor of this type of sundial, but the French astronomer Lalande explained the theory behind the analammatic sundial in 1757. The sundial is situated at 54.05 North, 2.78 West. Calibrated and laid out by Peter Ransom, teacher of Mathematics, Romsey. |
How it works To tell the time, stand upright with your head above the date on the scale. Estimate the time of your shadow from the hour plaques. The sundial has been adjusted for British Summer Time and the longitude. You need to - correct it for the time of year and - subtract 1 hour if it is not British Summer Time. The first adjustment is known as the 'equation of time'. This correction is shown on the central scale. For example, on 1st July you need to add 4 minutes onto your shadow time to get the time your watch shows. For other days you may want to use the 'equation of time' in the graph form (displayed next to the Sundial). This shows the adjustment needed for any day of the year. If your shadow shows 2:30p.m. on September 15th the true time is 2:30-0:05 The graph shows suntime to be fast on September 15th.). This works out to be 2:25p.m. If your shadow shows 2:30p.m. on January 15th the true time is 2:30 - 1:00 (it's not BST) + 0.10 (The graph shows suntime to be 10 minutes slow on January 15th.). This works out be 1:40p.m. |