Sundial Simulator
An animated diagram showing how a sundial's gnomon casts a moving shadow to mark the hours, with the shadow rotating live to approximately the current time.
About this tool
A sundial tells the time using a gnomon, the raised blade in the middle, which casts a shadow onto a marked dial face. As the Earth turns, the shadow sweeps round the dial at 15° of the sun's movement per hour. On a flat (horizontal) dial the hour lines aren't evenly spaced: the angle of each line from the noon line follows tan(angle) = sin(latitude) × tan(hour angle). That's why a dial has to be made for the latitude where it stands, and why this simulator redraws its hour lines when you move the latitude slider.
A sundial shows apparent solar time, which differs from your clock for three reasons, each worked out and listed under the controls. Summer time puts clocks an hour ahead of the sun. Your longitude matters too, because a time zone covers a wide strip of the Earth and solar noon in Plymouth comes about 16 minutes after London's. Finally there's the equation of time: because the Earth's orbit is an ellipse and its axis is tilted, the sun runs up to about 16 minutes fast in early November and 14 minutes slow in mid-February. Enter your longitude (west is negative) to see the dial for where you are.
- Why doesn't the sundial agree with my watch?
- It isn't meant to: a real, perfectly built sundial would show the same difference. The line under the controls breaks it down into summer time, your longitude and today's equation of time.
- What does the latitude slider do?
- It sets where the dial is built for. Near the equator the hour lines are spread almost evenly; further north they crowd round noon, and on long summer days early-morning and late-evening hours appear, because the sun is up for longer.
- Does it work in the southern hemisphere?
- The slider covers the northern hemisphere, where the shadow moves clockwise. South of the equator a sundial's shadow moves anticlockwise and the gnomon points south instead.