0
Visit Your Local PBS Station PBS Home PBS Home Programs A-Z TV Schedules Watch Video Donate Shop PBS Search PBS
0
Rough Science
Explore the Challenges
Solve the Web Challenge
Meet the Rough Scientists
About the TV Series
Discover More
Feedback
Site Map
Explore the Challenges

The Challenge: Build accurate clocks



Another solution is the equinoctial, or equatorial, sundial. The hour marks are drawn on a circular dial which is not flat on the floor but at an angle to the ground. This angle varies depending on where you are in the world but always results in the dial being parallel to the equator. The gnomon points at the North star, which means that it is at right angles to the dial.

Equinoctial Sundial

This type of sundial is very easy to set up. Once the gnomon is pointed at the North star and the dial is constructed at right angles to the gnomon, the sundial is set up in the right direction and elevation to give us equal gaps between the hour marks. The shadow falls on the lowest point of the dial at noon and will be a quarter of a turn counter-clockwise from the bottom at 6am and a quarter turn clockwise from the bottom at 6pm. We decided to make a big equinoctial sundial, because the bigger it is, the more accurate it will be. On top of that, the only suitable metal rings knocking around on the island were very big! Of course, experts will know that there are still small inaccuracies in measurement. Local Apparent Time is usually slightly different from Mean Solar Time because of the path that the earth uses to orbit the sun. These inaccuracies can be corrected by using the Equation of Time.