Instructions for Reading our Equatorial Sundial
Understand how our Sundial is Set Up
- The gnomon is properly oriented to point toward the celestial pole (North Star in the Northern Hemisphere or South Celestial Pole in the Southern Hemisphere).
- The dial plate is tilted at an angle equal to your latitude and is aligned with the equatorial plane.
Understanding the Movement of the Shadow
- The shadow of the gnomon will change position as the Earth rotates throughout the day.
- The gnomon’s shadow will follow a circular arc on the dial plate, moving from west to east as the day progresses. The shadow will be longest in the morning and evening and shortest at solar noon.
Reading the Time
You’ll estimate the time based on the position of the shadow relative to a known reference (e.g., a physical object or a line you can mark). Here’s a method you can follow:
- Solar Noon (12:00 PM): At solar noon, the shadow will be at its shortest and point directly to the 12:00 position (due south in the Northern Hemisphere, due north in the Southern Hemisphere). This indicates the peak of the sun’s daily journey.
- Marking the Hour Intervals:
- Start by observing the shadow at solar noon, when it’s shortest, and mark this position as the 12:00 reference.
- Then, every hour, observe the shadow and note the position where it falls. You can estimate the time by comparing the shadow’s position to the solar noon mark, estimating roughly how far it has moved.
- For example, if the shadow is at a certain distance from the 12:00 position, you can estimate if it’s around 1:00, 2:00, etc., based on how much time has passed since solar noon.
Adjusting for Time Corrections (if necessary)
- Equation of Time: The sundial shows solar time, and this might differ from the time shown on your clock due to Earth’s elliptical orbit. If you need to match the sundial’s time to your clock, you might need to adjust using the equation of time.
- Time Zone Adjustment: Since the sundial shows local solar time, there may be a difference between this and standard time for your location. To adjust for this, you may need to add or subtract hours based on your longitude and the standard time zone.
Key Things to Remember:
- You’ll need to estimate the time by observing the shadow’s position relative to solar noon.
- The shortest shadow marks solar noon (12:00 PM), and the shadow gradually lengthens throughout the day.
- You can mark your own hourly positions based on the observed movement of the shadow after solar noon.
- Adjust for equation of time and time zone if you need precise clock time.
With this method, you’ll be able to read the time on your equatorial sundial, by estimating the shadow’s position and referencing solar noon.