There are some really clear ‘analemma’ images at this link, showing the apparent path of the Sun through the sky from a point on Earth over the course of a year. This helps to explain why the shadow made by a sundial needs the EOT correction:
Here is a photo of a sundial at Abingdon School taken at 15:23BST on 11 October 2021:
The shadow shows the time as approximately 2:31 which is not the same as clock time.
Correction 1 – adjust for BST so 2:31+1 hr = 3:31
Correction 2 – the equation of time shows that for 11 October, which is Day 284 of the year, a sundial will be approximately 13 minutes fast. This means the clock should read 12 minutes earlier which is 3:18
Correction 3 – the longitude of Abingdon is 1.288 degrees west and the Earth takes 4 minutes to turn every 1 degree so 4 x 1.288 = 5.152 minutes. This must be added to the sundial time as it is a westerly longitude, giving the final, corrected time as approximately 3:23 compared to the actual clock time of 3:23.
An example of a horizontal sundial at RHS Gardens, Wisley Surrey. The image was taken at 10:33 BST on 23 Sept 2023.
How accurate is the time given by the sundial? Not very accurate at first sight – the shadow is crossing the dial at approximately 9:43!
After corrections however, the result is a lot better.
Step 1 – convert the dial’s GMT reading of 9:43 to BST by adding one hour. Corrected time = 10:43
Step 2 – look at the lovely Equation of Time curve inscribed on the dial. For 23 September the dial reads FAST by about 8 minutes. Corrected time = 10:35
Step 3 – the longitude of RHS Wisley is 0.473 degrees west of Greenwich. It takes 4 minutes for Earth to rotate each degree so Wisley Mean Time will be 0.473 x 4 = 1.89 minutes LATER THAN Greenwich Mean Time. Final corrected time = 10:33 (rounding 1.89 minutes to approximately 2 minutes)
So this is a pretty well aligned and accurate sundial!
In this image students are demonstrating ways of observing the Sun safely:
1.Right of image – using a commercially purchased solar projector (see the next image to see the projection)
2. Centre – using a home made pinhole projector, a long box with a pinhole punched in tinfoil at the ‘Sun’ end and white paper stuck inside the opposite end to make a screen. A viewing aperture has been cut in the top of the box nearest the screen. NOTE that the student is facing away from the Sun to avoid accidentally looking directly at it.
3. Left of image – using a specialised solar telescope with a Hydrogen-Alpha filter safe for direct observation of the Sun.
An image of the projection screen inside the commercially purchased solar projector during a partial solar eclipse in June 2021.
Stellarium: an excellent, free to download, planetarium package. It runs on various platforms and allows you to find your way around the night sky in real time, the past and the future. We hope to post some simple Stellarium tutorials before too long. (The app version may not be free of charge and other planetarium packages are available, but you can also use the web version without any download here Web version of Stellarium)