Try this solar-eclipse experiment as you’re watching the Moon cover up the Sun.

In the run-up to the 2017 North American eclipse, I wondered if there was a way to tell time from the rotation of the eclipsed Sun’s crescent shape, caused by the passing Moon.

With the help of the NASA Jet Propulsion Laboratory’s free on-line Horizon ephemeris calculator, a copy of Astronomical Algorithms, 2nd edition by Jean Meuus, and a heaping helping of spherical trigonometry, I developed a unique sundial that can be printed on a flat piece of paper. It tells time by tracking the rotation of the Sun’s crescents as they are projected through a simple pinhole viewer or inverted binoculars.

I partnered with fellow math hobbyist and web designer Dan Axtell to create eclipsesundial.com, where users can create and print a free sundial specific to their location, now for the April 8, 2024, eclipse.

This is a particularly unusual sundial in that each dial works for only about 2 hours on just one particular day, at just the user's location — and it casts no shadow.  Additionally, this is an eclipse activity that favors observers outside the path of totality. The most rapid rotation of the Sun’s crescent (and the best time-telling resolution of the sundial) isn’t visible inside the path of totality.

Will you be projecting the eclipse through a refracting telescope?  Try this version: eclipsesundial.com/telescope.php.

Will you be watching the eclipse with a Sunspotter?  Try this version: eclipsesundial.com/sunspotter.php.

We developed this eclipse sundial just for the fun of it.  There are no cookies and no advertisements on the eclipsesundial.com website.

I (Bill Gotttesman) am a card-carrying member of the North American Sundial Society and have been fascinated with sundials and sun-oriented sculpture of all sorts since stumbling across a copy of “Sundials, their theory and Construction” by Albert Waugh in 1977.  Dan Axtell and I have been math hobbyists since high school long ago– his goal at the time was to discover the order of primes, and mine was to build a perpetual motion machine.  Neither of us has yet succeeded.

Solar Eclipses