Photo courtesy of Elaine Hunt
If you can read a clock, then you can read a sundial. If you can’t read a clock, I would recommend prioritizing your learning and starting there.
This Sunday, which marks the autumnal equinox, you can leave your watches and clock necklaces (sorry, Flavor Flav) at home as you walk by the sundial on McKeldin Mall. The McKeldin sundial reads the correct time on four occasions a year — during the autumnal and vernal (spring) equinoxes and the winter and summer solstices.
Sundials tell time by using shadows cast by the “gnomon,” the large metal rod sticking up from the center of the sundial. As the sun moves across the sky and hits the gnomon from different directions, the shadows move across the sundial. Where the shadow falls corresponds with the correct time marked on sundial.
The McKeldin sundial’s time is written in Roman numerals, which you can brush up on here.
Based on the Earth’s tilt at different times of the year, the sundial’s time may be slightly off from the actual time.
If you read the small plaque on the sundial, you will see a series of dates with corresponding minutes to add or subtract to determine the actual time. Over the course of a year the Northern Hemisphere, or more specifically, College Park, tilts slightly towards and away from the sun. This causes the sun to take higher or lower paths through our sky, slightly changing the shadows it casts on the sundial. The change in shadows cause the sundial’s time to vary.
The sundial will always read a time relatively close to the actual time but it may be as far as 15 minutes off. During the equinoxes and solstices however, the sundial reads the correct time.
The word equinox comes from two Latin words and it means “equal night,” which is fitting because the day and night are almost the exact same length. Equinoxes take place when the Earth is tilted neither away nor towards the sun. Sundials are calibrated to read the correct time when the Earth is most and least tilted, or in other words, at the equinoxes and solstices.
A final, major, thing to account for when using the sundial is Daylight Savings Time. Because we are currently in DST, you have to add an hour to the time you read on the sundial. However, there’s no shame in blaming the sundial for making you an hour late to class.
