What you seek is but a shadow.
– the motto on the University of Washington sundial.
With all the sunshine we had this week, I thought it was high time for me to understand how the sundial on the Physics building at the University of Washington works!
In the picture below, the shadow of the gnomon (ball) moves from left to right as the day progresses. The sun crosses lower in the sky in winter time, and then the path on the wall is higher. The sun crosses higher in summer time, and then the path on the wall is lower. The equinox was in March, so we have already crossed to below the line marked EQUINOX on the sundial.
The only other thing that seemed out of whack, was that the dial seemed a little off: it showed 12.30 pm PDT on the nose, when it was already 12.39 pm when I took the picture. Should the gnomon ball shadow not have moved at least a little bit off the 12.30 pm line, towards the 1.00 pm line?
We in Seattle, and all others in the Pacific Standard Time zone, keep a clock time based on the solar time at the arbitrary longitude of 120° W (which happens to pass through the town of Chelan). However, in Seattle we are located some 2° 19′ to the west of this longitude, and the sundial in Seattle indicates a time 9.2 minutes earlier than the sun would in Chelan. Here is the full explanation from the UW Dept. of Physics.
P.S. Look for the slender figure-eight-shaped curve in the sundial’s center by the 12, called the analemma. It is a plot of the location on each day at noon, throughout the year, of the gnomon ball’s shadow.