How to Read the time and the date on the Sundial
by Larry Kavanagh

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  August 15, at 1:45 PM EDT

  August 20, at 4PM EDT

Sundial painters
Damian and Moira,
July 20, 1985 at 1:30 PM EDT
(fisheye lens)

Looking at the north wall inside the sundial on a sunny day, you will see a pattern of light that looks like a parabola (i.e., like the letter “U” but wide at the top) Find the spot that is the base of this parabola – the lowest point on the wall that the light reaches. This spot is your mark.

Your mark lies somewhere in the midst of the seven long black Date Lines that run up and down the sundial wall. Except for the top and bottom line, each of these Date Lines is marked with two dates: July 21 and May 21, August 21 and April 21, and so forth. Estimate where your mark lies relative to the two nearest Date Lines, and you should find that it correlates closely to today’s date!

Next, use your mark to tell the time. The large red dots on the Date Lines correspond to the hours, and are marked in Eastern Daylight Time. The red stripes connect dots of the same time. Find which two hour dots your mark lies between, and estimate their relative distance to arrive at the correct time.

Now, your wristwatch. How accurate were you? You should have come within about fifteen minutes of actual time, and within about four days of the actual date.

Without further corrections, fifteen minutes is about as accurate as a sundial can be. Due to eccentricities in the earth’s orbit, there are some days that are actually shorter than 24 hours, and some that are actually longer. The cumulative effect (which is called the Equation of Time) can throw a sundial reading off by up to 16 minutes. However, sundial time is accurate on four days each year. These days are (approximately) April 15, June 14, September 2, and December 25.

Also, further corrections would be necessary to make your date reading closer than about four days. For example, the date labeled “June 21” on the sundial is actually meant to be the First Day of Summer (i.e., the Summer Solstice). However, due to things like Leap Year and orbit eccentricities, the Summer Solstice can actually occur on June 20, 21, 22, or 23 in any given year.

Finally, the yellow dots mark actual Sun Time: High Noon, three hours before and three hours after noon. These are always correct. Note that when it is High Noon in Louisa, it is about 1:15 EDT!


  Diagram of full sundial