Sundials are an historic reminder of how man used to measure the time of day and are frequently used to inspire us about how time is actually measured. New York City has many distinctive sundials on public display and this article will mention some specific examples that are easy to find.
First, however, we need to understand a few basics about time keeping and the rotation of the Earth. The earth rotates once on its axis approximately every 23 hours and 56 minutes. This is literally the time it takes to rotate 360 degrees and complete one rotation and is called a “sidereal day.” Because the Earth revolves around the sun and is therefore moving in a large circle, it takes approximately an additional 4 minutes each day for the earth to rotate so that it is aligned with the sun again. Think of a very large flagpole pointing directly at the sun at noon. After the Earth rotates on its axis through exactly one rotation, the flagpole will not be pointing directly at the sun because the Earth is not in the same place as it was the day before in its orbit. Those additional four minutes allow the Earth to rotate so that it is once again lined up with the sun. The 24 hours required for this to happen is called a “solar day.”
Because the Earth is in a slightly elliptical orbit around the sun, at certain times of the year the Earth moves faster and other times it moves slower as it travels around the sun. This means that the length of a solar day changes as the time needed for the Earth to rotate so that our flagpole points at the sun again varies. Therefore, using a sundial to measure time accurately over the course of a year does not produce days of equal length. Timekeepers around the world have agreed to use the average of a year’s worth of solar days to come up with the “mean solar day” which is the time scale that everyone on the planet uses today to keep time. These differences between the length of one solar day (as measured by a sundial) and mean solar day are very small, generally only a few seconds but they accumulate over many weeks, until a sundial could indicate a time up to 15 minutes either fast or slow. Fortunately, this error is well known, and a graph called the “equation of time” or “analemma” will give the correction factor for any day of the year.
An additional problem with sundials is that they do not account for your location within a time zone. A sundial in the Eastern Time zone will only read correctly if it is on the meridian line called 75 degrees west longitude. Here in New York City we are at 74 degrees west longitude. That means that any sundial in New York City will read 4 minutes slow every day of the year unless the sundial is made expressly for its location.
Sundials come in many forms, but the four most common are: horizontal, vertical, equatorial and armillary spheres. In all cases the gnomon (shadow producing indicator) must be parallel with our Earth’s axis and therefore pointing at the North Star (Polaris). The horizontal sundial is by far the most common, located in many gardens for example. There are well known formulas for determining where to position the hour lines and these sundials can be mass produced. A good example is the sundial in the Shakespeare Garden in Central Park. The vertical sundial should ideally face due south, but other orientations are possible. The hour lines are calculated in a similar manner to the horizontal sundial and a good example is located on a church wall on 77th St. between Broadway and West End Ave. The third type is the equatorial sundial. In this case, the gnomon is surrounded by a partial ring or curved surface which is parallel to the Earth’s equator and perpendicular to the gnomon. This is the easiest type to construct because each hour mark is exactly 15 degrees in separation from the previous hour mark. A pleasing example of this design is on the Columbia University campus just south of 120th St. The final design is the armillary sphere. This is basically a fancy equatorial sundial. With the equatorial ring making a full circle and other rings used to give support to the structure and make it durable. A nice example of this design is located at 54th St. and Sutton Place South. There is an excellent web site (sundials.org) which lists many important public sundials throughout the USA and there are about a dozen interesting ones in the city.
My favorite is not technically a sundial but a sun triangle. It is a huge stainless steel triangle in the sunken plaza of the former McGraw Hill Building on 6th Ave. between 48th and 49th St. Each leg of the triangle points directly at the sun at solar noon on either March 21, June 21, September 21 or December 21. I will meet you there this December 21 to see the shadow (if it is a sunny day). Happy hunting!
“Sundials, Their Theory and Construction,” Albert Waugh, Dover, 1973