The Shadow Knows!
Sundials - The Oldest of Scientific Instruments
The use of standard time zones, GMT , and UTC, serves us very well. But Standard Time applies the solar time at a Standard Meridian to the entire time zone, which is around 15 (more or less) of longitude wide. This is necessary for practical use, but unless you are located directly on the Standard Meridian for your time zone, your watch time and your local solar time will not be in agreement. To use the sundial we must again remember that it is truly only the same time along the same lines of longitude (meridians). And to tell local, solar time, one needs a sundial!
Sundials are one of the oldest of scientific instruments. They may, in fact, be the oldest of scientific instruments. There is evidence for their use as early as 1500 B.C.E. Sundials are "living objects", requiring no winding, batteries, or other power source. It is said that a clock or watch may keep time, but only a sundial can findtime. Sundials allow us to tell time by the constant movement of a shadow that is cast by an object called agnomon ("KNOW'-mon" - "the one who knows"). The shape and position of a gnomon on a sundial varies depending on the type of dial, of which there are many. Most gnomons are rods that are permanently fixed on the dial. But in some dials, the gnomons are re-positioned daily. This is the case for the dial at Northwest Florida State College, where you, as the observer, become the gnomon as you stand on the dial.
It may be frustrating at first to discover that the time indicated on a sundial and the time indicated on a watch or clock almost never agree. There are three basic reasons for this. The first is what may be called the longitude problem, and it has already been alluded to. Most places in the world do not lie exactly on one of the Standard Time Meridians, so the local time by the sun is not the same as the time used for the entire time zone. Niceville, Florida is in the Central Time Zone, which uses the solar time of the 90 West Standard Meridian (9 West passes through New Orleans, Louisiana, and Memphis, Tennessee). But Niceville is located at 86.48 West Longitude (or, 86 28' 29" West), which is 3.52 east of the Central Time Zone Principal Meridian of 9 West. Since the Earth turns 1 of longitude every 4 minutes, it is solar noon in Niceville 14.08 minutes earlier than indicated by an accurate clock set on Central Standard Time (4 x 3.52 = 14.08 minutes). So when the sundial shows Noon, an accurate watch would read 11:45:52 sec AM. And when your watch reads 12:00 Noon, the sundial would show 12:14:08 PM.
The second reason that the sundial only rarely agrees with an accurate clock has to do with the Earth's orbital velocity around the Sun, which varies throughout the year. The orbital path (eccentricity) around the Sun is slightly elliptical, and the Earth moves faster when it is closer to the Sun and slower when it is farther. This change in orbital velocity affects the actual length of days throughout the year, and can be as much as 16 1/2 minutes difference (in early November) when compared to the length of an average day.
The third reason has to do with the Earth's axial tilt (obliquity), which is leaning about 23.5 from vertical, and causes slight variation in the time of noon throughout the year. The combined effect of Earths eccentricity and obliquity cause sundials to either "run fast" or "run slow" when compared to a standard-time clock. This daily difference is called the equation of time.
Equation of Time = Mean (standard) Time - Apparent (solar) Time
Figure 5 is a chart of the Equation of Time throughout the year. Figure 6 is a more accurate, daily chart showing the Equation of Time. These charts indicate how much a dial is "fast" or "slow" when compared to an accurate watch or clock. Some very sophisticated sundials can partially correct for the Equation of Time, but most simply require the use of a chart with one simple time correction so that the dial and watch or clock can be compared.
The Equation of time and the longitude problem are both accounted for in the Standard Time Correction Chart printed at the back of this booklet. To use the chart, use the month and day to find the time correction for that day. This chart tells you the difference between sundial time (Local Apparent Time or Solar Time) and Central Standard Time for any day of the year. Most of the numbers have a negative (-) sign in front of them. That means that Central Standard Time (on your watch, clock, or phone, if it is accurate), will be behind("slower" than) what the sundial reads. In other words, for most days of the year, the sundial "runs fast" compared to your watch. (Only on a few days, from February 6th to 18th, does the sundial read slow, and that by only a few seconds, so it will not be noticeable on the dial. On these days, the Sundial and your watch will agree!) This difference in time can be as much as 30 minutes in parts of October and November.
There remains one additional, annoying, factor - Daylight Savings Time. If you are on Daylight Savings Time, you must ADD an hour to your corrected Sundial time to get the same time as your watch (or subtract an hour to begin with to make the comparison). Sundials do not understand Daylight Savings Time (nor should they).