Hermograph Press

Educational Materials and Informative Publishing


Some Sundial Resources

From The Classroom Astronomer Magazine,
Issue 7
Other Resources

Gnomon Knowledge - Sundials Are A Global Activity  - The article in the Magazine

Korean Sundial Template  - From the article.

The North American Sundial Society  sundial computer programs

The British Sundial Society sundial computer programs page

http://www.bobswatches.com/sundials.html
A good general history and description of the types of sundials

More About Sundials

What is that funky Figure 8 on the sundial?
How can I make the time readings more accurate, using better gnomons?
Using the Sundial....as a Compass!

What is an Analemma?

The analemma is a graphical design which looks like a figure 8 that allows us to correct the sundial time to account for the Earth's tilt and orbital speed.

The Earth rotates at a constant speed, taking 23 hours, 56 minutes, 4 seconds to turn once around its axis (okay, it actually does change speeds slightly due to some internal changes, effects of the planets, etc.....these are extremely minute changes over human lifetimes). But we are also moving around the sun in our orbit so when we've turned once around, the sun isn't where it was in the sky when we started. It takes 4 more minutes to catch up. This 24-hour steady noon-to-noon motion is called the motion of the mean sun and its steadiness is the origin of mean solar time, the time of the sundial.

But the real sun's speed across the sky actually varies. The Earth's orbit is elliptical. This non-circular path causes Earth's orbital speed to increase or decrease over the course of a year. When we are nearer the sun we move faster in our orbit, when farther out than our average 92.9 million miles, we move slower. This in turn causes our sun's apparent position to be "fast" or "slow" (i.e. it gets due south some minutes sooner or later than usual). Thus the apparent solar time can vary from that steady digital watch or sundial time by as much as about 16 minutes. One could add or subtract the values (known as the values of the Equation of Time) to the sundial time to get the correct time or simply adjust the position of the gnomon on the sundial by shifting its place left or right for the appropriate months. On the Hermograph sundial shirt, we do the latter.

However, the sun's position also changes during the year in a vertical way, because of the tilt of the Earth's axis. It is higher in northern summers, lower in northern winters. The sun sets early in winter, say at 4:30PM, while it can be up past 8PM in summer. The gnomon shadow should be horizontal at sunrise or sunset but if we didn't adjust the gnomon's position on this dial it would record the same sunrise and sunset times for all 12 months, and that clearly isn't the way it is in nature. So the analemma not only adjusts for the changing speeds of the Earth in its orbit but also adjusts the gnomon's position on the dial to account for its changing seasonal north-south altitudes (height in the sky). A 4PM shadow reading should be 4PM no matter what season it is.

More About Gnomons

  • The thinner and longer the gnomon, the narrower will be the shadow it casts. It then will go more easily between the lines and numbers for more accurate determination of the minutes than a shadow that is as broad as the space between the hours on the dial.
  • Your house clocks are counting the hours in a time zone. But the sun's actual position in the sky varies within the time zone. From a sundial's perspective, the sun's sky position at 12 noon at the center of the time zone isn't the same sky position as a point 200 miles to the west, even though your digital watch in both places says 12 noon. Time zones are artificial, made by mankind to make railroad schedules easier nearly one hundred years ago. So if you do not live in the middle of the time zone (In the US, longitudes 75, 90, 105 or 120 degrees West), you have to make corrections.

    For every degree of longitude west of the time zone's central longitude meridian you are, you must add 4 minutes to the reading of the time on the Sundial. For every degree of longitude East, you must subtract 4 minutes, the sun is early!

    Here are some sample time corrections for US locations:

    Atlanta 	+ 37 minutes
Boston 		- 16 minutes
Chicago 	-10 minutes
Denver		no correction
Kansas City	+18 minutes
New Orleans	no correction
New York 	- 4 minutes

    If you don't know your longitude, click here.

    • Using the Sundial....as a Compass!

    In these modern times we are much less aware of our surroundings than our ancestors. Streets are our guides, not the sky. So many of us lose track of the directions. But with a sundial you can carry or move, you can regain that lost knowledge. If you already know the correct time you can then use the shirt to find your directions:

    Use a gnomon (stick, finger, etc.) and hold it horizontally and forward from the analemma. Rotate until you make the gnomon's shadow cross the dial at the correct time. You then are facing South! Behind you will be North, to your left will be East, and West will be to your right.