How to Make a Sundial


How to Make a Sundial

A sundial is a device that indicates the time of day using the position of the Sun. There are many types of sundials, but the simplest one uses an upright stick called a gnomon. The shadow cast by the gnomon shows the time on a sundial face.

Materials

For an easy sundial, all you really need is a straight stick, like a dowel or a pencil, some pebbles that mark the hours, and a watch or phone so that you can match the shadow cast by the stick to the time.

  • Straight stick
  • Markers for the hours
  • Watch, phone, or clock

Write numbers on the pebbles (or whatever you are using for marking hours). Chalk, crayons, waterproof paint, or permanent markers are all good options.

Make a Sundial

  1. Select a sunny location. You don’t want any shadows cast by buildings or trees at any time during the day.
  2. Place the stick (gnomon) upright into the ground. For ground that is too hard for inserting a gnomon, an option is putting the stick in a small bucket of sand or soil.
  3. In the Northern Hemisphere, slightly tilt the gnomon toward the north. You want true north, not magnetic north, and ideally the angle corresponds to your latitude. But, for this project, your only goal is to angle the stick enough that it casts a shadow at noon! In the Southern Hemisphere, slightly slant the gnomon toward the south. If you don’t know which direction north/south is, you can use a compass, a compass app on your phone, or you can keep the gnomon straight until it’s noon. At noon, the angle that gives you the best shadow is north (northern hemisphere) or south (southern hemisphere).
  4. Calibrate your sundial. You can do this in one day or over a few days. Check your clock and when it’s a new hour, place a marker that indicates the position of the shadow. Keep doing this until you have all of the hours filled in.

Did You Notice?

  • The completed sundial face is a curve or arc and not a complete circle like a clock face. You’re counting the daytime hours, so some numbers might repeat (like 6 or 7). You don’t use the nighttime part of the circle because the Sun is not up!
  • The shadow moves in a clockwise direction from sunrise to sunset.
  • When the Sun rises, the shadow is long and points west. When the Sun sets the shadow is long and points east.
  • At noon, the shadow is very short and point north in the Northern Hemisphere or south in the Southern Hemisphere.
  • The marks between the hours are not the same distance apart (unless you live on the equator). Typically, the Sun appears to move faster around noon when it is at its highest position in the sky (marks closer together toward noon) and more slowly near sunrise and sunset when the Sun is near the horizon.
Ancient Egyptian Sundial
World’s oldest Sundial, from Egypt’s Valley of the Kings (c. 1500 BC)

History

Sundials have been used since ancient times, with the earliest known examples dating back to ancient Egypt and Babylon around 1500 BC. The Greeks and Romans further refined the design and use of sundials. Throughout history, various cultures developed different types of sundials, adapting them to their specific needs and environments.

Some Types of Sundials

The type of sundial in this project is a horizontal sundial, but there are many other versions:

  • Horizontal Sundial: This sundial features flat dial with a gnomon mostly perpendicular to the surface. It is the most common type and is placed horizontally.
  • Vertical Sundial: This version is mounted on a vertical surface like a wall. The gnomon sits at an angle to the vertical plane.
  • Equatorial Sundial: The dial plate aligns parallel to the Earth’s equator, and the gnomon is perpendicular to this plate.
  • Analemmatic Sundial: This is a more complex type where the gnomon is movable and placed based on the date. The hour markers form an elliptical pattern.
  • Polar Sundial: The dial aligns parallel to the Earth’s axis and the gnomon points directly towards the celestial pole.

Common Misconceptions About Sundials

  1. Sundials Work Anywhere Without Adjustment:
    • Misconception: If you move a sundial to any location it will still tell accurate time without any changes.
    • Reality: Sundials require calibration for the latitude and longitude of the new location for accurate timekeeping.
  2. Sundials Show the Same Time as Modern Clocks:
    • Misconception: A sundial shows the same time as a modern clock or watch.
    • Reality: Sundials show solar time, which differs from standard clock time due to the equation of time and the difference between solar noon and standard noon.
  3. The Gnomon Should Always Be Vertical:
    • Misconception: The gnomon (the stick or rod that casts the shadow) is vertical or points straight up.
    • Reality: Ideally, angle the gnomon towards the celestial pole (north in the Northern Hemisphere, south in the Southern Hemisphere) at an angle equal to the local latitude.
  4. All Sundials Are the Same:
    • Misconception: All sundials operate the same way and are interchangeable.
    • Reality: There are many types of sundials (horizontal, vertical, equatorial, etc.), each designed for specific conditions and purposes.
  5. Sundials Are Always Accurate:
    • Misconception: Sundials provide accurate timekeeping throughout the year.
    • Reality: The equation of time, seasonal variations, and even local topography affect sundials.
  6. Sundials Only Work on Sunny Days:
    • Misconception: Sundials are completely useless on cloudy days.
    • Reality: Some advanced types, like heliochronometers, provide rough estimates on overcast days by tracking the sun’s position through cloud cover.
  7. Sundials Were Only Used in Ancient Times:
    • Misconception: Sundials are outdated and were only used by ancient civilizations.
    • Reality: Sundials are still appreciated today for their simplicity, educational value, and as decorative garden features.
  8. The Sundial’s Shadow Moves Uniformly Throughout the Day:
    • Misconception: The shadow on a sundial moves at a constant rate throughout the day.
    • Reality: The shadow moves at different rates due to the Earth’s tilt and orbit.
  9. Sundials Only Show Time:
    • Misconception: Sundials can tell time, but nothing else.
    • Reality: Some sundials indicate seasons, solstices, equinoxes, and even geographical latitude.
  10. Reading a Sundial Is Like Reading a Clock:
    • Misconception: The face of a sundial looks and works like the face of a clock.
    • Reality: Sundials take many forms, but generally their numbers are not arranged like those on an analog clock.

References

  • Daniel, Christopher St. J.H. (2004). Sundials. Osprey Publishing. ISBN 978-0-7478-0558-8.
  • Depuydt, Leo (1998). “Gnomons at Meroë and Early Trigonometry”. The Journal of Egyptian Archaeology. 84: 171–180. doi:10.2307/3822211
  • Michnik, Hugo (1922). “Theorie einer Bifilar-Sonnenuhr” [Theory of a bifilar sunial]. Astronomische Nachrichten (in German). 217 (5190): 81–90. doi:10.1002/asna.19222170602
  • Walker, Jane; Brown, David, eds. (1991). Make a Sundial. The Education Group of the British Sundial Society. British Sundial Society. ISBN 0-9518404-0-1.
  • Waugh, Albert E. (1973). Sundials: Their Theory and Construction. New York, NY: Dover Publications. ISBN 0-486-22947-5.