Sanford Center for Sky Awareness
Observing Solar Cycles Using Sundials

Equatorial Sundials

What is an Equatorial Sundial?

Setting the Sundial

    Similar to setting the correct time on an analog clock or wristwatch (by moving the hands of the timepiece into proper position), properly orienting an equatorial sundial will move the shadow of the gnomon (or style) into position so that the dial face displays the correct time.

    The sundial may be used to align itself with your local meridian. Obtain the exact time of "Sun transit" from the U.S. Naval Observatory Complete Sun and Moon Data for One Day Web page. Set your wristwatch for the exact time of day.1 At the precise moment of Sun transit, align the sundial so that it reads exactly 12 noon (regardless of the time shown by your wristwatch); the sundial is now aligned with your local meridian. A slightly more complicated but more accurate procedure would be to use the shadow cast by a plumb bob to strike a north-south line at the time of Sun transit. For example, a tripod, nylon cord, and boat anchor were used to find the local meridian (see shadow of cord) during SUNdays in September, Huntley Meadows Park, Fairfax County, VA.

    Disclaimer: The preceding statements regarding the proper alignment of an equatorial sundial are based upon the assumption that the sundial does not feature a built-in correction for longitudinal offset from the Standard Time Meridian (see the following section, "Solar Time Versus Standard Time").

    1   Determine the exact time of day using one of the following methods: visit The Official U.S. Time Web page; use a radio-controlled atomic clock (such as the ExactSet™ RM806 from Oregon Scientific, Inc.); or use a relatively inexpensive Global Positioning System receiver (such as the Garmin GPS 12) that displays both the exact time (precise to the nearest second) and location (latitude & longitude) of the sundial.

Solar Time Versus Standard Time

    Solar Time, technically known as Local Apparent Time (L.A.T.), is sundial time--when the Sun crosses your line of longitude (is due south of the observer for northern mid-latitude locations), it is solar noon. In contrast, when the Sun crosses the Standard Time Meridian for your time zone, it is 12 noon Standard Time (regardless of the Sun's position relative to your meridian). Standard Time or wristwatch time is the worldwide time-keeping standard based upon Mean Solar Time for selected lines of longitude (located in the middle of each time zone) known as Standard Time Meridians. Around the world, there are 24 Standard Time Meridians, beginning with the Prime Meridian (0° longitude). In the continental United States, the Standard Time Meridians are 75°W, 90°W, 105°W, and 120°W for the Eastern, Central, Mountain, and Pacific Time Zones, respectively.

    Nominally, each time zone is one hour wide (15 degrees of longitude), extending 30 minutes (7.5 degrees of longitude) to the east and west of the Standard Time Meridian. Unless you live along a Standard Time Meridian, Solar Time (sundial time) is different from Standard Time (wristwatch time) by as much as 30 minutes (earlier or later). In the real world, the location of a given time zone boundary is determined by geopolitics as well as geography. The net result is that some time zones are wider than one hour, therefore the difference between Solar Time and Standard Time is greater than the theoretical 30-minute maximum.

    To correct Solar Time (sundial time) for Standard Time (wristwatch time), one must compensate for both the difference in longitude (between the location of the observer and the Standard Time Meridian) and the Equation of Time. The Solar Noon Calendar calculates tables showing either the exact time of Solar Noon for your location for each day of the year, or the Standard Time Correction -- the amount you have to add to, or to subtract from, the Solar Time shown on your sundial to get the Standard Time shown on your wristwatch. Add one hour for Daylight Saving Time.

Telling Time Using Shadows

    The gnomon is the part of a sundial that casts the shadow used to tell time. For equatorial sundials, the gnomon is a pole (or rod) of varying thickness. Some equatorial sundials are designed so that time is told by estimating the center of the gnomon shadow (see example); for others, time is told by reading one edge of the gnomon shadow, technically known as the "style shadow." For example, in the close-up photograph of an equatorial sundial dial face (shown left), time is told by reading the upper-left edge of the gnomon shadow. The thickness of the gnomon determines the way in which the hour lines are drawn on the two dial faces, and how time is told from either the gnomon shadow or the style shadow.

    "If the [gnomon] is less than 1/8" (3.175 mm) in diameter or if the rod tapers to a point at the top, [then] all of the hour lines will be drawn from the center...." [Quote courtesy Sundials: Their Construction and Use, Mayall & Mayall, Dover Publications, Inc., ©2000, p. 98.] For example, look closely at the StarDate Equatorial Sundial template; notice that the hour lines radiate from the exact center of the dial face. A thin gnomon should be used with this type of dial face design, otherwise time is told by estimating the center of the gnomon shadow.

    In contrast, the Sandburg Planetarium Equatorial Sundial is designed for use with a slightly thicker gnomon, e.g., a pencil approximately 1/4" (7 mm) in diameter. Notice that the hour lines radiate tangentially from a small inner circle representing the diameter of the gnomon. A similar design is used by both the St. Petersburg, FL Equatorial Sundial and the Boulder, CO Equatorial Sundial. Tell time by reading the style shadow on the dial face--the time-telling edge of the gnomon shadow should be parallel to one of the hour lines (see dial face close-up, upper left).

Making the Sun-Earth Connection

    "The fundamental units of time are set by the cycles in the sky, and people have been measuring them since prehistoric time. ... The sundial is a link between the sky and our need to measure time, and it's actually a model of the apparent movement of the Sun. The [equatorial] sundial charts the progress of the Sun across the sky during the day [as well as throughout the year]." [Quote courtesy Griffith Observatory.] Two motions--the rotation of the Earth around its axis, and the revolution of the Earth around the Sun--cause daily- and annual cycles in the Sun's apparent path across the sky that can be observed indirectly using an equatorial sundial.

Earth's Rotation and Solar Time-Keeping

    Planet Earth is a magnificent timepiece! The Earth rotates counterclockwise once every 24 hours. One complete rotation equals 360 degrees. The rate of the Earth's rotation equals 15 degrees per hour:

      360°/24 hr = 15°/hr or 15°/60 min, which reduces to 1°/4 min

    Therefore, all of the hour lines on the dial face of an equatorial sundial are spaced exactly 15 degrees apart [Graphic source credit: Francois Blateyron]. Similarly, there are 24 time zones around the world; each time zone is one hour or 15 degrees of longitude wide.

    Sun shadows fall in the opposite direction as the Sun. Because the Earth rotates counterclockwise (as viewed from above the Northern Hemisphere), shadows cast by the Sun move in a clockwise direction. Therefore, morning times are located on the right side of the upper dial face; afternoon times are on the left. The reverse is true for the Southern Hemisphere (shadows cast by the Sun move in a counterclockwise direction), which is modeled by the lower dial face. [See a time-lapse animation (565 KB) of seven images (560x320 pixels) spanning three hours on 02 March 2004, archived by ED-11, South Pole, Antarctica.]

Earth's Revolution Around the Sun and the Annual Cycle of Change in the Sun's Apparent Path Across the Sky

Time for Learning

    The equatorial sundial is by far the best type of sundial for teaching a wide range of fundamental concepts in astronomy, geography, and mathematics. Most upper-elementary students are somewhat aware of the Sun's apparent daily motion across the sky; fewer students realize that the Sun's apparent path across the sky changes in a predictable annual cycle. Experience working with equatorial sundials will increase students' awareness of both the Sun's daily and annual motions caused by the Earth's rotation around its axis and revolution around the Sun.

    In education as in life, timing is everything! Ideal times to work with equatorial sundials include a week-or-so before and after the equinoxes (SEP & MAR) so that students have the opportunity to indirectly observe the Sun crossing the Celestial Equator -- one of the real reasons for the seasons. Also, there are four days during the year when the Equation of Time equals zero (0 minutes): on average SEP 02; DEC 25; APR 15; and JUN 14. On these four days, Solar Time is coincident with Standard Time (after correcting for the difference in longitude between the location of the observer and the Standard Time Meridian). From an educator's point of view, April 15 appears to be the best of the four dates with respect to the school calendar (SEP 02 is too early in the school year, JUN 14 too late, and DEC 25 is a holiday).

    The SCSA recommends the following instructional resources, appropriate for use with upper elementary, middle, and high school students:

    1. From NASA Liftoff to Space Exploration, a set of sundial Web pages:
      Editor's Note: Hyperlinks to the following NASA resources are broken. Archived versions of these Web pages may be accessed by visiting the Internet Archive; enter the URLs for the broken hyperlinks. Be advised that some of the features on the archived pages may not work properly. You may also visit the Kid's Space Home Page and follow the link to Build a sundial. (The only document currently available is the sundial template for the Northern Hemisphere.)
      1. Sundials
      2. How Sundials Work - Editorial Commentary: Be aware of a fundamental factual error on this Web page: "...the base plate is titled [sic] at an angle equal to the latitude." In fact, the dial plate of an equatorial sundial should be inclined (tilted) at an angle equal to the complement of the observer's latitude (also known as the co-latitude).
      3. Building a Simple [Equatorial] Sundial - Editorial Commentary: From the March Equinox to the September Equinox, use the Northern Hemisphere equatorial sundial template. From the September Equinox to the March Equinox, use the Southern Hemisphere template. Be sure to fold the Southern Hemisphere template so that the dial face and gnomon point downward rather than upward (as directed). Also, be aware that the Southern Hemisphere dial face is misnumbered.
      4. Pondering Sundials
    2. From StarDate Online and the University of Texas McDonald Observatory/SCOPE (Southwestern Consortium of Observatories for Public Education), the Solar Poster educational activities and resources, including:
      1. Dial Face Template (print using cover stock)
      2. Equatorial Sundial Activity: PDF
    3. The SCSA Equatorial Sundial Activity
      1. The Sandburg Planetarium Equatorial Sundial template (print using cover stock) - Designed for use with a pencil-sized gnomon, approximately 1/4" (7 mm) in diameter. See also, FCPS Equatorial Sundial template (featuring a compass rose for orienting the sundial and a chart for the Equation of Time).
      2. Assembly instructions (courtesy John Hoy)
      3. Equatorial Sundial Activity Questions - Provide differentiated instruction by assigning multiple choice questions only, as appropriate. [Teacher's Answer Key available upon request.]
    4. Among many good geography lesson plans from George F. Cram Company, Inc., SCSA specifically recommends the following sundial-related lessons (for use with the Horizon Ring Globe): Lesson 11 - Rotation of the Earth; Lesson 12 - Global Time; Lesson 14 - Earth and Sun; Lesson 15 - The Changing Seasons; and Lesson 16 - Daylight Hours (see section regarding the analemma).

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