Hevelius Sundial Generator

This webpage draws the face of a Hevelius sundial. You design the other components yourself. For background, see the articles in The Compendium, Journal of the North American Sundial Society, by Claude Guicheteau [29(1), Mar. 2022], Patrick O'Hearn [29(2), June 2022], and Steve Lelievre [29(3), Sept. 2022].

Latitude Use the Fetch button to load your location, or give the required latitude as decimal degrees; positive for Northern Hemisphere, negative for Southern Hemisphere.
Include Sundial Red curve. Solid markers indicate hours from solar noon. Hollow markers are for quarter hours.
Include Compass Green curve. Solid markers represent 22½° steps (two compass points) from South. Hollow markers are for intermediate steps (11¼°, one point). For the southern hemisphere, angles are from North. Labels are not provided for this option.
Calendar Scale



Modified Gregorian

  • Zodiacal: Astrological months based on 30° of ecliptic longitude. Each month is divided in thirds (10° of ecliptic longitude).
  • Gregorian: based on the ordinary (civil) calendar. The scale runs from June 21st to December 21st and back, with ticks at the 1st, 11th, and 21st day of each month.
  • Modified Gregorian: This scale works the same way as the Gregorian option, except that the tick positions are calculated by splitting the difference between the pairs of dates with similar solar declinations (that is, 1st-11th and 21st-21st pairs). This approach halves the number of marks needed, producing a less cluttered appearance while keeping the ticks close to their correct positions. As well, all the ticks are placed on the inside of the scale (but calendar labels are not provided). This type of scale's impact on accuracy is equivalent to setting the date wrong by 2 days in late January / early Febrary and late October / early November, and by 1 or 0 days during the rest of the year. Note, even if the option is selected, the accuracy limits shown below are calculated using the actual daily declination values.
  • Calendar offset This setting is used to rotate the calendar scale. Doing so changes the shape of the device, affecting accuracy (depending on latitude, this can be an improvement or an impairment). Altering the setting allows you to change the geometry of the device, possibly making it easier to read near noon or near mid-summer sunset. Positive for clockwise; negative for counter-clockwise.
    Calendar span This setting controls the anglular span of the calendar scale. Changing it changes the shape of the device, affecting accuracy. Using it may simplify construction of your device, or make it easier to read near noon or near mid-summer sunset. Set it to greater than 1 to widen the scale; less than 1 to shrink it.
    Compensate for Atmospheric Refraction This phenomenon changes the angle of the sun's rays, particularly when the sun is low in the sky. The consequence is a subtle change the shape of the diagram and the daily limits of operation. Adjustment for refraction follows Jean Meeus' book "Astronomical Algorithms", ultimately derived from Sæmundsson, Þorsteinn (1986). "Astronomical Refraction". Sky and Telescope 72:70. Using this option makes the accuracy estimate shown on this page more realistic.
    Treat the sun as a point If not checked, the sun is treated as having ½° angular diameter with sunrise and sunset happening when the upper limb is on the horizon.
    Show construction lines for major points
    Mark today Draws an orange dot to indicate today's position on the calendar scale.

    To save the diagram as PDF, click the Print button then look for an option called something like Save to PDF for the printer name.

    Warning: The Offset and/or Span settings cause invalid diagram geometry at the chosen latitude. Try altering their values.

    The hours of operation had to be reduced because of diagram geometry.

    Your latitude is unavailable. Try reloading the web page.

      Accuracy (worst-case intrinsic error, approximate):
    • Sundial:
    • Compass:
    • In operation, accurancy will be reduced, particularly near noon and near summer sunrises and sunsets, due to factors such as plumbness, interpolation of scale positions, construction tolerances, and visual acuity.

    Hevelius Sundial Generator © Steve Lelievre 2022. Please do not copy or redistribute.