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EarthDial Calculator and Instructions for Near-Equatorial Locations

1. To find your latitude and longitude, consult a good map, use a GPS receiver or visit:

Entire world: http://www.getty.edu/research/conducting_research/vocabularies/tgn/

USA Only: http://www.census.gov/cgi-bin/gazetteer

2. Enter your (less than 14-degree) latitude below, then click on the "Calculate!" button. The EarthDial Calculator will calculate all of the angles and distances that you need to construct the lines on your EarthDial. Note that the calculator does not work for latitudes less than 0.3 degrees. In this case, please use the "At the equator" method described below.

Measuring an equatorial region EarthDial
Click to enlarge >
Measuring an equatorial region EarthDial

Enter your latitude in degrees
(positive values only):

Point P at cm

Note: For northern latitudes, P is north of G (example shown below). For southern latitudes, P is south of G.

5. Through P draw an east-west line perpendicular to the north-south line; this is the Equinox line.

6. Along the Equinox line, measure eastwards and make marks (such as point E in the diagram) for the 1, 2, 3, and 4 hour lines at PE values shown in the following table.

7. Through point E draw a slanted line as shown in the diagram. It should deviate from perpendicular by the angle B given in the EarthDial calculator: these angles B for the 1, 2, 3, and 4 hour lines are shown below.

  Hour Line Angles B PE
12 noon: degrees cm
11 am, 1 pm: degrees cm
10 am, 2 pm: degrees cm
9 am, 3 pm: degrees cm
8 am, 4 pm: degrees cm

8. This slanted line extends north and south to the two solstice curves, as shown in the diagram above. The following table gives (1) the distance ES to the summer solstice line (southwards in the Northern Hemisphere and northwards in the Southern Hemisphere) at point S, and (2) the distance EW from the equinox line (at point E) to the winter solstice line (northwards in the Northern Hemisphere and southwards in the Southern Hemisphere) at point W.

Date Curve Points
  Distance
ES:		 Distance
EW:
12 noon: cm cm
11 am, 1 pm: cm cm
10 am, 2 pm: cm cm
9 am, 3 pm: cm cm
8 am, 4 pm: cm cm

This completes the afternoon portion of the EarthDial; the morning portion is exactly symmetric. Use the same values as above for the 11, 10, 9, and 8 hour lines. Be sure that the morning hour lines slant in the opposite sense from those in the afternoon. On your completed EarthDial all nine hour lines (8, 9, .... 3, 4) should appear to radiate from a point, which is off the board on the southern side (for a northern latitude dial) or northern side (for a southern latitude dial).

At the Equator (or less than 0.3 degrees)

EarthDial face at the equator
EarthDial face at the equator

The pattern is drawn for 0 deg latitude, not for the latitude you entered in the calculator. At the equator (latitude = 0 deg), the hour lines are exactly parallel in a north-south direction and the above method does not work. Instead, use the equation:

PE = 10 cm * tan (15 deg * HA),

where HA = hour angle of the Sun = the number of hours before or after noon.

Please contact tps@planetary.org if you have any questions.