Sundials - World's Oldest Clocks

# North American Sundial Society

To make a dial that correctly tells standard time you will need to know your latitude for dial construction and longitude to determine the correction (time zone offset) between your dial and the time zone meridian (usually set every 15 degrees of longitude).

IMPORTANT Some maps will indicate longitudes in North American as negative, indicating that they are west of Greenwich.  Other maps will indicate longitudes in North American as postive, declaring them "west longitude".

Correcting for your position in your Time Zone

When solar time became inadequate for railway timekeeping, a system of time zones was initiated creating 24 time zones, each 15° wide - within which zone all clocks would tell the same time. (Basic arithmetic: The earth's 360° circumference was divided into 24 one hour segments, and the result was 24 time zones each 15° wide.)

This correction for your sundial is determined by the relationship between the longitude of your sundial and the longitude of your time zone meridian. The central meridians of all 24 time zones can be found in the table below. Note: some time zones are perturbed by geographical and political boundaries.

The correction is the difference between your longitude and the time zone meridian, converted into minutes. For example:

Dial Longitude: -78° 15' (note the minus indicates west longitude)
Time Zone Meridian:-75° 00' (this is -75/15 = 5 hours west of Greenwich)
---------------------------
Longitude Offset - 3° 15' = -3° 15/60 = -3°.25
Time Zone Offset 4 x (-3°.25) = -13 minutes of time

That is, for our example of where we are 3 1/4 degrees west of the time zone meridian, we are -13 minutes or 13 minutes earlier in time according to the sun. If the time zone (your watch) says it is exactly noon, 12:00, then the longitude correction requires us to subtract 13 minutes and we would expect the dial to show 11:47.  If this dial shows the shadow at exactly the XII mark, then we add 13 minutes and we would expect a clock to show 12:13

Even after the time zone offset is applied, your dial will not consistently tell standard or daylight savings time. Seen the Equation of Time tab.

 Time Zone Number (offset) Central Meridian Locations GMT-11hrs 11 165 Midway, Samoa GMT-10hrs 10 150 Hawaii, Society Is. NORTH AMERICA GMT-9hrs 135 Alaska, Pitcairn Island US Pacific 8 120 Whitehorse, Seattle, Los Angeles US Mountain 7 105 Denver, Phoenix, Yellow Knife, W. Mexico US Central 6 90 Chicago, New Orleans, Churchill, E. Mexico US Eastern 5 75 New York, Montreal, Washington D.C., Peru Atlantic 4 60 Nova Scotia, Venezuela, (Newfoundland -30m) GMT-3hrs 3 45 Greenland, E. Brazil, Argentina GMT-2hrs 2 30 Georgia & S. Sandwich Islands GMT-1hrs 1 15 Cape Verdes, Azores Greenwich 0 0 WET, British Isles, Iceland, W. Africa Middle Europe -1 -15 MET, Europe, Scandinavia, C. Africa Eastern Europe -2 -30 EET, OEZ, Europe, Mid East, E.C. Africa GMT+3hrs -3 -45 Moscow, E. Africa GMT+4hrs -4 -60 Gorki, Oman, Mauritius GMT+5hrs -5 -75 Pakistan, (India +30m) GMT+6hrs -6 -90 Bangladesh, Burma GMT+7hrs -7 -105 S.E. Asia, Sumatra GMT+8hrs -8 -120 China, Philippines, W. Australia GMT+9hrs -9 -135 Korea, Japan, (C. Australia +30m) GMT+10hrs -10 -150 P. New Guinea, Guam, E. Australia GMT+11hrs -11 -165 Sakhalin Peninsula, Solomon Islands GMT+12hrs -12 -180 New Zealand, Wake, Marshall Islands