How to Read a Sundial

Or, Converting Between Sundial Time and Clock Time

Many would say that sundial time is truer and provides more insight into the nature of our world than our artificially constructed clock time.  A sundial indicates local solar time, which is time measured by the Sun at a given location; in general, this time differs from clock time.  For example, it is local solar noon when the Sun is highest in the sky. At local solar noon the Sun is generally due south for observers in the Northern Hemisphere, or due north for observers in the Southern Hemisphere. Between the Tropics of Cancer and Capricorn, however, the noontime Sun is to the south on some dates and to the north on others (depending on the latitude).

If you wish to read clock time with your EarthDial (or any sundial), there are three adjustments you need to make:

1. Adjustment for Location Within Your Time Zone

The Earth’s rotation causes the Sun to appear to move from east to west. So the Sun arrives on the eastern edge of a time zone long before it arrives on the western edge. Consider Seattle, which keeps Pacific Standard Time (PST) on its clocks. At Seattle’s latitude, the two edges of the Pacific Time Zone are more than 800 kilometers (500 miles) apart. When sundials on the eastern edge correctly read 12:30 pm, sundials on the western edge (also correctly) read 11:30 a.m. The sundials differ by an hour, while everyone’s watch in the Pacific Time Zone reads exactly 12:00 noon. Yikes! Imagine two people at the Pacific Time Zone’s edge. They are shaking hands and admiring a sundial correctly telling local solar time, while their watches disagree by one full hour. (In a sundial-admirer’s sense, the sundial is the only trustworthy timepiece available.) We need a rule to compensate for the width of time zones.

The Rule.  For every degree of longitude you live west of the center of your time zone, add 4 minutes to the sundial reading to adjust to clock time.  If you live east of the center, then subtract 4 minutes for each degree.

Most time zones are one hour or 15 degrees of longitude wide.  With 60 minutes in an hour, it thus takes 4 minutes for the Sun to sweep through 1 degree.  Seattle is at 122.3° West longitude, so dials in Seattle are 2.3° x 4 = 9.2 minutes behind clocks and watches.  One should add 9.2 minutes to the dial reading.

You can find the center longitude of your time zone by noting how many hours your clocks are ahead of or behind Greenwich Mean Time GMT (also called Universal Time), where longitude is defined to be 0°.  For each hour that you are ahead of (behind) GMT, the center of your time zone is shifted 15° of longitude to the East (West).  For example, PST is 8 hours behind GMT, and thus its center longitude is 120° West.  For another example, India Standard Time is 4.5 hours ahead of GMT, and thus its center longitude is 67.5° East.

2. Adjustment for Summer Time

If Daylight Saving Time (Summer Time) is in force, add one hour to the sundial time to get clock time.

3. Adjustment for the Non-Uniformity of Solar Time

We have created clock time so that it proceeds absolutely uniformly -- a second is a second, an hour is an hour, day after day after day.  But the rate of solar time turns out to be slightly variable.  The interval from one solar noon to the next averages exactly 24 hours over a year, but it can be as much as 30 seconds longer or shorter on any given day.  This behavior of the Sun’s apparent motion is caused by the slight non-circularity (1.7%) of the Earth’s orbit and by the 23.4 degree tilt of the Earth’s rotation axis.  The net effect is that dial time can differ as much as 15 minutes from clock time (even if you are at the center of your time zone).  This adjustment is called the Equation of Time.  This is an old usage of the word “equation”; think of it as equating one system of time to another.  The required time to add or subtract for each day of the year is given at this website:

http://www.go.ednet.ns.ca/~larry/orbits/jsjdetst.html

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