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n the two previous sections, we have seen how the difference between sundial
time and standard time depends on two effects: the eccentricity of the Earth's
orbit and the inclination of the Earth's orbit.
The combination of these two effects, which is the
true equation of time, is plotted in Fig.6. In December and
January these two effects are both working to slow the
sundial time, while in June and July the two effects are
opposed to each other. The sundial lags only six minutes
during June when the two effects are opposed, but lags 13½
minutes during December. The equation of time expresses the
relationship between the sundial and standard time, and the
standard time is then available from the sundial by
applying the proper value, plus or minus, from the equation
of time. But such conversion yields true standard time
only if the sundial is on the standard meridian. One must
know one's distance east or west of the standard meridian in
order to make the remaining correction to the sundial time.
The Earth turns through one time zone in an hour.
The time zone is 15 degrees wide (one twenty-fourth of 360
degrees), so each degree of longitude within the time zone
is equivalent to four minutes of time (60 min./15o). This
then is the correction to make for each degree of longitude
away from the standard meridian: minus if east or plus if
west of the standard meridian
As an example, suppose that you are located at
longitude 155 degrees west. What is the correction to
arrive at standard time for your time zone? The standard
meridian is the 150 degree west meridian, so you are located
5 degrees west of that. Every degree is 4 minutes of time,
so the sun passes overhead at your longitude 4 x 5 = 20
minutes later than at the standard meridian. Thus, you must
add 20 minutes from your sundial time to get the standard
zone time. This, of course, is in addition to the time that
must be added or subtracted according to the equation of
time. See Appendix B for additional examples.
Figure 6 The Equation of Time
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