Why doesn`t my sundial show the same time as my watch?

Why doesn`t my sundial show the same time as my watch?
Why doesn't my sundial show the
same time as my watch?
by Don Mickey
The reason is that the sundial and the watch have slightly different ideas of what time means. The sundial
measures "apparent solar time." This means that noon on the sundial is when the sun is on the "meridian",
the imaginary line that divides the east half of the sky from the west half. And noon the next day occurs the
next time the sun is on the meridian. The length of a solar day varies a little from one part of the year to
another. And the location of the meridian depends on exactly where on Earth you are standing.
A watch, on the other hand, measures time in seconds, minutes, and hours so that each day has 24
hours and on the average the sun is in the same position each day at a given time. Watches are set to time
zones (generally an hour apart) so that everyone in a nearby area can set their watch to the same time.
This is called "standard time."
So if you want to read your sundial and figure out how to set your watch, you have to do two things. The
first is to know where you are on earth, compared to the standard longitude for your time zone. In Hawai‘i,
our standard longitude is 150 West, that is 150 degrees west of the zero longitude line. But the Hawaiian
islands are farther west than the 150W line, so the sun appears to pass overhead later than noon on the
watch. The correction is four minutes of time per degree of longitude. Hilo is about at longitude 156 W, so
solar noon is 24 minutes after noon standard time, and Honolulu is about at 158 W, so solar noon is 32
minutes late.
Adjustment in minutes
There's a second effect that is a little smaller for us in Hawai‘i. It is caused by two things: eccentricity
(the Earth moves at a varying speed in its orbit around the sun), and obliquity (the Earth's equator is tilted
compared to its orbit). This means we need another correction of up to about 15 minutes depending on
the time of year. The correction can be calculated, but it's simplest to keep a little chart like the one below
to read the correction from. The chart shows the effect of eccentricity as a blue line, the effect of obliquity
as a green line, and the total correction as a red line.
The equation of time chart
20
Here's an example... Suppose you're in HonoSundial faste
fasterr than watch
lulu, near the beginning of May, and your sundial
15
Total
tells you it's 10:00 in the morning. To get from
correction
10
this to standard time, first you need to correct for
due to
due to
eccentricity
obliquity
your location. You add 32 minutes for Honolulu,
5
giving 10:32 AM. Then you read the "equation
0
1 week interval
of time" correction from the red curve on the
–5
chart; it says the sundial is 3 minutes fast. Subtracting 3 minutes tells you standard time is
–10
Sundial slowe
slowerr than watch
10:29 AM. That should agree with your watch
–15
(unless you just came from California or your
Jan 1 Feb 12 Mar 26 May 7 Jun 18 Jul 30 Sep 10 Oct 22 Dec 3
Time of year
battery has given out).
www.ifa.hawaii.edu
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