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Sundial - Isan
Lunar calendar
The sundial can not be used as a lunar watch. When marking the
shadow of the moon it soon becomes evident that the moon and the sun
follow different rhythms. The sun's shadow is predictable on the dial;
the moon's is not. The distance between the daily path from full moon to
full moon varies, so does the hour-length and the position. But after
having made some of the hour-marks one can extrapolate.
The sundial can be used as a lunar calendar predicting
the full moon; which is regarded as useful in the countryside of
Thailand and Laos, where the lunar-month still is in daily use. After
new moon there are 15 "ascending" days until full moon, after which there are 15 or 16 "descending" days. The lunar
month has four holy days (wan sin): New moon day, 2 half-moon days, and
the full-moon day, of which the latter is the most important.
Present days Cambodians are most likely to
adhere to the same kind of lunar month as they share the same Theravada
Buddhism as in Thailand and Laos nowadays Cambodia.
The ancient Khmer would have followed Hindu practice
of lunar system and operating with 27 or 28 asterisms. |
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When marking the position of the
moon's shadow with one hour intervals on six days around full moon, the
pattern comes out like this. The moon-shadow is moving more
"slowly" than the shadow of the sun - nearly one hour a' day.
A lunar day is shorter than a solar day, making a lunar month 29.5 days.
When the shadow of the midnight (mean time) has passed the north-south
oriented line (solar noon) it is the 15th day.
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Lunar analemma
Observations from around the
full-moon in January 2001.
The marks represent the position of the shadow of the moon at full clock-hours (mean
time = watch time - BBC) over 10 days.
Linking the marks following a 25
hour interval a part of an analemma shaped figure appears.
The moon goes more slowly than the sun. It rises app. 1 hour later every
day: 25 = 24 + 1
The time between successive new moons is 29.5 days (709 hours)
The green line shows the path of the moon's
shadow the 14th of January 2000 at the first quarter.
The shadow of the half-moon is close to the north-south projection of the
gnomon at 18.00 o'clock, indicating that there is 7 days to full moon.
The red lines connect marks made at similar mean-time, here starting
behind the stick, which is 18.00 o'clock.
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Positions of the moon shadow at full
mean-time hours. As above, but made by ASTRONOMIC CLOCK.
Remember that the sundial is located
3 minutes from local time-zone (west of time zone), so solar time and mean
time position are very close to one another (2 cm on the sundial floor).
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Using moon shadows
to predict sun shadows
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Around
winter solstice the shadow of the sun will follow its northernmost path.
If there is a full-moon around winter solstice, the shadow of the moon
will follow a path close to the sun's southernmost path!
In the weeks around any full-moon the daily path of the moon's shadow
will change declination, which takes the sun a week or more (see photo
below):
The moon's path "3 days before full-moon" followed the sun's
path the 18th of March.
The moon's path "1 days before full-moon" followed the sun's
path the 14th of April.
The moon's path "2 days after full-moon" followed the sun's
path the 10th of May.
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This
has practical implications for house-construction! When the author for
example extents the roof of his cottage, he always makes a frame first,
and at night-time gets an impression of how the roof will shadow in
various months.
The months around the solstices are the best. At day-time the sun will
tell the shadows. At night-time the moon will show the shadows 6 month a
'head.
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Using moon shadows
to predict sun alignments
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The
moon's shadows can also be used to substitute the sun's shadows when
checking alignments of constructions.
For example: The author would like to check some carved alignments
observed at That Phanom Rung. This temple is aligned at azimuth 84.5
deg., and the carved lives are aligned at azimuth 90 deg. - indicating,
that the inhabitants were aware of, that the temple was not aligned at
the equinox point, but anyway focused on it. But the author has no time
to visit That Phanom Rung at equinox, because he will have to work on
his Equinox Path at those days. So instead he plans to visit That Phanom
Rung on August
26, 2002, at 21:06:26, when the moon will have azimuth =
90*00’00’’, altitude = 4*30’16’’ and apparent size =
87,6 % (Source: ASTRONOMIC CLOCK, see picture below).
For visibility
of the moon through all 15 gates of That Phanom Rung, see The
Four Days
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Last updated:
24 November 2003 |
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