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Hi diallists!
This is a hint for teachers and those who
want to
make an analemmatic sundial in their gardens. You do not need to draw the whole ellipse,
just the
hour marks, and for that purpose the method of the evolute works better. Suppose you have calculated the major (a)
and minor (b)
semiaxis of the ellipse for your latitude. Then proceed as follows: 1. Take a thin wooden lath or a courtain
bar (even
a string could do, if you don't have anything else) as long as the major semiaxis (a). 2. Place a mark into it at a distance b from
one of its tips.
Then the bar has two arms, so to say: the long one whose length is b and the short one whose lenght is a - b. Right? 3. OK. Now place marks on the major axis at
the following distances:
D_i = b
* sin(HourAngle_i)
=
= a * sin(Latitude) * sin(HourAngle_i) where the HourAngle's are 0 deg at noon, negative
in the
mornings and positive in the evenings. (It is also very easy to make a graphical construction to determine these dots). 4. Nearly finished. Then we just have to
place the mark into the
bar on one of these dots, rotate the bar so that the closest tip (ie., the one at distance a - b) just touches the short axis. Then the other tip lays in the corresponding hour mark. 5. Repeat the latter step for every hour
angle and that is it!
Anselmo Perez Serrada
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