A sundial is a way of showing the passage of time by the
shadow cast on a graduated scale by a gnomon
(a solid object of some sort, such as a rod or a triangular plate attached to
the dial). As the planet turns, the sun appears to move across the sky, and
shadows move.
A sundial can be accurate, within
limits caused by the fuzziness of the shadow, variations in day length, and the
annual ‘equation of time’ which must be explained. As we look at it from here
on earth, the sun sometimes runs “fast”, and sometimes it runs “slow”. Around
November 2 each year, the sundial is 16 minutes fast, but by February 12, it
will be 14 minutes slow. If you ever visit a really accurate sundial, you will
be advised to make a correction for this oddity.
This sundial on the Schürstabhaus in Nuremberg in Germany is often described as “ancient”, but it is probably more correct to say it is “of ancient design”.
The variation happens because our planet’s orbit is an
ellipse, not a circle. If you really want to know more than this matter, look
up Kepler’s Laws of planetary motion. Look at the second of his laws.
The advertisement
above comes from an 1837 Sydney newspaper. It shows how early settlers could
keep their clocks right.
People (especially “Gentlemen”, meaning rich people) who
owned a clock or a watch used the sundial to set it, but why did the sundial
need to be adjusted? There was no radio, no telegraph, and no internet, so if
you wanted to keep the same time as the main or capital town there was a
problem. By definition, noon is when the Sun is at its highest point in the
sky, so it is always noon somewhere in the world, with a noon line sweeping
along through one degree of longitude every four minutes. Logically, you should
be setting your clock forward or back by a minute for each 20 kilometres or so
going east or west!
The sundial had to point north, though
magnetic north wouldn’t really be good enough.
To make life easier, we have split the world into time
zones, usually 15 degrees across, where everybody keeps the same “official
time”. If you are trying to set up a very accurate sundial, you need to make
allowance for your position east or west of the official time longitude in your
time zone.
Time is a very hard thing to explain,
but measuring time is a lot easier. We know that people have been measuring
time with sundials for at least 4000 years, and some people may have been
measuring, estimating and recording time for something more like 40,000 years.
You need a pencil,
a ruler, a square piece of board, a compass and some woodworking tools. Draw
diagonals on the board, mark its centre, and drill a hole for the pencil. Set
the pencil in the hole, and poke a piece of paper over the pencil. Rule a line
along one edge of the paper and take the sundial outside.
Use the compass to
set up the sundial with the line pointing north-south. Carefully mark where the
shadow of the tip of the pencil falls, and write in the time. Repeat this a
number of times during the day, and record what you see. Did the shadow move
equal distances each hour?
A
better sundial
This design is based on this one seen at Den Gamle By (that
means ‘The Old Town’) in Denmark. I have no background information on this
item, but as Den Gamle By is a ‘living museum’, I suspect that it is an old
design. Metalworking requires special tools and expensive materials, so I chose
a cheaper design, using strips cut from a manila folder to make a ring, about
12 cm across and 6 cm high:
Making the ring.
All I needed was a folder, a ruler, scissors, sticky tape, a
paper punch and string. I’m tired, so you
work out the rest from a close look at the pictures…
Completing the sundial.
To search this blog, use this link and then use the search box
Another way: use the index!
No comments:
Post a Comment