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The acrylic Spot-On Sundial
The sundial has 4 parts: the black baseplate, the white dialplate
with the hour lines, the black gnomon, and the clench pin. To
assemble, slide the foot of the gnomon (the end with the small
drilled hole) into the dialplate, and then insert the clench
pin into the drilled hole. This will hold the gnomon in position
at the correct angle. Then put the dialplate on the baseplate.
To set up the Spot-On Sundial on a windowsill, print out the
Solar Noon calculator on www.solar-noon.com. This will give
you a table which will show you the exact time of solar noon
(when the sun is exactly overhead) for every day of the year.
Make sure your windowsill is level. Wait till a day of clear
sun, and, at the exact time of solar noon, rotate the dialplate
till the line of light shines between the slit in the gnomon.
Your sundial is now pointing accurately to true North, and will
tell the current "sun time" without any further adjustment.
You can if you wish lock the dialplate in this position, either
with a piece of sellotape or by putting a dab of glue on one
of the dimples in the baseplate.
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Please note this Spot-On Sundial is not designed to be left
outdoor permanently. You would probably find someone walked
off with the dialplate.
The Spot-On Sundial has been designed to be as robust as possible,
but it will not withstand rough treatment or dropping, so please
treat it with care.
The basics of sundials are not very complicated, and they are
simply explained on the Sundial Information website at www.sundials.co.uk
Below, we give the answers to some common questions. For a full
explanation of how sundials work, we recommend a visit to the
website www.sundials.co.uk.
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Frequently asked questions:
(To know more than we can tell you in this brief summary, please
go to www.sundials.co.uk)
What is the gnomon? Gnomon is a Greek word meaning "indicator".
The gnomon of a horizontal sundial has to be parallel with the
axis of the Earth - this means that the angle between the gnomon
and the horizontal has to be equal to the latitude of the place.
(If the sundial were at the North Pole, the gnomon would have
to point straight up to be parallel with the earth's axis)
Which part of the shadow indicates the time? It is the
shadow of the top part of the gnomon which indicates the time.
You will notice that all the hour lines radiate from the point
where the top edge of the gnomon meets the dialplate; when this
shadow lies directly along a line, that is the hour shown
Why does the Spot-On Sundial have Roman numerals and Arabic
ones? The Roman numerals are for winter time and the Arabic
ones for summer time.
What do the short lines and dots round the edge indicate?
The lines are for every 10 minutes of the hour, and the
dots are for 5 minutes. So you can read the time to an accuracy
of o
ne minute if the shadow is over a line or dot, and within two
minutes if the shadow is between a line and a dot.
How do I align my Spot-On Sundial to true North? We recommend
that you print out a Solar Noon Calendar for your latitude and
longitude from www.solar-noon.com. Then, at the exact time of
solar noon from the calendar, rotate the dialplate till the
line of light shines between the two plates of the gnomon and
over the circle at the foot of the dialplate (between the 12
and 2 markings) |
Can I line up my Spot-On Sundial to true North using a compass?
Yes, if you allow for magnetic deviation, which varies from
place to place; you can find the current value for your location
(in the UK) from an Ordnance Survey map. You will find the "line
of light" method is more accurate.
What do I need to do if my latitude is not exactly 51.5 deg?
If you are more than 2 deg. away from 51.5 deg N, and if
you are very concerned to get accurate readings from your Spot-On
Sundial, you will need to tilt the North or South edge of the
baseplate very slightly. (see www.spot-on-sundials.co.uk/advanced.html
for more details if needed)
Why isn't solar noon at 12 o'clock? There are two reasons.
Our watches are set to the time at Greenwich. If you are in
Bristol which is some 2.5 deg. W of Greenwich, the sun will
be overhead at Bristol 10 mins. later than at Greenwich, so
you have to allow for the longitude correction. Also, watches
tell "mean" time - that is the time based on a "mean" or "average"
day of exactly 24 hours. But days in fact vary by up to 20 seconds
from 24 hours, depending on the position of the earth in its
orbit round the sun. So you need to correct for this too (known
as the Equation of Time). If you print out the Solar Noon Calendar
for your latitude and longitude at www.solar-noon.com, both
of these corrections are taken into account.
Why does the sundial need to be on a level surface? If
it is not on a level surface, the gnomon will not be at 51.5
deg to the horizontal, but at some other angle. So your sundial
may not be giving correct readings, depending on your latitude
and the slope of the sill. |
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