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Equatorial sundials are basically the same as normal horizontal
dials, except that rather than the shadow being cast onto the flat base
plate, here the hour scale is formed into a ring set at right angles to
the gnomon. This results in the hour lines being evenly spaced
around the ring, unlike the different sized "wedges" on a horizontal
dial's base plate. The advantage of this is that the gnomon and hour
scale are now one unit which can be adjusted for the latitude at which the
dial is used. Should the sundial later be moved, this unit can be
re-adjusted to the new latitude and will still tell the correct time
because the gnomon and hour scale are still in the same orientation
relative to each other.
This dial [ref ED] is 22.5cm (9") diameter and is made of bronze. It is priced at £290. | |
The dial to the right is not adjustable for latitude, but has specially shaped gnomons (one for spring and one for autumn) which automatically correct for the Equation of Time (see Vertical Dials), so that it reads "watch time" directly. It is of solid brass, 38cm (15") diameter and with a base 28cm (11") diameter. [ref EDLS] £785. It can also be offered with a plain rod gnomon [ref EDL] for £490. |
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A sundial will
normally tell "local time" - local noon is when the sun is directly
overhead. This means that at the same time, a sundial in London will
say it is noon, whilst one set up in Truro will say it is 11:40am.
This is because it will take another 20 minutes for the Earth to rotate
the further 5 degrees necessary to have the sun directly overhead at Truro
(5 degrees west of London).
In the days when people travelled
by horse from one town to another, this discrepancy made little
difference. Pocket watches were not that accurate and would have to
be reset regularly by their owners, so they would merely reset their watch
to whatever was the local time. However, with the coming of the
railways, this state of affairs could not continue. |
| If a train
journey from London to Bristol took 2 hours then a London traveller
setting off at noon would arrive at 2pm "London time", but the Bristol
timetable would show the arrival due at 1:50pm ("Bristol time")! The
opportunity for confusion and missed trains was just too great, so the
idea of timezones was born and the globe sliced up into 15 degree segments
which would each take the local time at their centre and use this
throughout.
This now meant that sundials, which display local time, would not agree with watch time (regardless of the Equation of Time issue) unless they were located at the centre of their timezone. Every degree they were west or east of the centre resulted in a 4 minute difference in time. The sundial above is the perfect solution! Not only is it adjustable for latitude and self-correcting for the Equation of Time, but it can be set to make allowance for longitude too, by means of a moveable hour scale mounting. The dial is 22.5cm (9") diameter and of solid bronze. [ref EDS] £425 | |
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