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.
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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