Scratch dials - a closer look

Updated 11 June 26

This replaces an earlier post.

As noted in the previous post, between the Spring and Autumn equinoxes, such dials, being South-facing, are in shadow nearer sunrise and sunset. Let's use Stellarium for a few examples at 51 degrees North:

- at the Summer solstice, the Sun enters the Southern half of the sky (i.e. lies due East) 3 hours and 40 minutes after sunrise, when its altitude is about 30 degrees and the dial therefore indicates the end of the second hour and the start of the third (no good if we need to know when the second hour starts and similarly for the final hour); the dial is illuminated by the Sun for just over half the daytime,

- about the start of Summer, May 4th say, the Sun enters the Southern half of the sky 2 hours and 20 minutes after sunrise, when its altitude is about 20 degrees and the dial therefore indicates part way through the second hour (again no good if we need to know when the second hour starts, but perhaps ok for the start of the final hour, which can be guessed at); the dial is illuminated by the Sun for just over two thirds of the daytime,

- even in mid-April, less than 4 weeks after the equinox, the Sun enters the Southern half of the sky 90 minutes after sunrise, when its altitude is about 13 degrees and the dial therefore indicates nearly the end of the first hour (and so is likely still usable); the dial is illuminated by the Sun for just over three quarters of the daytime. 

This demonstrates the limited usability of this type of dial when the Sun is North of the celestial equator - surely there were complementary dials for these months? Perhaps something as simple as a North-facing dial of the same type? In this case, it would need marks for the first two and final two hours only of the daytime. In the morning, the shadow cast by the gnomon would be horizontal at sunrise and would rotate until reaching a particular angle at which time the entire dial would go into shadow and the South-facing one would then be illuminated, continuing at the correct angle and hour. Later in the day, the process would be reversed as the South-facing dial went into shadow and the North-facing one became illuminated.

A scratch dial

Updated 11 June 26

Also known as a mass dial, although I seem to recall that strictly speaking there is a distinction. I came across this one by chance at St. Catherine's Church, Montacute, Somerset. It's a vertical sundial that surely pre-dates the Church clock of around A.D. 1400. The hole centre bottom would have held a horizontal gnomon to cast a shadow for telling the time. 

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Originally, I wrote: There are 12 lines radiating from the gnomon hole for indicating the hours from 6 am to 6 pm, albeit inaccurately, as they are spaced regularly (15 degrees) and with no regard for latitude.

However, having read the section Medieval Times and Canonical Hours here on the excellent GAUDIUM SUB SOLE website, I was reminded that centuries ago, day and night were divided into 12 hours each, an hour in the day-time being longer than at night-time in the Summer, for example. 

Then I wrote: There are 12 lines, spaced at 15 degrees, radiating from the gnomon hole for indicating the 12 daytime hours from sunrise to sunset, the length of the hour depending upon the time of year. While perhaps crude in execution, it and similar dials were sufficiently accurate for the needs of the period.

However, on further reflection, this is still only partially correct! These dials show sunrise and sunset only when the Sun is fully in the Southern sky, i.e. between the Autumn and Spring equinoxes. Between the Spring and Autumn equinoxes, for part of the day, after sunrise and before sunset, the Sun is in the Northern sky and such dials are in shadow. More to follow...

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The dial is unusual, being upside down, and hence unusable! Presumably the stone was re-used when the dial was no longer needed. 

More info. on this dial from GAUDIUM SUB SOLE here, along with another dial at the church that I missed!

Note - the design was already ancient when it was made, being the same type as this Ancient Egyptian dial.