The following table, called an “Equation Table,” gives the difference in minutes, and you will notice that the sundial and clock both agree on four occasions during the year: 15th April, 14th June, 1st September, 25th December.
| EQUATION TABLE. | |||||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| FAST means that the Watch should be Faster than the Dial. SLOW, Slower. | |||||||||||||||||||||||||||||||||||
| JAN. | FEB. | MARCH. | APRIL. | MAY. | JUNE. | JULY. | AUG. | SEPT. | OCT. | NOV. | DEC. | ||||||||||||||||||||||||
| Days. | Mins. | Days. | Mins. | Days. | Mins. | Days. | Mins. | Days. | Mins. | Days. | Mins. | Days. | Mins. | Days. | Mins. | Days. | Mins. | Days. | Mins. | Days. | Mins. | Days. | Mins. | ||||||||||||
| 2 | Fast | 4 | 3 | Fast | 14 | 4 | Fast | 12 | 1 | Fast | 4 | 2 | Slow | 3 | 4 | Slow | 2 | 4 | Fast | 4 | 4 | Fast | 6 | 1 | 0 | 1 | Slow | 10 | 11 | Slow | 16 | 1 | Slow | 11 | |
| 4 | 5 | 20 | 14 | 8 | 11 | 5 | 3 | 15 | 4 | 10 | 1 | 10 | 5 | 12 | 5 | 5 | Slow | 1 | 4 | 11 | 17 | 15 | 4 | 10 | |||||||||||
| 7 | 6 | 27 | 13 | 12 | 10 | 8 | 2 | 28 | 3 | 14 | 0 | 19 | 6 | 17 | 4 | 8 | 2 | 7 | 12 | 22 | 14 | 6 | 9 | ||||||||||||
| 9 | 7 | 16 | 9 | 12 | 1 | 20 | Fast | 1 | 22 | 3 | 11 | 3 | 11 | 13 | 25 | 13 | 6 | 8 | |||||||||||||||||
| 11 | 8 | 19 | 8 | 15 | 0 | 24 | 2 | 26 | 2 | 13 | 4 | 15 | 14 | 29 | 12 | 11 | 7 | ||||||||||||||||||
| 14 | 9 | 23 | 7 | 20 | Slow | 1 | 29 | 3 | 29 | 1 | 16 | 5 | 20 | 15 | 13 | 6 | |||||||||||||||||||
| 17 | 10 | 26 | 6 | 25 | 2 | 19 | 6 | 27 | 16 | 15 | 5 | ||||||||||||||||||||||||
| 20 | 11 | 29 | 5 | 22 | 7 | 17 | 4 | ||||||||||||||||||||||||||||
| 24 | 12 | 25 | 8 | 19 | 3 | ||||||||||||||||||||||||||||||
| 28 | 13 | 28 | 9 | 21 | 2 | ||||||||||||||||||||||||||||||
| 23 | 1 | ||||||||||||||||||||||||||||||||||
| 25 | 0 | ||||||||||||||||||||||||||||||||||
| 27 | Fast | 1 | |||||||||||||||||||||||||||||||||
| 29 | 2 | ||||||||||||||||||||||||||||||||||
| 31 | 3 | ||||||||||||||||||||||||||||||||||
The sundial, as already mentioned, shows solar time, but by adding or deducting the differences, as shown above, local mean time is found, and by again adding or deducting the difference of longitude, Greenwich mean time is the result. For instance, a sundial at Wrexham (3° West longitude) on the 11th March indicates 11 o’clock, and we want to find Greenwich mean time. We proceed as follows:—
| H. | M. | |
|---|---|---|
| Wrexham Sundial | 11 | 0 |
| Difference of Equation, add | 0 | 10 |
| Local Mean Time | 11 | 10 |
| Difference of Longitude, add | 0 | 12 |
| Greenwich Mean Time | 11 | 22 |
We see by the above, that the watch should be 22 minutes faster than the dial.
A well-made sundial should have engraved upon it an equation table and the longitude of the place where it is fixed, and the consequent allowance of time to be added or subtracted to find Greenwich mean time.
SUNDIALS SHOULD BE FIXED on a bright, sunny day, a horizontal dial being fixed as follows:—First see that the stone pedestal on which the dial is to be fixed is perfectly rigid, also flat and level on top. Remove with a pair of pinchers or plyers the three button-headed studs that are usually fitted on the back of the dial, then place the dial approximately in position by moving it about until the shadows show the time within a few minutes, and with a pencil, mark the positions of the studs on the stone through the holes in the plate. Remove the dial and replace the studs in the dial; cut the three holes about twice as large as the heads, so that the dial has plenty of play to the right and left to facilitate final adjustment. Mix with water a little Portland cement, which is known as “grouting,” damp the holes in the stone and pour the grouting in and place the dial in its place, turning it until it shows correct solar time of the place, taking care that the plate is level, and then allow the cement to set. The dial is now fixed, and requires no further refixing at any time.
To ascertain correct solar time for fixing purposes proceed as in the example given here, and for the purpose we will take Andover (1½° West) as the place where the dial is about to be fixed on 1st October.
| H. | M. | |
|---|---|---|
| Greenwich Mean Time | 10 | 30 |
| Difference of Longitude, deduct | 0 | 6 |
| Local Mean Time | 10 | 24 |
| Difference of Equation, add | 0 | 10 |
| Andover Solar Time | 10 | 34 |
So we see that on 1st October the dial must be fixed 4 minutes faster than the watch.