Prob. XVIII. The Latitude of any place, not exceeding 66½ degrees, and the day of the Month being given; to find the time of Sun-rising and setting, and the length of the Day and Night.
Having rectified the globe according to the latitude, bring the Sun’s place to the meridian, and put the hour index to 12 at noon; then bring the Sun’s place the Eastern part of the horizon, and the index will shew the time when the Sun rises. Again, turn the globe until the Sun’s place be brought to the Western side of the horizon, and the index will shew the time of Sun-setting.
The hour of Sun-setting doubled, gives the length of the day; and the hour of Sun-rising doubled, gives the length of the night.
Let it be required to find when the Sun rises and sets at London on the 20th of April. Rectify the globe for the latitude of London, and having found the Sun’s place corresponding to May the 1st, viz. ♉ 10¾ degrees, bring ♉ to 10¾ degrees to the meridian, and set the index to 12 at noon; then turn the globe about ’till ♉ 10¾ degrees be brought to the Eastern part of the horizon, and you’ll find the index point 4¾ hours, this being doubled, gives the length of the night 9½ hours. Again, bring the Sun’s place to the Western part of the horizon, and the index will point 7¼ hours, which is the time of Sun-setting; this being doubled, gives the length of the day 14½ hours.
Prob. XIX. To find the length of the longest and shortest Day and Night in any given place, not exceeding 66½ degrees of Latitude.
Note, The longest day at all places on the (North/South) side of the equator, is when the Sun is in the first point of (Cancer/Capricorn) Wherefore having rectified the globe for the latitude, find the time of Sun-rising and setting, and thence the length of the day and night, as in the [last problem], according to the place of the Sun: Or, having rectified the globe for the latitude, bring the solstitial point of that hemisphere, to the East part of the horizon, and set the index to 12 at noon; then turning the globe about ’till the said solstitial point touches the Western side of the horizon, the number of hours from noon to the place where the index points (being counted according to the motion of the index) is the length of the longest day; the complement whereof to 24 hours, is the length of the shortest night, and the reverse gives the shortest day and the longest night.
| Longest Day. | Shor. N. | |||
|---|---|---|---|---|
| Deg. | Hours. | Hours. | ||
| Thus in Lat. |  | 45 | 15½ | 8½ |
| 51½ | 16½ | 7½ | ||
| 60 | 18½ | 5½ | ||
If from the length of the longest day, you subtract 12 hours, the number of half hours remaining, will be the Climate: Thus that place where the longest day is 16½ hours, lies in the 9th Climate. And by the reverse, having the Climate, you have thereby the length of the longest day.