Navigation and Daylength
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In which one shall learn how to know the length of the day, and to know how much the day is shortened or lengthened by the Sun's declination.
The Day varies by the Suns Declination
The common people fall into a number of errors concerning the length of the day, holding an opinion that every 30 days the day is an hour longer or shorter. The truth is this; the day keeps no such rule in its lengthening and shortening, but lengthens and shortens according to the swiftness or slowness of the sun's declination, for when the sun has a swift declination, then the day lengthens and shortens apace; and when the declination is slow, then the days lengths and shortens slowly.
Length of Days in Seagate Port
And yet the best part of the common people of Seagate hold the opinion that by Twelfth Night the day needs to be a full half hour longer than the Solstice, and yet the sun has declined or come towards the Equinoctial less than 1 degree, which will not make even a quarter hour in the length of the day. Wherefore, I think it good to declare throughout the whole year when the day is an hour longer or shorter here in this port of Seagate for the Latitude of 37 degrees. Our longest summer day is 14 hours and 34 minutes long, and this day is the 14th or 16th of Heat, and then the sun has the least declination southwards; and our shortest winter day is 9 hours and 26 minutes from sunrise to sunset, and this shortest winter day is the 14 or 16th day of Snow, and then the sun has the greatest declination southwards.
- Day Length at Seagate
| Summer | Autumn | Winter | Spring | |||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Beltane | 13:45 | Lugnasad | 13:45 | Samhain | 10:15 | Candlemansa | 10:15 | |||
| 9 Meadow | 14:00 | 6 Fruit | 13:30 | 9 Frost | 10:00 | 6 Thaw | 10:30 | |||
| 3 Heat | 14:30 | 19 Fruit | 13:00 | 3 Snow | 09:30 | 20 Thaw | 11:00 | |||
| Solstice | 14:34 | 2 Harvest | 12:30 | Solstice | 09:26 | 3 Seed | 11:30 | |||
| 27 Heat | 14:30 | Equinox | 12:00 | 27 Snow | 09:30 | Equinox | 12:00 | |||
| 21 Breeze | 14:00 | 27 Harvest | 11:30 | 21 Ice | 10:00 | 28 Seed | 12:30 | |||
| 10 Vintage | 11:00 | 11 Blossom | 13:00 | |||||||
| 24 Vintage | 10:30 | 24 Blossom | 13:30 | |||||||
And you will find this matter more thoroughly spoken of for all places throughout the world, in the rest of this chapter.
Length of Days outside the Arctic Circles
I think it is convenient for Seafarers to know the length of the day in any place that they have occasion to go to, for they have occasion to travel into all the climates and places of Alusia, transporting themselves many times quickly from one place unto another. And although the ancient writers have described various climes, and have made tables very exact for the longest or shortest day in any of those climes, and other places of similar elevation of the pole, yet they have not provided any way to give a method for travellers to know when the day is an hour longer or shorter, whereby they might at all times know the length of the day, which is very necessary for those that are abroad under sail both night and day, and likewise for those that must keep account of hours and times exactly, as should anyone keeping a Ships Log. Wherefore it is most necessary for Navigators to know the true time of the Sun rising and setting, which you may know from the length of the day.
First, under the Equinoctial, the sun rises twelve hours from its setting, regardless of its declination, so that it rises at 6 am and sets at 6 pm, forever. And the further you get from the Equinoctial, the more the length of the day varies. The day length can be found by knowing the declination of the Sun and your Latitude. At any given Latitude, there is a fixed change in Declination which results in a day being one hour longer. This Declination per hour and the length of the longest day at various Latitudes is shown here.
- Day Length
| Latitude | Longest Day | Decl. /hour |
|---|---|---|
| 0°00' | 12 hours | N/A |
| 10°00' | 12 hrs 35 min | 40°05' |
| 16°43' | 13 hours | 23°30' |
| 20°00' | 13 hrs 13 min | 19°21' |
| 30°00' | 13 hrs 56 min | 12°07' |
| 30°46' | 14 hours | 11°45' |
| 37°00' | 14 hrs 34 min | 9°10' |
| 40°00' | 14 hrs 51 min | 8°14' |
| 41°21' | 15 hours | 7°50' |
| 45°00' | 15 hrs 26 min | 6°50' |
| 48°59' | 16 hours | 5°53' |
| 50°00' | 16 hrs 10 min | 5°39' |
| 54°28' | 17 hours | 4°42' |
| 58°25' | 18 hours | 3°55' |
| 60°00' | 18 hrs 31 min | 3°36' |
| 61°16' | 19 hours | 3°21' |
| 63°20' | 20 hours | 2°56' |
| 64°48' | 21 hours | 2°37' |
| 65°46' | 22 hours | 2°21' |
| 66°19' | 23 hours | 2°8' |
| 66°30' | 24 hours | 1°58' |
| 70°00' | 62 days | 1°40' |
| 80°00' | 127 days | 0°50' |
| 89°00' | 177 days | 0°05' |
From this method you may know at all times the true length of the day in any Latitude between the Equinoctial and the Arctic circles, by knowing the declination of the Sun, which you may know by the Regiment in a previous chapter.
What a Day is
And you must note that a day is from the rising of the Sun, where it is but half above the Horizon, until the setting of the same half under the Horizon, and not from daylight until daylight. For before the Sun rises, and after it has set, does not count as part of the day, even thought the day light will appear by the time that the Sun reaches 17° below the Horizon before sunrise, and will not be clean gone until the Sun is more than 17° under the Horizon after sunset. And you may perceive this if you travel but to 50° North in mid Meadow, when the Sun has the greatest declination to the North, the daylight remains all night, as the Sun goes under the Horizon a maximum of 16°30'.
