A “like” button, or an “OK Fine” button?
I should elaborate, treat venereal diseases without rubber gloves.
:dubious:
So, the watch, as in “portable timepiece”, became indispensable for working out longitude. Meaning it had to run steadily and consistently. Does this mean that the watches, meaning shifts, were always reckoned according to GMT, or whatever port time was? If the ship from Southampton is picking up a hold full of durians in Borneo, would make the daylight/darkness aspect to the watch shifts inverted?
I could put in a good word for you with these guys.
I believe ship’s time was tied to a daily determination of “noon”, based on direct astronomical observation (of the sun at its zenith).
No. iIRC, for the longitude calculations to work, you had to compare GMT as preserved by the ship’s chronometer with local time, typically at midday. It was the difference that allowed yo to find longitude.
You can identify local midday at sea tolerably easily with a sextant or equivalent, and always could long before the chronometer came along. Watches on a ship during the 24 hours between midday sightings were calculated by turning hourglasses. They were good enough for a day or so, and given that they were effectively recalibtated every day when they found noon, that was good enough. Overcast weather got in the way, but not usually for long.
In harbour, you can’t see a horizon (a necessary part of finding noon with a sextant) and so the harbour master was responsiblele for firing a noon gun. Even more accurately, a large metal ball visible in the whole harbour, was dropped at midday, so that the lag caused by the sound delay from the cannon was eliminated. This is preserved in the dropping of the ball ceremony at midnight on NYE in places like New York.
I suspect that the wrist and pocket time keepers were name after the naval term.
That doesn’t seem as if it would be precise enough. You know approximately when the sun will be at its zenith; you can get out your sextant and measure the angle above the horizon, and get another reading a couple minutes later, etc. At its peak, the angle is not gonna change much over the course of minutes. Seems to me that trying to keep your sextant constantly on the sun, while on a moving ships’s deck, and to measure it so precisely that you can tell the instant it starts back down would be impossible. By the time it’s noticeably descending it’s too late, you’ve already missed it.
On the other hand, the moment in the morning when the first ray of sunlight clears the horizon is very distinct. Seems like that would be a much more accurate time check to use in calculating longitude.
I was at an exhibit a couple months ago on this very subject, at the Mystic Seaport. I wish I’d thought to ask.
Don’t know how easy it was (never tried it myself), and of course it probably entailed some degree of day-to-day error - but that was how it was done, not only to determine local noon but the ship’s latitude.
That timepiece is known as a chronometer (never as a “watch”). Typically, three are needed (so if one goes bad, you know which).
No, watches were and are based on local time, as Noel Prosequi notes. In the old days, this would have been referenced to local noon; nowadays, it would likely be referenced to a chosen time zone.
It’s a simple calculation: the difference in hours between local noon and GMT noon, multiplied by 15, equals longitude in degrees.
I actually have tried it. Only did it once, and I’m sure I’d get better with practice; I figured the latitude and was off by about 20 miles. The second mate was off by less than a mile.
If you chart the sun’s angle as a function of time, it would be a parabola. It goes up (relatively) quickly at sunrise, tops out at local noon, and then is descending quickly at sunset. Finding the exact top of that parabola is gonna be really tough. But if you take your siting a minute before noon, or a minute after, the actual angle (for determining latitude) is not going to be off by very much.
When I learned and tried it, I was very impressed with how precise the process really was. You match the bottom of the sun’s disk with the horizon, then you apply correction factors for the radius of the sun and the height of your eye above the water level, which are both fractions of a degree. I’d think that missing noon by a minute (to calculate longitude) would be unforgivably sloppy.
But doesn’t the idea of making a time-based calculation of your longitude depend on also knowing your latitude (which you can observe more-or-less accurately at noon, but not at all at sunrise)?
Yes, but I think you could interpolate. When you see the first ray of sunlight, record the time shown on the chronometer. At noon, use your sextant and calculate your latitude. If you know yesterday’s noon latitude, and today’s, you can figure out what your latitude was at sunrise (or a pretty darn close approximation). With that and the chronometer reading you can calculate your longitude (or at least what it was at sunrise).
I found this page that describes the problem and one way of solving it. In essence, take a sighting about 30 minutes before estimated noon (and record the time), leave the sextant set to that angle, record the time when the sun hits that same angle on its descent. Halfway between those two times is Local Apparent Noon.
And without washing or changing your clothes
Timekeeping at sea was tricky back then. The rough conditions; temperature variations and climate rendered the longitude calculation pretty hit or miss after a few days at sea, even with three clocks on board.
John Harrison, a Yorkshire carpenter, solved it but he only got part of the reward, partly, it’s said, because of his relatively humble background, and partly because it looked so simple.
I believe they compared the height of the sun not to the ship, but to the horizon. Perhaps I need more coffee.
Well according to Sobel’s book it was probably mostly because the board that awarded the prizes was stacked with astronomers, because it was assumed the solution would come via astronomy. A simple astronomical solution would have been better, frankly, because - if it existed which it doesn’t - it would work even if your chronometer failed or was lost.
The board probably didn’t like Harrison’s solution because it solved the problem by engineering rather than astronomy.
How is “2 or 3 parts water to one part rum” not watered?
It IS impossible. So you record several observations within a few minutes (before and after) of local noon. Set your sextant to the same angles that you got the best measurements for before local noon, and record the times when the sun goes through them again after noon. Obviously, the closer to exact local noon, the better, but if you do it well, taking the time halfway between two equal heights within a few minutes of noon, or especially averaging several halfway times, will be very close to right.