The Noon Shot for Latitude–VIDEO
August 15, 2012
Given we have nice, clean sun again, it’s time you started learning how to use a sextant…
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Murre and the Pacific 
Cooooool! Love the instructional vids. How did our forefathers determine when noon was, without the fancy book that helps you calculate “Grinitch” time, and the digital time piece?
Surprisingly easy. Grab your sextant and go on deck well in advance of what you think noon might be. Measure the height of the sun. This is done by moving the arm and/or micrometer dial of the sextant until the super-imposed image of the sun is exactly on top of the horizon, a process called “Racking down the sun”, not “pulling down the sun” as I stated in the video. Wait a few minutes and then find the sun in your sextant again but without making any adjustment to the micrometer. If the sun is still climbing toward its zenith, it will now appear above the horizon in your viewfinder, not on top, where you had put it just a few minutes before. Now rack it down again and wait. Continue this process. As the sun gets on toward noon it slows. The adjustments you make between events will now be small ones, and then a few minutes before and after noon you won’t be making any adjustments at all–the sun will just seem to hang there and not move. At some point as you look to find the sun again it will appear incrementally BELOW the horizon because it has now moved beyond the zenith. Don’t move the micrometer…that last time you racked down the sun was your noon shot.
In short, if you’re racking down the sun before noon, it will appear to climb each time you raise your sextant; if after noon, it will appear to sink each time.
No watches required.
(Not so for the longitude shots.)
Clever indeed. I’m guessing the guy who had to do all the sun spotting was inevitably the same guy who ended up with the patch on one eye. So…it sounds like you have maybe 5 minutes in which to take your reading and still be accurate, yes? Somehow I’d gotten the idea you had to be within a few seconds. And also, what’s with this “enter your longitude” stuff? Given the scant attention you give this question, I assume there are markers or buoys placed in the ocean by our good friends at Grinitch, or perhaps printed fish trained to hover in particular spots, so’s you can simply read off your longitude as you sail by?
I tried calculating local noon as per your instructions and I am pleased to report: Success! Today (August 16) local noon (in Los Angeles) occurred at precisely 8:93 a.m. this morning.
Now that’s a local noon time of magnitude! I think I can say with some confidence that your noon was unique amongst all other local noons.
Let’s see, if I run the numbers here that puts your latitude (in LA) somewhere between Liberal Kansas, Paris Texas and the Republic of Tea.
It’s interesting how very easy things can be very difficult to explain. I knew in the video that my explanation was but general; how to make it more accurate from the deck of a rolling boat I could not figure.
So here:
Meridian Pass (i.e. noon in Greenwich England) on Aug 16, 2012 = 12:04 gmt
Los Angeles longitude = 118 degrees west (whole numbers OK. this can be rough: see explanation for finding noon without a clock to understand how slowly the sun moves at/near its zenith)
118 x 4 (4 minutes in a degree of longitude) 472 (this is the number of minutes it will take the noon Greenwich sun to move west and be noon at 118W)
472 minutes = 7 hours 52 minutes
12:04 +7:52 19:56 gmt (this is what time it will be in Greenwich when it’s local noon in LA)
I stop here. All calculations are done in GMT. But for you, LA is eight hours behind GMT, so
19:56 gmt -8:00 11:56 pst (local noon in LA in local time on Aug 16, 2012)
OK, I think I get it. I was unable to find “meridian pass” on a quick search so I got distracted by an alternate method I found that got me (almost) exactly one hour off of your calculation. The alternate method was to obtain sunrise and sunset data, and calculate the midpoint. I managed to foul even that up. Sunrise at 6:16, sunset at 7:39 (Tevya dancing on a roof top), convert to decimals that’s 6.266 and 19.65. Subtract, divide by two, then add the result to 6.266 and I got 12.958, or 12:57 and three quarters, just an hour off your calculation. What did I do wrong? I remember from Raiders of the Lost Ark you’re supposed to take “take back one Kadan, to honor the Hebrew God, whose ark this is.” I assume the same reasoning applies here?