Last time we established that the yardarm is not the whole yard, but just the tip of it outboard of the leeches, and moreover the commentators are all over the map concerning which yard to use. Yet any part of the rigging, which is constantly in motion, is a poor gauge for the altitude of a celestial body compared to the sextant. And why use such a clumsy measurement to tell time anyway, since by 1899 when our phrase first appeared, the chronometer had been in use well over 100 years?
The one thing the pundits all seem to agree on is that the sun rises over the yardarm at 11:00 AM, sometimes adding for good measure “in the northern hemisphere,” though they should know the hemisphere makes no difference.
As the builders of Stonehenge understood, the sun’s altitude varies by the date. At 1100* at the Tradewinds dock the sun was at a height of 50 degrees, roughly the angle of the main yardarm viewed from the mainmast, on March 24 and again on September 21. The yardarm will give the correct time twice a year. But by October it would not rise to this angle even at noon, while at summer solstice it would reach 50 degrees by 0916.
And as Eratosthenes demonstrated in about 240 BC, the sun’s altitude varies with latitude. So on March 24, the day the sun was at 50 degrees at 1100 here, it reached 50 degrees by 1014 local time in Hawaii, and never rose that high in Vancouver.
Were 18th and 19th century navigators such boneheads that they were not aware of these fundamentals? Methinks not.
If you’ve ever worked for a living or stood watch, you know the end of your shift is not a matter taken lightly, particularly if grog is in the offing. For regimentation of life aboard, for establishing the ship’s speed in dead reckoning, and for longitude, the accurate measurement of time was crucial. Nothing as vague as the sun’s proximity to a yardarm could ever have served.
But don’t get me wrong. If I overhear you on the dock one day saying, “The sun’s over the yardarm, mate,” I’ll know you’re not doing celestial navigation.
*Ignoring daylight savings, which would confuse things