Watching the Heavens change copyright Bob Graham 2002
That determination of latitude was made from observations
of polaris--the north star. Why on earth would someone,
after an exhausting day of traveling for miles down a steep
mountain covered with ice and snow, get up at three in the
morning and stand shivering in the cold to shoot polaris?
Hadn't it been there all night? And wouldn't it still be
there for hours yet before dawn. Why not after dinner, or
before breakfast, instead of getting out of a warm sack in
the middle of the night?
But why did Frémont get up at such an ungodly hour to look at a star that was there to see throughout the hours of darkness? Edward Wright, in CERTAIN ERRORS IN NAVIGATION, 1590 wrote, "I must for the present only give the mariner warning that he not trust to it [the Pole-Star reading], being very erroneous, and grounded on two false positions." What was the problem with polaris? Part of the problem, in a time before optics, was in determining just how far polaris actually was from the celestial pole. In Drake's time, various tables had put it at from 4°9' (Cortes) to about 3° (Bourne). That would be the maximum correction a sighting would require if the star was above or below the pole--without correction a sighting would be off that much (a lot!).
Of course, in the mid 19th Century, Frémont had the advantages of very accurate instruments and tables of ephemerides which give the polaris correction for any day, hour, minute, and second that the sighting might be made. So why three in the morning? From his Ephereris he would have been able to calculate the time during the hours of darkness that that position would require no correction--a direct reading of latitude. But that was not what he wanted; he wanted to know the time of the position requiring the maximum correction. Frémont was after a precise determination, and because he was not absolutely sure of his Greenwich Time, he wanted the apparent change of position to be at its slowest--adding or subtracting the correction in the process of the reduction of the reading presented no difficulty at all.
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February
24, 1844. Frémont: We rose at three
in the morning, for an astronomical observation, and
obtained for the place a latitude of 38° 46' 58";
longitude 120° 34' 20" . The sky was clear and pure,
with a sharp wind from the northeast, and the thermometer
at 2° below the freezing point.
Because
of precession (wobble of earth's pole), in
Frémont's day polaris was nearly 1.5 degrees from the
celestial pole. Today it is only approximately 1 degree from
that pole. Going back further in California history, polaris
was nearly 3 degrees from the pole in 1579 when Francis
Drake spent 37 days on the coast making repairs to his ship.
Having spent as much time as I have following the trail of
Frémont and the wake of Drake, it is almost as if I
have been watching it move!
There
is a way around this. When polaris is left or right of the
celestial pole (exactly 9 or 3 on a virtual clock dial) the
error is zero. If one has time, one waits until the handle
of the Big Dipper and Cassiopia on the opposite side are
horizontal to polaris, and at that time the altitude of
polaris is the latitude. This is the sort of rule-of-thumb
published in survival manuals. The other way around is a
little nocturnal-like device (
This
phenomenon is most readily observable in the rising and
setting of the sun. It appears to jump up fast at daybreak,
hang in the sky at noon, and plunge rapidly to the horizon
at sunset. The sun does not, of course, actually speed up
and slow down in its transit of the sky. (Yes, I accept that
it is really the earth that rotates, but I get dizzy
thinking that way.) A simple diagram (rise over run)
will graphically demonstrate the reason for the apparent
rate of change. Notice how rapidly the sun rises in the sky
in the first hour between 6:00 and 7:00 and how little it
rises between 11:00 and Noon. It is just the reverse in the
afternoon hours. And it is exactly the same with
polaris.
So
on February 24. 1844 Frémont and Kit Carson did get
up at 3:00 a.m., The determination of latitude is very good.
You can go there and find the only place they could have
stood to take this sighting. The canyon itself is used as
the second line of position, because of problems he had been
having with the chronometer. His longitude determinations on
this part of the journey are off by over 20 miles. Not bad,
considering that it had stopped altogether near Bridgeport,
and he had been able to re establish Greenwich Time this
well from telescopic observations of the moons of Jupiter.
Twenty miles is good enough to make a landfall at sea. He
later was able to refine this time to make very good
determinations of longitude on the return leg of the
expedition.
The
results are shown on this page from the
Astronomical Tables of his Report. The altitudes
are doubled, because he used an artificial horizon
in his sextant sightings.
