Octant Octants
were used principally to determine latitude by measuring the
angle between the sun and the horizon. The instrument had a
simple “A” shape and its legs formed an angle of 45°, hence
its name (from Latin octans = one-eighth). Because the octant’s
two-mirror system used the principle of reflection it could
measure an angle of 90°, thus the instrument sometimes was
also called a quadrant. Most were made of hard, tropical wood.
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| Copyright 2007 and prior by Land
and Sea Collection, All Rights Reserved |
The index
mirror was attached to the index arm usually
12 to 14 inches long. The fore-horizon glass (half
mirror-half clear glass) was affixed to the leg nearest the
reflecting surface of the index mirror. A peep site was located
opposite the horizon glass on the other leg of the “A”.
The octant that Lewis and Clark used
was also equipped with a back-horizon peep sight and mirror.
This allowed them to measure the altitude of the sun when it
was greater than 60° above the horizon (early April until
late August along their route.)
This octant, purchased in Philadelphia for $22,
is similar to the one used by Lewis and Clark.
Sextant The
sextant, invented about 1757, had the same “A-frame” configuration
as the octant, but it's circular arc spanned an angle of
60°, that is, a sixth (Latin sextant) of a circle
and its index arm usually was 9–10 inches long. With
its two-mirror system the sextant could measure the angular
distance between objects that were as much as about120° apart.
The sextant was made of brass and had a telescope for sighting.
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Photo courtesy
of Smithsonian Institute |
Lewis
and Clark used the sextant to measure the angular distance between
the moon and sun or a star for longitude. They also used it for
latitude when the sun's noon altitude was less than 60°.
Although the sextant could measure an angle of up to 120°,
the captains could not use the sextant to find the altitude of
the sun when it was greater than 60° above the horizon because
they needed to use an artificial horizon, which doubled the angle
to be measured .
Artificial Horizons Navigators
at sea could determine the altitude of a celestial body by
measuring the angle between it and the true horizon. Inland
away from the sea or large body of water, however, one rarely
has a true horizon. Therefore, on land, it generally is necessary
to use an artificial horizon. Lewis and Clark carried three
different types of them. Each design had its own advantages
depending upon observing conditions
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Drawing by Bob Bergantino, MBMG |
On
bright days when the temperature was above freezing and the sun
was being observed, Lewis and Clark commonly used a tray filled
with water as an artificial horizon; the water made a level reflecting
surface. The captains, however, used artificial horizons made
from leveled mirrors when the temperature was below freezing.
They also used them when observing stars because the image reflected
from the mirrors was more distinct than from water.
Robert Patterson, a Philadelphia mathematician
and astronomer, had devised several types of artificial horizons
using either a regular (single-coated) mirror or the double-coated
index mirror from a sextant. The mirror was cemented onto a wooden
ball and the ball was set in a wooden frame and adjusted by three
screws used as legs. It was leveled with the aid of a spirit
level.
 Two-Pole
Chain English
and American landsurveyors of the 18th and early 19th
century generally used a surveyor’s chain constructed
of iron links each 7.92 inches long.Surveying chains,
called Gunters chains, normally were 66 feet long and
were comprised of 100 links. Eighty Gunters chains
equaled one statute mile and ten square chains (example:
5 chains x 2 chains) equaled one acre. A distance of
25 links (16½ feet) was called a “pole.” Clark
used the expedition’s two-pole (50-link, 33-foot)
chain primarily to measure a base line when determining
a river’s width by triangulation.
Two-pole chain similar to one used by Lewis and Clark.
Image from The Theory and Practice
of Surveying, 1911
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Chronometer A
chronometer is a precise watch used to determine the time at
which an observation is made. Chronometers usually are set to
keep Greenwich Mean Time (the time at 0° longitude) because
almost all nautical tables are based on Greenwich Time. Lewis
and Clark’s chronometer, however, showed Local Mean Time
and, because it stopped on several occasions and ran erratically
at others, they needed to make special observations of the sun
to determine the true Local Mean Time.
Checking the chronometer’s
time No chronometer
kept perfect time; they either lost or gained time,
though usually at a highly uniform rate. The captains
knew that their chronometer lost time, but unlike most
chronometers, its rate of loss was notconsistent. To
determine the chronometer’s error on Local Time, Lewis
and Clark made observations called “Equal altitudes
observations of the Sun.”
Using the sextant and artificial horizon,
they would take three measurements of the altitude of the sun
in the morning, noting the time shown by the chronometer for
each. In the afternoon, when the sun was at the same altitude
as it had been in the morning, the times again were recorded.
The average of the times, after adjusting for sun’s changing
declination, gave the time that the chronometer would have shown
at noon. The difference between this calculated time and 12 noon
was the “error of the chronometer.” From another Equal altitudes
observation the next day or some days later, the captains could
determine the chronometer’s daily rate of loss during that period
of time.
At a purchase price of $250
(plus 75 cents for a winding key), the chronometer was
the single most expensive piece of scientific equipment
taken on the journey.
“… her
balance-wheel and escapement were on the most improved construction.
She rested on her back, in a small case prepared for her,
suspended by an universal joint, she was carefully wound
up every day at “twelve o'clock.” (Lewis,
July 22, 1804) ).
Surveyor's Compass or Circumferentor Lewis
and Clark used the surveyor’s compass to record their
direction of travel and determine the bearing of celestial
objects such as the sun and stars. Because the bearings they
observed were based on magnetic north they often took observations
of Polaris or the sun at the junction of major streams or
other important points to determine the difference between
magnetic north and true north. Most maps are based on true
north which does not change through time or with location.
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Photo
courtesy of Smithsonian Institute |
The
circumferentor was simply a six-inch-diameter magnetic compass,
but was much more precise than the usual pocket or hiking compass
and could be read to one-half of a degree. The circumferentor
was equipped with front and rear sight vanes to help improve
the observer’s ability to sight precisely on an object.
On the underside of the compass housing was a receptacle to allow
it to be fastened on a tripod or to a shaped pole that was placed
firmly into the ground. Attached to that receptacle was a ball-and-socket
joint that allowed the user to level the circumferentor with
great accuracy. A spirit level would have been used in conjunction
with the surveyors compass to establish a level plane from which
to work.
The circumferentor Lewis and Clark used was six inches
in diameter and was divided into degree and half-degree increments.
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| Photo courtesy of Smithsonian
Institute |
Pocket
Compass
Lewis purchased three pocket compasses for $2.50 each and also
the compass shown in this picture. The pocket compasses were
less cumbersome than the surveyor’s compass and were used to
take the bearings when impractical to use the circumferentor
and when traveling on land.
This pocket compass was one of the few items that survived
the journey. After the expedition returned to St. Louis, it
was purchased by William Clark. The compass is on display at
the Smithsonian Institution in Washington, D.C.
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| Drawing by Bob Bergantino,
MBMG |
Spirit
Level A spirit level
is similar to a miniature carpenter's level, though made
with greater precision. It was used to level the surveying
compass (circumferentor) for precise angular measurements
and to level a mirror to produce an artificial horizon
when water was not used. Clark also used the spirit level
as a hand level when he measured the height of the falls
and rapids of the Missouri from Lower Portage Camp to White
Bear Islands (see Great Falls of the Missouri).
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| Image from The
Nautical Almanac and Astronomical Ephemeris, 1805 |
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Lewis carried with him The
Nautical Almanac and Astronomical Ephemeris … for
the Year 1803, 1804 and 1805 (it is not certain that he
had the 1806 almanac). The almanacs were published several
years in advance of the year indicated on them and gave the
celestial coordinates of the sun, moon, planets, and navigational
stars at various increments of time at Greenwich, England.
A
companion book to the Nautical Almanac was: Tables Requisite
to be Used with the Nautical Ephemeris for Finding the Latitude
and Longitude at Sea (Nevil Maskelyne). These books provided
Lewis and Clark with valuable information on how and when
to take observations, what corrections they needed to make
to obtain valid results, and how to calculate geographic
data from most of their observations. In addition, Lewis
brought along a copy of A Practical Introduction to Spherics
and Nautical Astronomy (Patrick Kelly) — a practical
guide to spherical trigonometry and how to compute celestial
observations and convert them into geographic information. |
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