Lewis Observes Latitude

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Lewis's Observations at Fortunate Camp

On August 19th, Lewis dutifully made and recorded a series of celestial observations of the sun (sun symbol) and moon (moon symbol) for longitude and a meridian2 altitude observation of the sun's lower limb (L.L.) at noon for latitude:

today I observed time and distance of the sun symbol's and moon symbol's nearest limbs with sextant3sun symbol East. it being the Point of Observation No. 43

Time (A.M.) Distance
h  m  s
°  '  "
11 37 11
65 53 15
 " 39 50
"  52 15
 " 44 15
"  50 45
 " 46 18
"  49 –
11 51 37
65 47 15
"  54 43
"  45 30
"  55 53
"  44 15
"  57 40
"  43 30
 " 59 30
"  42 30

Observed Meridian Altitude of sun symbol's L.L. with Octant4
by the back observation 69°15'00"
Latitude deduced from this observation. N. 44°37'57.4"

And, on the 20th, Lewis:

observed time and distance of the sun symbol's. and moon symbol's nearest Limbs with Sextant. sun symbol East.

Time (A.M.) Distance
h  m  s
°  '  "
11 37 11
65 53 15
 " 39 50
"  52 15
 " 44 15
"  50 45
 " 46 18
"  49 –
11 51 37
65 47 15
"  54 43
"  45 30
"  55 53
"  44 15
"  57 40
"  43 30
 " 59 30
"  42 30

Longitude deduced from this observation West of Greenwich [blank—not calculated]
Latitude N. deduced from the Hor 5 of the P.M. Observation of sun symbol's center 44° 33' 50.5"

Observed Equal Altitudes6 with Sextant of the Sun.

table of data derived from celestial observations

Observed Meridian Altitude of sun symbol's. L.L. with Octant by the back observation 70° ' "
Latitude deduced from this observation N. 44° 39' 43"

Finally, on August 21st, Lewis:

observed Equal Altitudes of the sun symbolwith Sextant.

table of data derived from celestial observations

Also observed Meridian Altd of sun symbol's L.L. with Octant by the back observation 72°00'00"
Latitude deduced from this observation North. 44°30'21.7"
Mean Latitude of the Forks of Jefferson's river deduced from three observations of the Meridian Altd of the sun symbol's L.L. with Octant, and one calculation by means of the hor: of the sun symbol's center in the P.M. observation for equal Altitudes on the 20th Instant N. 44°35'28.1"


1. The "No. 43" stands for "Point of Celestial Observation Number 43," their forty-third such point of observation since leaving Fort Mandan.

2. On the earth a meridian is any line that passes through the north and south poles and is perpendicular to the equator. The half of the meridian circle that is on the observer's side of the globe is called the upper meridian. A meridian altitude is a celestial observation taken when the object observed (usually the sun) is on the observer's upper meridian and thus at the highest point that it reaches above the plane of the equator that day.

3. Lewis's sextant was in the form of an arc encompassing one-sixth of a circle, hence the name sextant. Because a sextant uses an index mirror to reflect objects to a second mirror and then into the eyepiece, the angles (degrees) marked on its graduated arc are double the actual. That is, a sextant actually could measure an angle of 120°, but this compresses the angle (degree) marked on the graduated arc very close together even though the radius of that arc on Lewis's sextant was ten inches. Each of the degree marks, in turn, was equally divided into four parts of 15' each. The instrument also had a vernier (Lewis called it a nonius), which allowed him to interpolate the angle to 15". Finally, there was a micrometer screw that allowed him a further interpolation down to 7-1/2". To read this angle, even with the attached magnifying glass (Lewis called it a microscope) was no easy task, especially when it had to be done in a short time and under difficult conditions. Errors sometimes did occur. Like all precise scientific instruments, a sextant could become out of adjustment, especially when it had been banged about on a storm-tossed ship or in a canoe on the Missouri or Jefferson River. Lewis recorded that his sextant had an index error of 8' 45". What he actually meant was that the sextant read too high by 8' 45" and, to correct for this error the person doing the calculations had to subtract 8' 45" from the angle measured. Usually an instrument's index error is corrected before any other calculations are made.

A vernier is a scale for measuring smaller subdivisions of a scale or circle than can be done by estimation; Lewis called this device a nonius, but that term is obsolete now. On a sextant or octant the vernier measures an of arc of about 20° and is so graduated as to have one line less (or more) than the equivalent distance or angle on the main scale or arc. Lewis said that with the vernier he could read his sextant to 15 seconds (15") of arc. Therefore, its vernier contained 60 equally spaced lines that encompassed an arc of 14° 45' or (less likely) 15° 15'; that is, 60 lines on the vernier spanned the same arc as 59 (or 61) spaces of 15' each on the main scale. The 60 lines on the vernier were blocked into groups of four. The first line was marked with a symbol to indicate the 0 mark or starting point. The 20th, 40th and 60th lines were marked 5, 10 and 15, respectively.

4. The octant also was a device to measure the angle between two objects. Although its graduated arc spanned 45° (an eighth of a circle, hence octant) it could measure an angle of 90° (when used in the fore sight method) because it utilized the principle of reflection of a ray of light from two mirrors. Lewis's octant, with a radius of 14 inches, was fitted with an additional peep sight and mirror on the "back" limb. Using the octant in the "back sight method" Lewis, with the help of geometric principles, could thus "measure" an angle of 180°.

5. It appears that Lewis uses the abbreviation "Hor" to mean "horizon." Combining it with the geometric symbol for angle creates the expression "horizon angle." It refers to the altitude of the sun above the horizon at the time of the PM observation for Equal Altitudes.

6. An equal altitudes observation is a two-part process. The first set of measurements is taken when a celestial object is east of the observer's meridian and the second set is taken when the object is west of the observer's meridian. The equal altitudes observation provides information from which calculate several types of navigational information may be calculated. Lewis and Clark, however, used the observation exclusively to check the time of their chronometer.

This page is supported in part by a grant from the National Park Service CCSP Program.