Lewis and Clark's Celestial Observations
Clearwater Canoe Camp (Idaho) to the mouth of Snake River (Washington)
On the morning of September 27, 1805, wood chips flew as expedition axes bit into five large pine trees near Clearwater Canoe Camp. When the trees finally crashed to the ground, workers cut the trunks into lengths suitable for canoes then they began to hollow the tree trunks with axes, hatchets—and, not least, with fire—a method they learned from the Nez Perce. By the afternoon of October 6, the work parties had completed five dugout canoes. During this same period of time, Lewis and Clark took at least seven sets of celestial observations and would take two more sets several hours after sunset on the 6th. (see Clearwater Canoe Camp celestial observations). Loading the canoes began early the morning of October 7, and by afternoon the expedition started down the Clearwater River. Rapids, riffles and rocks were plentiful, and it required both careful watching and quick maneuvering to avoid these dangers. Nevertheless, incidents and accidents occurred, and repairs to the canoes had to be made. On October 10, after traveling about 40 river miles from its canoe camp, the expedition reached the junction of the Clearwater River and the Snake River. By the afternoon of October 16, the Snake's swift current and the men's well-plied paddles had brought the expedition another 140 miles downstream to the mouth of the Snake River. Before them flowed the largest river they had seen since wintering at Camp Dubois seventeen months earlier. There was no doubt in the captains' minds that this was the Great River of the West, the Columbia (or "Oregon," as it was known before Captain Gray renamed it)—and that they needed to take celestial observations here.
|Date||Type of Observation||Number of Measurements and Description|
|17||Equal altitudes of the Sun AM||3, altitude of the sun + the time (no equivalent PM observation)|
|17||Lunar distance from the Sun||12, angle between the moon and sun + the time|
|17||Magnetic declination with Sun||2, altitude and bearing of the sun + the time|
|17||Equal altitudes of the Sun AM||3, altitude of the sun + the time|
|17||Meridian altitude of the Sun||1, sun’s altitude at its highest daily point in the sky, octant|
|17||Equal altitudes of the Sun PM||3, altitude of the sun + the time|
|18||Double altitudes of the Sun||2, altitude of the sun + the elapsed time of the measurements|
|18||Lunar distance from the Sun||12, angle between the moon and sun + the time|
|18||Meridian altitude of the Sun||1, sun’s altitude at its highest daily point in the sky, sextant|
AM Equal Altitudes, October 17
On October 17, at the junction of the Snake and Columbia River, sunrise comes about 6:20 a.m. Local Mean Time. In 1805, on that date and at that time the sky was clear—perfect for taking celestial observations. The captains waited, however, until the sun had burned off some of the morning chill and had risen higher into the sky. Then, about 7:30 a.m., Lewis set out the artificial horizon and got the sextant out of its case. Meanwhile one of the men1 stood by with the chronometer, ready to read and record the time of Lewis's measurements of the sun's altitude. The first measurement was at 7:40:13 a.m.2, and the double altitude was 22°25'15" (this becomes 11°03'35" after halving the angle and correcting for sextant's index error and for refraction).3 Three and one-half minutes later Lewis had completed the last of the AM measurements for the Equal Altitudes observation. After taking this last altitude measurement, however, the captains must have reevaluated their observation plans. It was standard practice to leave the sextant's index arm locked at the AM setting until completing the PM set of observations. Those PM observations, however, could not be taken until about 5:30 p.m., during which time the sextant would be unavailable for other observations. If the sky clouded up later in the day or if it was cloudy the next day, the captains might miss an opportunity to take Lunar Distance observations for longitude and other observations of the sun for Magnetic Declination. The captains opted to reject the AM Equal Altitudes observation they had just completed and take the other observations.
Lunar Distance from the Sun, October 17
When Lewis began to take his first measurement of the angular distance between the moon and the sun, the sun was about 14° above the horizon and bore S60°E true. The moon (about two days past its last quarter) stood more than 54½° above the horizon and was nearly due south. Lewis completed the first angular-distance measurement for longitude at 7:51:43 a.m. The distance (uncorrected) between the near limbs of the two bodies, by his sextant, was 60°47'15". Sixteen minutes later (at 8:07:52) he obtained the twelfth angular-distance measurement. By that time the near limbs of the two bodies had moved 5'45" closer together. This rapid motion of the moon with respect to other celestial bodies is why Lunar Distance observations came to be the first practical observational method to determine longitude.
Magnetic Declination (Variation of the Compass Needle), October 17
The captains took a short break before beginning the observations to determine the magnetic declination. An assistant readied the chronometer while Clark set up and leveled the six-inch-diameter surveying compass. Meanwhile Lewis selected the proper sunshades on the sextant, then, looking at the artificial horizon through the horizon glass, he found the image of the sun on the water's surface. Next he moved the index arm with its mirror until his eye caught the image of the sun reflected from that mirror to the mirror on the horizon glass. When he had brought the upper limbs of the two images into contact he signaled to Clark and to the assistant at the chronometer, who recorded the time—8:15:45 a.m. Clark then read the bearing of the sun's center (S75°E), and this, too, was duly recorded together with the uncorrected double altitude of the sun's upper limb (33°04'30"). The captains took a second set of altitudes and bearings four minutes later, and the observations for magnetic declination at this junction were completed. The time was 8:19:43 a.m.
AM and PM Equal Altitudes Observation, October 17
Less than four minutes after completing the second observation for magnetic declination, Lewis began (at 8:23:00 a.m.) taking another set of Equal Altitude observations to replace the set taken earlier that morning. The final observation that morning was taken at 8:26:49 a.m. The sextant's index arm was locked at the altitude observed: 35°09'30" (true altitude of sun's center = 17°27'30.5" after corrections). The afternoon observations for Equal Altitudes were completed at 3:25:42 p.m.
Meridian Observation of the Sun with Octant, October 17
Neither Lewis nor Clark recorded taking a Meridian Observation of the sun on October 17, but is highly likely that they would have taken this observation using the octant in case the sky was cloudy the next day. Whitehouse, in the redraft of his journals for October 18, seems to confirm this: "Our officers delayed until after 12 o'Clock A.M. to compleat & prove the observation that they had taken Yesterday." This strongly suggests that Lewis took a Meridian Observation of the sun with the octant on October 17, but took the opportunity to check on that observation with the more precise sextant the next day.
Double Altitudes of the Sun, October 18
On the morning of October 18, the captains took two altitudes of the sun and noted the time (8:01:24 a.m. and 10:03:59 a.m.). Between the time of these two observations the sun had risen more than 15° higher in the sky. These observations could be used either to check the chronometer error or determine the latitude.
Lunar Distance from the Sun, October 18
In the interval between the two observations of the sun's altitude mentioned above, the captains took a Lunar Distance observation of the sun. This observation consisted of twelve sets of measurements (from 9:37:46 a.m. to 9:53:46 a.m.) of the angular-distance between the moon and sun's nearest limbs. At the mid-time of the observation (9:45:44 a.m.) the sun was at 27°27' above the horizon and bore about S35½°E true; the moon was 46°56' above the horizon and bore about S21°W; the uncorrected angular distance between the near limbs of the two bodies, was 47°12'33".
Meridian Observation of the Sun, October 18
The captains needed one last observation before the expedition set out down the Columbia River. This was the Meridian Observation of the sun for the latitude of this junction. At noon, Lewis took the double altitude of the sun's upper limb with the sextant and artificial horizon. The sun's uncorrected double altitude, as he measured it, was 68°57'30".
Calculations Made from the Expedition's Observations
The junction point between the Snake and Columbia rivers in 1805 was at or near 119°02½' W. On the Lewis and Clark's map of 1806 and that of 1814 (Moulton, Journals, vol. 1, Map 123 and 126), however, this junction is shown at about 119°40' W. If the captain's celestial observations had been calculated before the topographers of the Northern Railroad Surveys made their observations (1854), calculations from Lewis and Clark's observations likely would have been made by assuming a longitude of 119°40' or, more probably, 119°45'.4 The time at 119°45' W is 7 hours 59 minutes earlier than at Greenwich. Put another way, the Apparent (solar) Time at Greenwich for an observation made at Local Noon at the 1805 mouth of Snake River would have been 7:59:00 p.m.
1. Throughout the Expedition, the only celestial observations taken without an assistant were the Meridian observations of the sun. For observations where the chronometer's time was required, another person also read and recorded the time of the sextant measurements. Magnetic declination observations required two observers (usually Lewis at the sextant and Clark at the surveying compass) and one person to read and record the time shown by the chronometer. The captains never mention the name of the person or persons who read and recorded the chronometer time. Most of the time it may have been Clark, but when he was elsewhere or had other duties, it may have been one of the sergeants or possibly it was John Thompson (who may have had some surveying experience) or Reubin Field (who assisted Lewis to make celestial observations on the 1806 Marias River exploration).
2. Unless noted otherwise, all observation times are those shown by the chronometer, which was 9 minutes 15 seconds fast on Local Mean Time at noon on the 17th, based on the Equal Altitudes observation taken that day.
3. Refraction makes a light rays from an object in the sky bend upwards. The effect of refraction is especially strong near the horizon where the atmosphere is the densest; cold temperatures add to this effect. Most books on navigation caution against taking observations of celestial objects when their altitude was less than 15° above the horizon because the effect of refraction at lesser altitudes is highly variable.
4. The calculations for latitude, longitude and magnetic declination in this article were made using 119°45'.
Funded in part by a grant from the National Park Service's Challenge Cost Share Program