Challenges for Hassler

Page 12 of 13

It's not surprising that Ferdinand Hassler had a negative attitude toward calculating the celestial observations Lewis and Clark made. Most of them actually present little or no problem and are merely very time-consuming to interpret. Others, however, might have been much more challenging. For one example, let's look at the observations taken on 20 May 1805 at and near the mouth of the Musselshell River. Hassler never got anywhere close to having to decipher those data sets, but here's what he would have had to contend with:

For the Lunar Distance observation, Lewis gives just the mean of the time and the distance for 12 data sets. Clark gives the actual values for the data sets, but lists only 11, not 12; and the 8th and 9th distance are the same (not likely!).

Ferdinand Rudolph Hassler

Pencil sketch of Hassler
Pencil sketch by Charles Fenderich
(Date unknown)

Fenderich captured Hassler dressed in his favorite fashion, a high-collared ankle-length greatcoat, and a cap to hide his baldness.

For the Equal Altitudes observation to check the chronometer neither Lewis nor Clark mentions that the AM observation was taken at Point of Observation 20, a few miles downriver from the Musselshell River, whereas the PM observation was taken at Point of Observation 21 at the mouth of the Musselshell. Fortunately the 1'27" latitude difference and the 1'39" longitude difference make only about a second of difference in the chronometer error, but they should have recorded the fact and provided Hassler with their courses and distances so he could determine approximately how far apart the two points were.

The Meridian Altitude observation presents no problem, provided Hassler had access to information on the octant's index error in the back observation. Lewis, of course, uses an incorrect index error and derives a latitude about 27 minutes too far south without showing his work.

The observation for magnetic declination of the sun has some real problems. First of all, Lewis has the observation being taken in the AM, yet the bearing of the sun for those observations is SW! Clark gives no AM or PM with his values and has 4 data sets compared to Lewis's 3. In addition, in Moulton's edition of the journals (4:173) Clark's 2nd value is S85W whereas in Thwaites (2:55) it is S83W. Obviously, Hassler too would have had to struggle with reading the manuscript numbers. From Lewis's entry headed "Point of Observation No. 21" in Patterson's notebook it appears the bearing should be S83W.

But there's more. If the observation was taken in the afternoon, why is the bearing of the sun going from S85W to S80W? In other words, why is the sun's bearing tending eastwardly as the day progresses? Apparently Lewis has the numbers 1st, 2nd and 3rd reversed in sequence, yet the times and altitudes appear to be in the proper order.

And, on top of that, Lewis merely gives the altitude of the sun at the time of observation without saying whether it is the upper limb, lower limb or center. In Patterson's notebook it is given as lower limb and, using the corrected chronometer times, one can calculate that the altitudes Lewis gives are fairly close to what should have been observed for corrected PM times.

Poor Hassler! And to have to do all these calculations on top of his regular duties and family troubles! Maybe we've misjudged him.


1. In addition to two portraits of Hassler, Fenderich is known to have produced only two other lithographs: one from a painting of President William Henry Harrison by a Mr. Franquinet (1841), and his own original lithograph of President James K. Polk, "from life on stone" (ca. 1841).

Funded in part by a grant from the NPS Challenge-Cost Share Program.