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Jefferson read Mackenzie's account of that method's success and it's puzzling why the President didn't tell Lewis to use Jupiter too. Perhaps he feared a telescope might be too easily broken on a long trip, but the American choice of a trickier way of getting longitude didn't pay off. Lewis used a sextant to measure angles between the moon and nine bright stars, or the sun, as a way of knowing Greenwich time with the help of the Nautical Almanac. On the expedition he and Clark took 60 such "lunar distance" measurements, but postponed the tough calculations needed to produce actual longitudes in the field. Mackenzie took three lunar distance angles himself, also without recording a computed answer. Evidently he found that Jupiter gave easier results.

Astronomy aside, both parties carefully recorded "courses and distance," or miles traveled on each compass heading, to mark their progress by dead reckoning. At the outset neither group knew much about pioneering through world-class mountains like the Rockies. Mountain geography was a difficult learning process for these explorers, but they got pretty good at it.

In May , 1793, Mackenzie left the plains and wrestled his single canoe upstream through a deep canyon where the Peace River flows from the Rockies. Still moving west, he came to a place in modern British Columbia between two mountain chains where the Peace forms itself from tributaries coming from the north and south. Mackenzie wrote he would have taken the northern branch, "if I had been governed by own judgment." Likewise, his Canadian voyageurs looked with professional eyes at the angry current of the Parsnip River coming from the south, and agreed that the gentler Finlay was the way to go. But to the horror of his men, Mackenzie ordered the canoe southward up the Parsnip. As a result, he reported, "we were the greatest part of the afternoon in getting two or three miles."

Mackenzie took that unpromising turn because he had been coached in advance by "an old man"—unnamed, of course—met earlier on the trip. The Indian warned Mackenzie that when he reached the forks of the Peace, he was "not on any account" to go north. The southern branch, said his coach, would take him to a portage leading to a big new river the explorers were looking for. (Twelve years later Lewis and Clark faced a similar decision when their eight-boat flotilla arrived at a branching of the Missouri River in modern Montana. They had received no prior Indian advice. The men said go north up the Marias River, but the captains studied the waters and correctly picked the southern branch as the main Missouri.)