Charts, Diagrams and Timelines

The Longitude Problem

A ship's latitude — how far north or south it is — can be found from the height of the sun at noon. Sailors could do that for centuries. Longitude, how far east or west, was the problem. It has no natural marker. The solution was known in principle: compare the time on board with the time at a fixed place. The Earth turns fifteen degrees an hour, so every hour of difference is fifteen degrees of longitude. That required a clock that stayed accurate on a rolling, salty, temperature-swinging ship. No such clock existed. John Harrison, a carpenter, spent forty-three years building four. The fourth, finished in 1759, was the size of a large pocket watch and lost five seconds in eighty-one days at sea.

The Wolves and the Rivers

Wolves were shot out of Yellowstone by 1926. Fourteen were brought back in 1995. With no wolves, elk had grazed the valleys freely for seventy years. Young willow and aspen were eaten as soon as they grew. The wolves killed some elk. More importantly, they changed where elk would stand still. Open valley floors became dangerous, so the elk stopped lingering there. Willow and aspen grew back along the rivers. Beavers, which need willow, returned. Their dams made ponds. Roots held the banks. The rivers narrowed, deepened and moved less. Some researchers describe this as wolves changing the shape of rivers, though others argue the effect has been overstated.