备考
Lecture3 (astronomy)
Narrator:
Listen to part of a lecture in an astronomy class.
Professor:
OK, last time we talked about ancient agricultural civilizations that observed the stars and then used those observations to keep track of the seasons. But today I want to talk about the importance of stars for early seafarers, about how the fixed patterns of stars were used as navigational aids.
OK, you’ve all heard about the Vikings and their impressive navigation skills, but the seafaring people of the pacific islands, the Polynesians and the Micronesians, were quite possibly the world’s greatest navigators. Long before the development of, uh, advanced navigational tools in Europe, pacific islanders were travelling from New Zealand to Hawaii and back again, using nothing but the stars as their navigational instruments.
Um, the key to the pacific islanders’ success was probably their location near the equator. What that meant was that the sky could be partitioned, divided up, much more symmetrically than it could farther away from the equator. Unlike the Vikings, early observers of the stars in Polynesia or really anywhere along the equator would feel that they were at the very center of things, with the skies to the north and the skies to the south behaving identically, they could see stars going straight up in the east and straight down in the west. So it was easier to discern the order in the sky than farther north or farther south, where everything would seem more chaotic. Take the case of the Gilbert Islands, they are part of Polynesia, and lie very close to the equator. And the people there were able to divide the sky into symmetrical boxes, according to the main directions, north, east, south and west. And they could precisely describe the location of a star by indicating its position in one of those imaginary boxes. And they realized that you had to know the stars in order to navigate. In fact there was only one word for both in the Gilbert Islands, when you wanted to the star expert, you ask for a navigator.
Um, islanders from all over the pacific learned to use the stars for navigation, and they passed this knowledge down from generation to generation. Some of them utilized stone structures called stone canoes, ah, and these canoes were on land, of course, and you can still see them on some islands today. They were positioned as if they were heading in the direction of the points on the sea horizon where certain stars would appear and disappear during the night, and, um, young would-be navigators set by the stones at night and turned in different directions to memorize the constellations they saw, so they could recognize them and navigate… by them later on when they went out to sea.
One important way the Polynesians had for orienting themselves was by using zenith stars. A zenith star was a really bright star that would pass directly overhead at particular latitude…at a particular distance from the equator, often at a latitude associate with some particular pacific island. So the Polynesians could estimate their latitude just by looking straight up, by observing whether a certain zenith star passed directly overhead at night, they’d know if they have rates the same latitude as a particular island they were trying to get to. Um, another technique used by the Polynesians was to look for a star pair, that’s two stars that rise at the same time, or set at the same time, and navigators could use these pairs of stars as reference points, because they rise or set together only at specific latitudes. So navigators might see one star pair setting together. And, uh…would know how far north or south of the equator they were. And if they kept on going, and the next night they saw the pairs of stars setting separately, then they would know that they were at a different degree of latitude. So looking at rising and setting star pairs is a good technique. Um… actually it makes more sense with setting stars; they can be watched instead of trying to guess when they’ll rise.
Uh, OK, I think all this shows that navigating doesn’t really require fancy navigational instruments; the peoples of the pacific islands had such expert knowledge of astronomy as well as navigation that they were able to navigate over vast stretches of Open Ocean. Uh, it's even possible that Polynesian navigators had already sailed to the Americas, centuries before Columbus.