early sailors discovered a gradual change in posistion of the north star.This discovery helped to prove that?

Question

a. earth is round
b.earth is flat at its poles
c.earth rotates on its axis
d.earth is approximately 40,074 kilometers in circumference

Answer 1

The early sailors discovered that they could figure their latitude by measuring the angle of the north star from the horizon.

Earth being round, flat at the poles or having a 40,000 Km circumference has nothing to do with it.

That leaves C, which is not a good answer either. The north star stays at its level above the horizon if one sails east or west and changes as one sails north or south.

Answer 2

Actually, the reason why the "pole star" slowly drifts is called precession. However, the early sailors that noticed the drift would not have known what caused it.

Earlier astronomers already knew of precession. However, they did not know what caused it. In order to understand what causes precession, you need to know:

1) Earth is (almost) a sphere, as opposed to a disk or a mound of dirt carried by elephants walking on the back of turtles...
2) Earth is a planet that rotates on its axis.
3) Earth has an equatorial bulge (i.e., Earth is a bit flatter at the poles)
4) the axis of rotation is tilted in relation to the plane of the orbit.
5) The Sun is massive enough for its gravity to pull a bit more on the equatorial bulge than on the rest of the planet.

This extra pull on the bulge tries to bring the tilt of the axis (presently 23.5 degrees) back to zero but the Earth's rotational momentum is so great the it acts like a gyroscope: its axis changes its orientation instead of its angle of tilt. This change is called precession.

It takes 25,800 years for the precession to "swing" Earth's axis all the way back to where it starts. (So, if we come back in 25,800 years, the present pole star will again be the pole star).

So, I probably would not have known which answer to pick. At the time that this drift was "discovered" -- a few thousand years ago -- your typical sailor thought the Earth was flat and that it was the celestial sphere that was rotating around him.

It was "rediscovered" again in the early 17th century, by which time some people were thinking that maybe, Earth was rotating on its axis and, scandal, may be in orbit around the Sun. Oh Dear. At the time, our present pole star (at the tip of the tail of the small bear) was still 5 degrees or so from the celestial north pole. It was slowly getting closer (not that you would notice over a few years).

At the end of the 20th century, Polaris was at its closest to the pole (within one degree). It is now slowly drifting away.

Answer 3

The answer is c