Celestial navigation


Celestial navigationThe celestial bodies used in sea ship and yacht navigation may be divided into two classes:

(1) the fixed stars and
(2) those bodies that belong to the solar system.

The fixed stars are those celestial bodies that are at such immense distances from the earth that they appear to remain in fixed positions relative to each other. The nearest star is 26 trillions of miles from the earth, and most of the stars are much farther away.

At such immense distances the real motions of these bodies are not evident to us, nor does the earth's motion about the sun cause them to have an apparent motion.

Their right ascension and declination remain nearly the same from one year to another. They have an apparent motion due to the earth's rotation upon its axis, which causes them to appear to revolve about the earth from east to west once every day.

The solar system embraces the sun and those bodies, called planets, that revolve about it, as well as the lesser bodies, called satellites, that revolve about the planets. These bodies are near enough to the earth so that their real motions in space are evident to us and they also appear to move, due to the earth's motion about the sun and rotation upon its axis.

The earth is one of the planets. Those planets which are nearer the sun than is the earth are called inner planets and those that are farther from the sun than is the earth are called outer planets.

The planets, in the order of their distances from the sun, are Mercury, Venus (both inner planets), the earth, and the outer planets, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Of these, the only ones of use to the nautical navigator are Venus, Mars, Jupiter, and Saturn.

Mercury is so near the sun as to be usually in the brightly lighted part of the sky and is therefore seldom visible, while Uranus, Neptune, and Pluto are not near enough to be visible without the aid of a telescope. See Fig. 833, which shows the navigational planets as viewed from the north pole of the ecliptic.

The earth's satellite, the moon, is the only satellite sufficiently bright for use in yacht navigation. Thus, for the sea ship navigator, the solar system narrows down to the sun, four planets, and the moon.