A FEW DEFINITIONS

What “Parallax” means. Since the earth revolves round the sun, the stars are apparently in slightly different directions from it at different times of the year. The difference in direction of a star as seen from two points on the earth’s orbit which are separated by the mean distance to the sun is the parallax of the star. In other words, the parallax of a star is the angle subtended by the major semi-axis of the earth’s orbit, as seen from the star.

The “Orbit” of a moving body is its more or less circular passage through space, usually around another larger body, as our earth revolves round the sun. The “eccentricity” of the orbit consists in the fluctuations or variations from its exact path.

The “Ecliptic” System. If we could see the stars near the sun, we should find that the Sun apparently moves eastward among them, completing one revolution in a year. Tracing such a path, it will be found that it more or less coincides with the celestial equator. The equator and the ecliptic intersect at two points; these points are the “equinoxes” the vernal equinox being the one at which the sun crosses the equator from south to north, and the autumnal equinox the other one.

“Satellites.” These are smaller bodies which revolve round large ones, and, so to say, attend them. All except two of the planets are known to have satellites revolving round them, just as they revolve round the sun. Mercury and Venus have none; the earth has the moon; Mars has two little moons, only a few miles in diameter; Jupiter has four large satellites and four small ones; Saturn has ten, one of which is larger than Mercury; Uranus has four satellites, and Neptune one.

The “Planetoids.” Between Mars and Jupiter a number of small bodies have been discovered, moving in a regular orbit; these have been called planetoids. If some planet has once occupied this mid-way position, and subsequently exploded, the fragments would occupy the position occupied by the planetoids. Whether or not this is their origin is a disputed point, which it would take us too far afield to consider here. They suggest the possibility.

“Planets.” These are the bodies revolving round a central sun. Aside from those constituting our own solar system, we see no planets in space; we see suns, or stars; but if the latter have planets attendant upon them, we cannot see them.

The point of the moon’s orbit nearest the earth is called the perigee; the furthest point, the apogee.