Practice will quickly enable him to distinguish a planet from the true stars, both by its greater apparent size and by the quality of its light. The planets do not twinkle as do the stars. This arises from the fact that they present measurable disks which reflect the sunlight, but do not shine with a light of their own. No star shows a real disk, even when viewed with a powerful telescope. The stars are mere points, and the larger and better the telescope the smaller they appear. This is not to say that they do not look brighter in a telescope, for the larger stars are dazzling when viewed with a glass of large aperture; but they are so distant that the mightiest of telescopes cannot reveal their real surfaces in the form of disks. The apparent disks which they present are due entirely to irradiation, and the higher the power the smaller these spurious disks appear. Another way in which the beginner may identify a planet is by observing its motion. No planet remains long in the same position with regard to neighboring stars. They all travel, at varying rates, from west to east through the sky. But this motion is not constant, and at times it is reversed. In the cases of Mars, Jupiter, and Saturn the reversal is due to the fact that when they are in opposition to the sun the earth, being nearer the sun than they are, outfoots them in eastward motion, so that they appear for a time to move backward on their orbits. It is like a fast train passing a slow one on a parallel track; to an observer on the fast train the slow one seems to be either standing still or moving backward. But Mercury and Venus, being nearer the sun than the earth is, have at times a backward motion which is real. Let us consider them only when they appear as “evening stars.” From “superior conjunction” (_i. e._, the point occupied by the planet when it is on the opposite side of the sun from the earth) to “greatest eastern elongation” (greatest apparent distance from the sun in the evening sky) both Mercury and Venus move eastward among the stars; from “greatest eastern elongation” to “inferior conjunction” (_i. e._, the point occupied by the planet when it is between the earth and the sun) they move westward among the stars, or, in other words, approach the sun. The motions of Mercury and Venus are comparatively swift, particularly that of the former. Few persons have ever seen Mercury, because of its nearness to the sun. When well seen it is brighter than any first-magnitude star. As an “evening star” it appears in the west immediately after sunset about once every four months (more precisely once every 116 days). It remains within view about twenty days, but can be easily distinguished only for a week or so when it is nearest eastern elongation. Every almanac gives the dates of its appearances.