How to Know the Planets

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Nature Magazine, March 1949

There are various ways of distinguishing the planets from the stars that lie far beyond them. Planets shine with a much steadier light than the stars, a light that is reflected light of the sun. Planets do not twinkle or scintillate appreciably. This is particularly true of the giant planet of the solar system, Jupiter, which shines with It remarkably calm and steadfast light. Venus, and, more especially, Mercury, do at times shine with considerable unsteadiness of light when the dense layers of atmosphere near the horizon may abnormally deflect their rays at low altitudes. Mercury was even at times called "The Twinkler" by the ancients.

With access to a telescope one has no trouble in identifying a planet by its characteristic disk. Mercury and Venus, moreover, show, in the telescope, phases similar to those of the moon because their orbits lie between the orbit of the earth and the sun. The albedo, or reflecting power, of Venus is greater than that of any other planet, and it is of dazzling brilliancy, very different in appearance from Mercury, which has faint markings on its disk. So dense is the atmosphere of Venus that light never penetrates to its surface, and there is no break in its veil of clouds. Mars, Jupiter, Saturn, Uranus, and Neptune all have their distinctive disks and peculiar surface features that readily distinguish them from other planets. Pluto is the only major planet that, because of its great distance from the earth and small size, shows no disk but appears like a small, faint star in the telescope. We are interested here only in the five brightest planets, however, all easily visible without telescopic aid.

Planets are easily recognized as such – if one observes them on a number of successive nights – by their motion relative to fixed stars among which they appear to move. It will soon be noticed, moreover, that the planets never appear outside of the Zodiac, that belt or zone in the heavens that extends eight degrees to either side of the ecliptic.

If one could look down upon the orbits of the planets from the direction of the north pole of the heavens, it would be noted that they are all inclined at very small angles to the plane of the earth's orbit. All of them, with the exception of Saturn, Venus, Mercury, and Pluto, are inclined less than two degrees to the orbit of the earth. The inclination of Saturn's orbit is only slightly less – two and one-half degrees – than that of Venus, which is about three and a half degrees, while the orbits of Mercury and Pluto are tipped to that of the earth at the comparatively large angles of Seven and seventeen degrees respectively. It would also be noted that all the planets were moving in their orbits around the sun in the counter-clockwise, or west-to-east direction. This is true of most of the satellites of the planets as well. This usual direction of motion of the planets is spoken of as direct motion. If the motion is clockwise, or in the east-to-west direction, it is called retrograde motion, as distinguished from direct motion, and the body is said to be retrograding.

The motion of the other planets as viewed from the earth is sometimes direct, sometimes retrograde. A planet will appear also to be stationary, as seen from the earth, when its apparent motion changes from direct to retrograde, or vice versa. All of the planets whose orbits lie outside of the earth's orbit are moving in retrograde, or in the east-to-west direction, at and near the time of opposition. It is easily seen why this is so. The nearer the planet is to the sun the more rapid its orbital motion. The mean motion of the earth in its orbit is 18 ½ miles per second; that of Mars about 15 miles; Jupiter about 8 miles; Saturn 6 miles, while the mean orbit velocities of Uranus and Neptune are, respectively, about 4 ¼ and 3 miles per second. Near time of opposition the planet and the earth are on the same side of the sun and both are moving around the sun in the same west-to-east direction, but the earth, being nearer to the sun, moves more rapidly than the outer planet and, as a result, there comes a time when the planet falls behind the earth and apparently moves in the opposite direction, or retrogrades, after first reaching a stationary point. As both planets move onward in their orbits there comes a time when the outer planet ceases to fall behind, reaches a stationary point again and then resumes direct motion. If the path of a planet, as seen from the earth, were traced on a sky map it would have strange loops and curves, although for most of the time its motion would be west to east, or direct. It would also be confined to the Zodiac. One need never look for planets among the circumpolar constellations, either northern or southern, or, in fact, more than thirty degrees north or south of the celestial equator. This fortunately limits the possibilities of mistaking some bright star in these regions for a bright planet.

Planets whose orbits lie beyond that of the earth always retrograde for some time around the date of opposition. They are always shifting westward in position relative to the sun, however. Shortly before conjunction, an outer planet – Mars, Jupiter, or Saturn – is seen, we will say, close to the western horizon in the evening twilight. It is nearing conjunction with the sun. Soon it will disappear in the rays of the setting sun. Shortly after it will be in line with earth and sun, with the sun between earth and planet. It will next be seen west of the sun close to the eastern horizon before sunrise. Its distance west of the sun will steadily increase until a point is reached, some days before opposition, at which the planet becomes stationary. From then on through opposition, and for some days after, it retrogrades, finally reaches a stationary point once more, and after that resumes direct motion. It soon passes into the western evening sky, draws in toward the sun once more, and another conjunction takes place. The planet's circuit of the heavens relative to the sun has been completed.

The inner planets, Mercury and Venus, of course, move more rapidly in their orbits than the earth. The mean orbital velocity of Venus is about 21 ½ miles per second, and that of Mercury is about 30 miles per second. Both of these planets appear to oscillate back and forth in their orbits relative to the sun. Unlike the outer planets they are never at opposition; they are at inferior conjunction when between the earth and sun, and at superior conjunction when the sun is between them and the earth. They also have points of greatest eastern and western elongation when at their greatest angular distance east or west of the sun. When at inferior conjunction, and for a short period before and after it, both Mercury and Venus are retrograding, but, for the remainder of the time, from before western elongation through superior conjunction until after eastern elongation, both planets are moving in the west-to-east direction. As with the outer planets, stationary points occur at the time the direction of motion relative to the earth changes from direct to retrograde, or vice versa.

It will be found instructive, as well as interesting, to follow the changing positions of the planets relative to bright or easily recognized stars in the background. When one speaks of the planets moving "among the stars" one means, of course, the motion of the planets as seen projected against the stellar background. So enormous are the distances that separate our solar system from even the nearest of the stars that the stars appear to us fixed in space, although actually they are moving with velocities of a number of miles per second.

One soon gets so well acquainted with the planets through continual observation that they often can be identified by their colors alone. Mercury always will be seen in the twilight hours, and then only for a period of ten days or two weeks, including the dates of greatest elongations from the sun. It will always be brilliant, brighter indeed than a star of first magnitude, when picked up in bright twilight near the ecliptic. Venus should be the easiest to identify of all the planets, since it is the most brilliant of all celestial objects except the sun and moon.

A brilliant white in color, it should never be mistaken for Jupiter, which is definitely a golden-yellow. Venus will be found alternately in the morning or evening sky, never more than about 45 degrees from the sun at most. Mars and Saturn are the two planets most easily mistaken for stars. Mars, when closest to the earth at a very favorable opposition, is for a short time more brilliant than Jupiter, but at an unfavorable time, when farthest from the earth, it is not even as bright as a second magnitude star. It also passes through the zodiacal constellations of Scorpio and Taurus, and may at times be mistaken for Antares, or Aldebaran, by one not familiar with it. Saturn is usually comparable with Capella, or Arcturus, or Vega, in brightness, and is a pale yellow in color. Once identified by its rather peculiar color, one is not apt to mistake Saturn for any other planet, or star. Saturn moves more slowly relative to the stars than any other bright planet, so, once identified, one can easily keep track of it.

In March both Mars and Venus will be too close to the sun for observation. Mercury may be seen early in the month in the morning twilight. Mars will be in conjunction with the sun on March 17, and will be too close to the sun to be seen throughout the month. Jupiter is now visible in the southeast in the morning sky for some time before sunrise. It is in Capricornus. Saturn is still in Leo, high in the eastern sky at sunset. This is a fine time to get acquainted with this planet and to observe its motion with respect to the nearby first magnitude star, Regulus, which lies to the southwest of it. Saturn is still retrograding, having been at opposition in February.

Spring will begin in the northern hemisphere, and fall in the southern hemisphere, on March 20, at 5:49 P.M., Eastern Time. The sun will then cross the equator from south to north.