Part 4

When an inferior planet is at greatest eastern elongation, it is, of course, east of the sun, and can be seen above the sun in the evening after sunset, and is an evening star. As it moves westward nearer and nearer to the sun, it is above the horizon a proportionately shorter time each evening, and is more and more obscured by the sun’s rays until it reaches inferior conjunction, when it is exactly between us and the sun, and hence at the point nearest to us. Here it becomes invisible, largely because it has its dark side toward us, but partly because the dazzling light of the sun entirely obscures it. Once in a while our relative positions are such that we see it pass like a black dot directly over the bright face of the sun. This is called a transit. But a transit does not occur at every inferior conjunction. It would so occur if the planet’s orbit and the earth’s were in exactly the same plane. But the small tilt that they have is sufficient to throw the planet, when it is passing the sun, into such an angle that it does not pass directly between the disc of the sun and us, but a little above or below. Thus transits are rather rare, though they occur periodically in the case of both Venus and Mercury, and will be spoken of elsewhere.

When the planet has passed inferior conjunction, it is then west of the sun, and rises in the morning before the sun is up, and is a morning star. For a few days it can be seen either not at all or with difficulty. Then, as it works its way out of the rays of the sun and on toward the west, it rises earlier each morning until it reaches its farthest point west.

As it starts back east again its distance from the earth increases daily until it reaches its greatest distance from us at superior conjunction. It is then the whole diameter of its orbit farther from us than when it was at inferior conjunction, and it is again invisible. The illuminated side of it is toward us; but it is at its smallest, because it is at its greatest distance from us, and even when it is not directly behind the sun the light of that luminary is too great for successful competition. After it has passed superior conjunction it is again in the evening sky, apparently moving farther from the sun each day. It is at the same time actually coming nearer to us each day, and these two facts cause a daily increase in its brightness.

But an inferior planet is not, like the superior planets and the stars, brightest when it is nearest to us. It is, in fact, darkest when it is nearest--that is, when it is at inferior conjunction--and we cannot see it at all. This is because an inferior planet passes through phases, like the moon, changing gradually during its rounds from full to crescent, and back again. Its full face is toward us when it is on the opposite side of the sun and farthest from us. The proportion of the face that is illuminated grows smaller as the planet approaches its eastern elongation. But the planet grows brighter because it is coming nearer to us and is getting out of the dazzling rays of the sun. One-half of its surface is illuminated when it is at greatest elongation; but it is brightest a few days later, when less than half of its face is illuminated, because it is enough nearer to compensate for the slight diminution in the proportion of light on its disc. It is brightest in the morning a short time before its western elongation, for the same reason.

This in a general way describes the motion of an inferior planet, and this is all that we need to know in order to understand its ordinary visible movements. If we watch it carefully, however, we may detect that shortly before inferior conjunction it pauses in its onward sweep and seems for a time to be stationary, and then to retrace its way among the stars until a short time after inferior conjunction, when it again pauses and appears stationary, and finally starts off again in its original direction on its way toward greatest western elongation. During this capricious sort of progress the planet usually describes more or less of a loop, sometimes almost a flourish, in its path. The appearance is wholly due to the planet’s overtaking and passing us in our journey around the sun. For a time it travels behind us, then beside us, and then beyond us; and, since we are both in motion, the effect is much the same as when one train passes another while they are both traveling in the same direction. The orbits of the earth and the planet are not exactly in the same plane, and, both bodies being in motion, we are not in a position to see the planet at the same angle more than once as it seems to pass back and forth, and so we get the effect of its making a flourish or loop. But this effect, while interesting, takes place only when the planet is so near the sun that to the ordinary observer it itself does not count for much. We can see but little of the inferior planets at that time, anyway, though it is important for us to know where they are, in order to keep track of them and to be ready for them when they are to be seen.

VIII

HOW THE SUPERIOR PLANETS SEEM TO MOVE

The movements of the superior planets, Mars, Jupiter, Saturn, Uranus, and Neptune, as they appear to us, are different from those of the inferior planets in some important respects. Instead of swinging back and forth east and west of the sun, and never appearing very far away from it, as the inferior planets do, the superior planets make an entire circuit of the heavens, and it is possible to see them at any distance from the sun, and at any time during the night. Sometimes they are, with relation to the earth, in that part of the sky exactly opposite to the sun, and hence in line with it and the earth. At such times they can be seen all night. They are then said to be in opposition, and are in the best position for our observation. The earth being, when in this situation, in a direct line between them and the sun, we have the sun at our backs, as it were, shedding its full rays on the disc of the planet under observation, which is then at its nearest to us, and also at its brightest. For, since the orbits of all the superior planets are outside of ours, the planets never get between us and the sun, and, in consequence, never turn a dark side toward us. Their entire discs are practically always illuminated, and their changes in brightness depend largely upon their changes in distance, which, as we have seen, is not the case with the inferior planets.

Mars, the nearest of them, is at times somewhat gibbous (that is, shows a little less than a full face, as the moon does when just beginning to wane), and, in less degree, Jupiter also. But in neither case is this departure from fullness sufficient to have any appreciable effect on the planet’s brightness, and, moreover, it does not occur when the planet is in the most favorable position for us to see it. At opposition, therefore, we always have the full face of the planet presented to us; and being, as we then are, on the same side of the sun with it, we are ninety-three millions of miles (our distance from the sun) nearer to it than the sun is.

Being, when in opposition, exactly opposite the sun, the planet rises just as the sun sets. After opposition it rises a little earlier each evening, and is higher up in the sky at each succeeding sunset. When we find it just half-way between the eastern and the western horizon at sunset, it is at quadrature. After quadrature it appears nearer and nearer the western horizon each evening at sunset, until it finally is too near the sun to be visible. It is then traveling in that part of its orbit which is beyond the sun from us. From opposition to this situation it has been an evening star.

When a superior planet is in line with the sun and the earth, and is on the far side of the sun from us, it is said to be in conjunction, and we are then one hundred and eighty-six millions of miles, or twice our distance from the sun, farther from it than we are when it is in opposition. But besides being placed at so much greater distance from it, we have in this situation the bright sun excluding the planet from our view. It will be readily seen, therefore, why the superior planets are in so much better position for us to see them in opposition than at conjunction.

From conjunction to opposition the planet is west of the sun, and will be below the horizon at sunset, and will rise some time during the night. At first it will appear just before sunrise as a morning star, but will gradually rise earlier each night until, when it reaches opposition again, it will rise just as the sun sets. Half-way between conjunction and opposition it is again at quadrature.

From opposition to conjunction the planet will be east of the sun and above the horizon at sunset. When a planet is in conjunction with the sun, it passes the meridian, or the point half-way between rising and setting, about noon, and is above the horizon with the sun during the day. When it is in opposition it passes the meridian about midnight, and is above the horizon during the night. When it is at quadrature and moving toward conjunction, it passes the meridian about six o’clock in the evening, and may be seen in the western half of the sky during the early evening, and will set before midnight. When it is at quadrature and moving toward opposition, it will rise some time between midnight and sunset, and will be in view in the east during a part of the first half of the night. The nearer it is to opposition, the earlier in the evening it rises and the longer it may be seen.

The main movement of the superior planets among the stars is from west to east, and this is known as their direct motion. But not far from opposition they seem to hesitate, then move more slowly, then finally stop, remain stationary for a time, turn back on their tracks, and start off in the opposite direction. This is their retrograde motion. They do not continue in it as long as in the direct motion; but after a comparatively short time they again hesitate, go more slowly, stop, remain stationary, then turn back and swing off in the original direction, and continue to move in this direction until they are again approaching opposition. It is exactly in the middle of this sweep toward the west that the planet is in opposition. Close observation will show that the superior planets also make something of the same sort of a loop in their path among the stars that the inferior planets make, and for the same reason. The only difference is that when a superior planet is retrograding we are passing it, and when an inferior planet retrogrades it is passing us.

In giving this rather rough outline of the way the planets in general move among the stars, reaching in their wanderings these various positions with relation to the sun and the earth, the intention is only to fix some definite situations from which to consider the movements of the individual planets. When we come to know each planet as an individual, and to follow it as it comes and goes in the heavens, and to watch its ever-wonderful changes in brilliancy, these situations will have a much more definite meaning to us and a relatively greater interest and importance. The planets as they appear to us all move along pretty much the same path; but each has its own way of gracing this path, and each its particular manner of changing in aspect.

IX

THE PATH OF THE PLANETS

Though the planets are called wanderers, they are not by any means the vagrants that the name might imply. They have a fixed course among the stars from which they never deviate, and the ways of all of them, and also of the sun and the moon, are confined to a comparatively narrow strip in the sky.

That strip is called the zodiac. It is only sixteen degrees wide, and extends like a narrow band all the way around the heavens. It lies so that it is always easy to observe; and, being so limited, very little observation is necessary to become familiar with every part of it. Within its limits all the movements of the sun, the moon, and the planets take place. Through the center of it is the ecliptic, the great circle that marks the annual apparent path of the sun through the heavens. It is the standard circle from which we measure the paths of the moon and the planets. Whatever degree their courses vary from the ecliptic is what we call the inclination of their orbits. If the plane of the orbit of a planet is tilted away from the ecliptic, the planet will travel half the time on one side of it, and half the time on the other.

The orbits are, in fact, very little inclined to the ecliptic, and all but one of the planets may always be found within three degrees of it, most of them nearer than this. The one exception is Mercury, which is sometimes as much as seven degrees from this central line of the zodiac, but ordinarily it is not so far as this. Uranus is so nearly on the ecliptic that an ordinary observer would not notice the deviation, and particularly as Uranus can rarely be detected with the naked eye, and can never be thus followed. Of the four planets which are the ones we ordinarily see, Mars and Jupiter are never as much as two degrees from the ecliptic, Saturn never more than two and a half degrees, and Venus never more than about three degrees. They are all usually nearer than these outside limits. The greatest distance of the moon from the ecliptic is about one and a half degrees.

Hence, with the exception of Mercury, all the planets and the sun and the moon travel in a path six degrees wide, which is only one degree wider than the distance between the pointers as we see them in the Great Dipper. The fact that the zodiac is sixteen degrees wide, or eight degrees on each side of the ecliptic, is due only to a very generous allowance for the vagaries of Mercury, which he really does not quite need. For Mercury is always as much as twice the breadth of the moon, or one degree, inside of the zodiac, and usually more than that.

Because the earth is tilted on its axis twenty-three and a half degrees from the perpendicular, the ecliptic runs through the heavens in an oblique circle, crossing the line of the equator at two points called the vernal and autumnal equinoxes. The equator in the heavens is the great circle extending around the celestial sphere half-way between the north and south poles. It is always practically ninety degrees from the north star, and the points at which the ecliptic intersects it are called the equinoxes. These are the only two points on the ecliptic that are just ninety degrees from the pole. The word equinox is derived from _equus_ (equal) and _nox_ (night), and when the sun is at the equinoxes the days and nights are of equal length.

From the vernal to the autumnal equinox the line of the ecliptic is north of the equator, and hence high in the sky, reaching its highest point midway between the equinoxes. It then crosses the equator again and runs obliquely south to the lowest point in its path, and then curves northerly back to the vernal equinox. The vernal equinox is the point at which the sun arrives when spring begins. This results in the sun’s being north of the equator from spring until autumn, and south of it from autumn to spring.

As the part of the zodiac that we can see best at night is that opposite where the sun is, so in summer, when the sun is high, we see best the part of the zodiac which is low in the southern skies in the evening; and in the winter, when the sun is in the southern half of his journey, the part of the zodiac best seen by us is high in the heavens. No part of it, however, is ever as high as the zenith, or directly overhead, and no planet is ever seen as far north as the zenith in any place whose latitude is more than twenty-three and one-half degrees from the equator.

To know the paths of the planets it is necessary to know only twelve constellations out of the seventy or more in the entire heavens; but it is difficult to imagine any one’s learning these twelve without becoming interested in and more or less acquainted with many of the splendid stars and constellations that lie on each side of them. The larger one’s acquaintance is with the appearance of the skies as a whole, the easier, naturally, it will be to distinguish the planets from the stars, and to follow their courses. But the planets themselves may be intimately known quite apart from any but the twelve constellations forming the zodiac. Happily, among them we shall find some of the most beautiful constellations in the heavens, and some of the most splendidly brilliant first-magnitude stars.[1]

[1] The reader will find fuller descriptions of the stars in the zodiac in _The Friendly Stars_, by the author of this book.

The twelve constellations of the zodiac are as follows:

Pisces, the Fishes. Aries, the Ram. Taurus, the Bull. Gemini, the Twins. Cancer, the Crab. Leo, the Lion. Virgo, the Virgin. Libra, the Scales or Balance. Scorpio, the Scorpion. Sagittarius, the Archer. Capricornus, the Goat. Aquarius, the Water-Carrier.

We shall begin at the point of the vernal equinox to trace the line of the ecliptic through these constellations, and that line will mark for us the path of the sun, the moon, and all the planets. It is convenient to begin at this point, because it is where the sun crosses the equator in the spring, and hence it is at the beginning of that part of the ecliptic which lies north of the equator.

The point of the vernal equinox is now situated in the constellation Pisces. It is not marked by any bright star, but is not very difficult to find. It marks the point on the eastern horizon where the sun rises about March 21st, and about the 21st of September it is on the eastern horizon exactly opposite that point in the western sky where the sun sets. It is always ninety degrees from the pole, and if one chances to know the constellation Cassiopeia, which is shaped like a chair and is on the opposite side of the pole from the Big Dipper, one can locate the vernal equinox by drawing a line from the pole-star through the star which marks the lower part of the front of the chair, and extending it until it is ninety degrees long. The ninety degrees can be estimated by using the distance between the pointers in the Dipper (which is five degrees) as a measure. The star mentioned in Cassiopeia is about thirty-two degrees from the north star.

Having once learned the constellations of the zodiac and, approximately, the line of the ecliptic, it is not necessary for the ordinary observer to keep in mind the exact location of the vernal equinox. It is, however, an important point for the student of mathematical astronomy.

Beginning at this point, the ecliptic runs through Pisces in a northeasterly direction for about thirty degrees to Aries, the second constellation of the zodiac.

ARIES

Aries is best seen in the autumn when the sun is in the opposite side of the heavens. It is marked by a small acute-angled triangle, with the apex toward the north and the brightest star of the three at the apex. This star is called Hamal, and, while not a first-magnitude star, is a rather bright one of the second magnitude; and the triangle itself is very distinctly marked. It is the only group of stars by which to distinguish Aries, and it is sometimes confused with the little constellation called Triangulum, which lies just west of it, or above it, as it rises. With this in mind, Triangulum may be made to serve as an identifying mark. They both rise just a trifle north of the exact east early in the evenings of late September and October. Triangulum rises first, with its apex toward the south. In less than an hour the triangle of Aries arrives with its apex pointed north. The ecliptic runs about five degrees below this triangle, and its path across Aries is about twenty-eight degrees long. When one sees any very bright star in Aries, one may be sure it is a planet. The sun is in Aries from April 16th to May 13th.

During the summer this constellation is not visible in the early evening; but it may be seen every evening from September to April, drawing all the time nearer to the sun, and setting earlier each evening until the sun blots it out. From this constellation the ecliptic runs into Taurus, the third zodiacal constellation.

TAURUS

This constellation may be identified by the brilliant first-magnitude star Aldebaran,[2] and the misty Little Dipper of the Pleiades. It is a very beautiful and large constellation. About an hour and a half after the triangle of Aries has risen, the soft-twinkling cluster of tiny stars which form the Pleiades comes above the eastern horizon, and about an hour later a V-shaped cluster of brighter stars, with a very bright-red one at the end of the lower half of the V, appears. This last cluster is the Hyades, and the bright star is Aldebaran.

[2] See “Aldebaran” in _The Friendly Stars_.

By these two clusters we may know the constellation. The ecliptic passes across Taurus about four degrees east of the Pleiades, and about seven degrees west of Aldebaran. The planets in passing through this region often come very close to the Pleiades, and parts of the group are sometimes occulted by the moon. Taurus is conspicuous in the eastern evening sky from September until nearly January. From that time on until May it may be seen in the evening, high up in the sky, a little farther west each evening, until it disappears in May. Among the four planets that we most see Mars is the only one that resembles Aldebaran in color. They are both reddish, but Mars is always west of Aldebaran near the line of the ecliptic, and also it does not have the same twinkling face that Aldebaran shows; hence the star and the planet need never be confused. Mercury, it is true, is reddish and twinkles, but so seldom needs to be taken into account that it will not be troublesome. The other planets when in Taurus will proclaim themselves by their color and size. There is no very bright star in Taurus except Aldebaran, which has been described. Any bright star north of it in the constellation is sure to be a planet.

Through Taurus the line of the ecliptic runs in a northeasterly direction, and about fifteen degrees east from Aldebaran it passes about half-way between two fairly bright stars which mark the tips of the horns of Taurus, and from there on into the fourth constellation.

GEMINI

Gemini lies northeast of Taurus, and is outlined by a box-shaped figure something more than twenty degrees long and about five degrees wide. The two stars marking the end of it farthest from Taurus are the famous twins, Castor and Pollux.[3] Pollux is a first-magnitude star, and Castor is very little less bright. They are both very charming stars, and too conspicuous to escape easy identification. Castor is greenish in tint, and rises between an hour and a half and two hours later than Aldebaran. About fifteen minutes after he appears, Pollux, with a yellow-tinted face, comes up over the eastern horizon. They rise about thirty degrees north of the exact east. The ecliptic has reached its highest point north just after passing through the horns of Taurus. It then runs through Gemini in a southeasterly direction, curving diagonally across the main figure and passing five or six degrees below Pollux. Gemini can be seen from October to early June. It is particularly charming in May in the northwest just after sundown, and when any of the planets are going along this part of their path at that season, they are sure to win one’s interest and admiration.

[3] See “The Heavenly Twins” in _The Friendly Stars_.

CANCER