Astronomy for Amateurs Part 15

If the Earth turns in twenty-four hours upon itself, a point upon the equator would simply travel at a rate of 465 meters (1,525 feet) per second. This speed, while considerable in comparison with the movements observed upon the surface of our planet, is as nothing compared with the fantastic rapidity at which the Sun and stars would have to move, in order to rotate round our globe.

Thus we have to choose between these two hypotheses: either to make the entire Heavens turn round us in twenty-four hours, or to suppose our globe to be animated by a motion of rotation upon itself. For us, the impression is the same, and as we are insensible to the motion of the Earth, its immobility would seem almost natural to us. So that, in last resort, here as in many other instances, the decision must be made by simple common sense. Science long ago made its choice. Moreover, all the progress of Astronomy has confirmed the rotary movement of the Earth in twenty-four hours, and its movement of revolution round the Sun in a year; while at the same time a great number of other motions have been discovered for our wandering planet.

The learned philosophers of antiquity divined the double movement of our planet. The disciples of Pythagoras taught it more than two thousand years ago, and the ancient authors quote among others Nicetas of Syracuse, and Aristarchus of Samos, as being among the first to promote the doctrine of the Earth's movement. But at that remote period no one had any idea of the real distances of the stars, and the argument did not seem to be based on any adequate evidence. Ptolemy, after a long discussion of the diurnal motion of our planet, refutes it, giving as his princ.i.p.al reason that if the Earth turned, the objects that were not fixed to its surface would appear to move in a contrary direction, and that a body shot into the air would fall back to the West of its starting-point, the Earth having turned meantime from West to East. This objection has no weight, because the Earth controls not only all the objects fixed to the soil, but also the atmosphere, and the clouds that surround it like a light veil, and all that exists upon its surface. The atmosphere, the clouds, the waters of the ocean, things and beings, all are adherent to it and make one body with it, partic.i.p.ating in its movement, as sometimes happens to ourselves in the compartment of a train, or the car of an aerostat. When, for instance, we drop an object out of such a car, this object, animated with the acquired velocity, does not fall to a point below the aerostat, but follows the balloon, as though it were gliding along a thread. The author has made this experiment more than once in aerial journeys.

Thus, the hypothesis of the Earth's motion has become a certainty. But in addition to reasoning, direct proof is not wanting.

1. The spheroidal shape of the Earth, slightly flattened at the poles and swollen at the equator, has been produced by the rotary motion, by the centrifugal force that it engenders.

2. In virtue of this centrifugal force, which is at its maximum at the equator, objects lose a little of their weight in proportion as they are farther removed from the polar regions where centrifugal force is almost _nil_.

3. In virtue of this same centrifugal force, the length of the pendulum in seconds is shorter at the equator than in Paris, and the difference is one of 3 millimeters.

4. A weight abandoned to itself and falling from a certain height, should follow the vertical if the Earth were motionless. Experiment, frequently repeated, shows a slight deviation to the East, of the plumb-line that marks the vertical. We more especially observed this at the Pantheon during the recent experiments.

5. The magnificent experiment of Foucault at the Pantheon, just renewed under the auspices of the Astronomical Society of France, demonstrates the rotary motion of the Earth to all beholders. A sufficiently heavy ball (28 kilograms, about 60 pounds) is suspended from the dome of the edifice by an excessively fine steel thread. When the pendulum is in motion, a point attached to the bottom of the ball marks its pa.s.sage upon two little heaps of sand arranged some yards away from the center.

At each oscillation this point cuts the sand, and the furrow gets gradually longer to the right hand of an observer placed at the center of the pendulum. The plane of the oscillations remains fixed, but the Earth revolves beneath, from West to East. The fundamental principle of this experiment is that the plane in which any pendulum is made to oscillate remains invariable even when the point of suspension is turned. This demonstration enables us in some measure to see the Earth turning under our feet.

The annual displacements of the stars are again confirmatory of the Earth's motion round the Sun. During the course of the year, the stars that are least remote from our solar province appear to describe minute ellipses, in perspective, in the Heavens. These small apparent variations in the position of the nearest stars reproduce the annual rotation of the Earth round the Sun, in perspective.

We could adduce further observations in favor of this double movement, but the proofs just given are sufficiently convincing to leave no doubt in the mind of the reader.

Nor are these two the only motions by which our globe is rocked in s.p.a.ce. To its diurnal rotation and its annual rotation we may add another series of _ten more motions_: some very slow, fulfilling themselves in thousands of years, others, more rapid, being constantly renewed. It is, however, impossible in these restricted pages to enter into the detail reserved for more complete works. We must not forget that our present aim is to sum up the essentials of astronomical knowledge as simply as possible, and to offer our readers only the "best of the picking."

The two princ.i.p.al motions of which we have just spoken give us the measure of time, the day of twenty-four hours, and the year of 365-1/4 days.

The Earth turning upon itself in twenty-four hours from West to East, presents all its parts in succession to the Sun fixed in s.p.a.ce.

Illuminated countries have the day, those opposite, in the shadow of the Earth, are plunged into night. The countries carried by the Earth toward the Sun have morning, those borne toward his shadow, evening. Those which receive the rays of the day-star directly have noon; those which are just opposite have midnight.

The rotation of our planet in this way gives us the measure of time; it has been divided arbitrarily into twenty-four periods called hours; each hour into sixty minutes; each minute into sixty seconds.

In consequence, each country turns in twenty-four hours round the axis of the Earth. The difference in hours between the different regions of the globe is therefore regulated by the difference of geographical position. The countries situated to the West are behind us; the Sun only gets there after it has shone upon our meridian. When it is midday in Paris, it is only 11.51 A.M. in London; 11.36 A.M. in Madrid; 11.14 A.M.

at Lisbon; 11.12 A.M. at Mogador; 7.06 A.M. at Quebec; 6.55 A.M. at New York; 5.14 A.M. in Mexico; and so on. The countries situated to the East are, on the contrary, ahead of us. When it is noon in Paris, it is already 56 minutes after midday at Vienna; 1.25 P.M. at Athens; 2.21 P.M. at Moscow; 3.16 P.M. at Teheran; 4.42 P.M. at Bombay; and so on. We are here speaking of real times, and not of the conventional times.

[Ill.u.s.tration: FIG. 60.--Motion of the Earth round the Sun.]

If we could make the tour of the world in twenty-four hours, starting at midday from some place to go round the globe, and traveling westward with the Sun, we should have him always over our heads. In traveling round the world from West to East, one goes in front of the Sun, and gains by one day; in taking the opposite direction, from East to West, one loses a day.

In reality, the exact duration of the Earth's diurnal rotation is twenty-three hours, fifty-six minutes, four seconds. That is the sidereal day. But, while turning upon itself, the Earth circulates upon its...o...b..t, and at the end of a diurnal rotation it is still obliged to turn during three minutes, fifty-six seconds in order to present exactly the same meridian to the fixed Sun which, in consequence of the rotary period of our planet, is a little behind. The solar day is thus one of twenty-four hours. There are 366 rotations in the year.

And now let us come back to the consequences of the Earth's motion. In the first place our planet does not turn vertically nor on its side, but is tipped or inclined a certain quant.i.ty: 23 27'.

Now, throughout its annual journey round the Sun, the inclination remains the same. That is what produces the seasons and climates. The countries which have a larger circle to travel over in the hemisphere of the solar illumination have the longer days, those which have a smaller circle, shorter days. At the equator there is constantly, and all through the year, a twelve-hour day, and a night of twelve hours.

[Ill.u.s.tration: FIG. 61.--Inclination of the Earth.]

In summer, the pole dips toward the Sun, and the rays of the orb of day cover the corresponding hemisphere with their light. Six months later this same hemisphere is in winter, and the opposite hemisphere is in its turn presented to the Sun. June 21 is the summer solstice for the northern hemisphere, and is at the same time winter for the southern pole. Six months later, on December 21, we have winter, while the southern hemisphere is completely exposed to the Sun. Between these two epochs, when the radiant orb shines exactly upon the equator, that is on March 21, we have the spring equinox, that delicious flowering season when all nature is enchanting and enchanted; on September 21 we have the autumn equinox, melancholy, but not devoid of charm.

The terrestrial sphere has been divided into different zones, with which the different climates are in relation:

1. The tropical zone, which extends 23 27' from one part to the other of the equator. This is the hottest region. It is limited by the circle of the tropics.

2. The temperate zones, which extend from 23 27' to 66 23' of lat.i.tude, and where the Sun sets every day.

3. The glacial zones, drawn round the poles, at 66 33' lat.i.tude, where the Sun remains constantly above or below the horizon for several days, or even several months. These glacial zones are limited by the polar circles.

We must add that the _axis_ of the Earth is a straight line that is supposed to pa.s.s through the center of the globe and come out at two diametrically opposite points called the _poles_. The diurnal rotation of the Earth is effected round this axis.

The name _equator_ is given to a great circle situated between the two poles, at equal distance, which divides the globe into two hemispheres.

The equator is divided into 360 parts or degrees, by other circles that go from one pole to the other. These are the _longitudes_ or meridians (see Fig. 62). The distance between the equator and the pole is divided into larger or smaller circles, which have received the name of _lat.i.tudes_, 90 degrees are reckoned on the one side and the other of the equator, in the direction of the North and South poles, respectively. The longitudes are reckoned from some point either to East or West: the lat.i.tudes are reckoned North and South, from the equator.

In going from East to West, or inversely, the longitude changes, but in pa.s.sing from North to South of any spot, it is the lat.i.tude that alters.

[Ill.u.s.tration: FIG. 62.--The divisions of the globe. Longitudes and lat.i.tudes.]

The circles of lat.i.tude are smaller in proportion as one approaches the poles. The circ.u.mference of the world is 40,076,600 meters at the equator. At the lat.i.tude of Paris (48 50') it is only 26,431,900 meters. A point situated at the equator has more ground to travel over in order to accomplish its rotation in twenty-four hours than a point nearer the pole.

We have already stated that this velocity of rotation is 465 meters per second at the equator. At the lat.i.tude of Paris it is not more than 305 meters. At the poles it is _nil_.

The longitudes, or meridians, are great circles of equal length, dividing the Earth into quarters, like the parts of an orange or a melon. These circ.u.mvent the globe, and measure some 40,000,000 (40,008,032) meters. We may remember in pa.s.sing that the length of the meter has been determined as, by definition, the ten-millionth part of the quarter of a celestial meridian.

Thus, while rotating upon itself, the Earth spins round the Sun, along a vast orbit traced at 149,000,000 kilometers (93,000,000 miles) from the central focus, a sensibly elliptical orbit, as we have already pointed out. It is a little nearer the Sun on January 1st than on July 1st, at its perihelion (_peri_, near, _helios_, Sun), than at its aphelion (_apo_, far, _helios_, Sun). The difference = 6,000,000 kilometers (3,720,000 miles), and its velocity is a little greater at perihelion than at aphelion.

This second motion produces the _year_. It is accomplished in three hundred and sixty-five days, six hours, nine minutes, nine seconds.

Such is the complete revolution of our planet round the orb of day. It has received the name of sidereal year. But this is not how we calculate the year in practical life. The civil year, known also as the tropical year, is not equivalent to the Earth's revolution, because a very slow gyratory motion, called "the precession of the equinoxes," the cycle of which occupies 25,765 years, drags the spring equinox back some twenty minutes in each year.

The civil year is, accordingly, three hundred and sixty-five days, five hours, forty-eight minutes, forty-six seconds.

In order to simplify the calendar, this acc.u.mulating fraction of five hours, forty-eight minutes, forty-six seconds (about a quarter day) is added every four years to a biss.e.xtile year (leap-year), and thus we have uneven years of three hundred and sixty-five, and three hundred and sixty-six days. Every year of which the figure is divisible by four is a leap-year. By adding a quarter day to each year, there is a surplus of eleven minutes, fourteen seconds. These are subtracted every hundred years by not taking as biss.e.xtile those secular years of which the radical is not divisible by four. The year 1600 was leap-year: 1700, 1800, and 1900 were not; 2000 will be. The agreement between the calendar and nature has thus been fairly perfect, since the establishment of the Gregorian Calendar in 1582.

Since the terrestrial orbit measures not less than 930,000,000 kilometers (576,600,000 miles), which must be traversed in a year, the Earth flies through s.p.a.ce at 2,544,000 kilometers (1,577,280 miles) a day, or 106,000 kilometers (65,720 miles) an hour, or 29,500 meters (18 miles) per second on an average, a little faster at perihelion, a little slower at aphelion. This giddy course, a thousand times more rapid than the speed of an express-train, is effected without commotion, shock, or noise. Reasoning alone enables us to divine the prodigious movement that carries us along in the vast fields of the Infinite, in mid-heaven.

Returning to the calendar, it must be remarked in conclusion, that the human race has not exhibited great sense in fixing the New Year on January 1. No more disagreeable season could have been selected. And further, as the ancient Roman names of the months have been preserved, which in the time of Romulus began with March, the "seventh" month, "September," is our ninth month; October (the eighth) is the tenth; November (the ninth) has become the eleventh; and December (the tenth) has taken the place of the twelfth. Verily, we are not hard to please!

These months, again, are unequal, as every one knows. Witness the simple expedient of remembering the long and short months, by closing the left hand and counting the k.n.o.bs and hollows of the fist, the former corresponding to the long months, the latter to the short: first k.n.o.b = January; first hollow, February; second k.n.o.b, March; and so on.[12]

[Ill.u.s.tration: FIG. 63.--To find the long and short months.]

Should not the real renewal of the year coincide with the awakening of Nature, with the spring on the terrestrial hemisphere occupied by the greater portion of Humanity, with the date of March 21st? Should not the months be equalized, and their names modified? Why should we not follow the beautiful evolution dictated by the Sun and by the movement of our planet? But our poor Earth may roll on a long time yet before its inhabitants will become reasonable.

CHAPTER IX

THE MOON