Part 15

With this velocity, one hundred and eighty-six thousand miles per second, we circumnavigate our globe in one-seventh of a second, reach the moon in one and a fourth seconds and the sun in eight minutes. In a little over four hours we pass the orbit of Neptune and are started on our journey to the stars, penetrating further and further into interstellar space. For a year we travel and reach not a single star though we are speeding ever onward with the velocity of light. We have now covered the distance of one light-year, which means that the waves of light from the sun we have left behind must travel for a year before they reach us. We continue our journey and find ourselves next at a distance of one parsec from the sun. We have traveled a distance of approximately three and a quarter light-years, and were it possible to see the earth as well as the sun at this distance, the two would appear to be but one second of arc apart, a distance that requires the most careful adjustment and manipulation of the telescope to measure accurately. We are still one light-year distant from Alpha Centauri, the nearest of the bright stars. A few of the stars will now appear somewhat brighter than they appeared to us on earth, but the majority of the stars appear just as we see them here and the forms of the constellations remain practically unchanged in appearance, for we are only beginning our journey through the sidereal universe and our position in it has only shifted by a very slight amount. If we should continue our journey to the immediate vicinity of Alpha Centauri, we would find that it is not like our own sun, a single star, but is a binary star consisting of two suns in revolution around their common center of gravity. The distance of this binary system from the solar system has been measured with considerable accuracy and is known to be four and a third light-years. Though there may be a few faint stars or non-luminous stars nearer to us than Alpha Centauri, this star has long held the distinction of being the nearest of the stars. As the sun continues his journey through the universe the two stars, Alpha Centauri and our sun, will finally draw away from each other after many ages have passed and some other sun of space will be our nearest star. The distances that separate the stars from each other probably average as great as the distance from the sun to Alpha Centauri. Within a sphere whose center is at the earth and whose radius is five parsecs, or about sixteen light-years, there are only about twenty known stars. There is, therefore, small chance of collision among bodies that are so small in proportion to the tremendous intervals of space that separate them from each other. There is ample room for the individual stars to pursue their journey through space without interfering with each other's motion so long as they are as widely scattered as they appear to be in this portion of the universe. The fact that our own sun has continued its journey through the universe for some hundreds of millions of years without any catastrophe such as would result from closely approaching or colliding with another sun of space shows how enormous is the scale upon which our sidereal system is fashioned.

Stars that are ten, fifty or even one hundred light-years from the earth are our nearest neighbors in space. They are the stars that show a slight displacement in the heavens or measurable parallax, viewed from opposite sides of the earth's orbit. There are probably a thousand stars among the hundreds of millions of stars within reach of the greatest telescopes whose distances have been determined in light-years by direct measurement of their displacement in the heavens resulting from the change of position of the earth in its orbit. The most distant of the stars are apparently immovable in the heavens showing neither the effect of the sun's motion or their own motion through space. Methods for finding the distances of many far remote stars and star-clusters have been devised, however, and some comparatively recent investigations have given results for the distances of these objects indicating that the diameter of the system of stars to which our sun belongs is approximately three hundred thousand light-years. It is difficult to grasp the full significance of this fact. It means that hundreds of millions of the suns of space throng the visible universe at distances from us and from each other running into hundreds, thousands and even hundreds of thousands of light-years. The light waves from some tiny object that we view today in one of our great reflectors may have started on their journey through space over one hundred thousand years ago when men of the Old Stone Age inhabited our planet earth!

Astronomers have found as a result of their investigations that the sidereal system to which our solar system belongs is in the form of a flattened spheroid with its longest axis in the plane of the Milky Way. The extent of this star system composed of hundreds of millions of individual suns in addition to nebulæ and clusters is probably something like three hundred thousand light-years along its longest axis, while globular star clusters lying above and below its central plane are estimated to be at distances from it ranging from ten thousand to two hundred thousand light-years. This entire organized system is our sidereal universe. Space beyond is unexplored. The globular star clusters are among the most distant celestial objects so far discovered. The spiral nebulæ may be entirely within the limits of this system or they may be even more distant than the globular clusters for their distances are not known as yet.

There is a possibility that our sidereal universe, vast as it is known to be, may be but a unit in some still greater unit and that other similar systems lie beyond the reach of existing telescopes at unimaginable distances.

The mind of man is overwhelmed by the thought of sidereal systems as vast as our own lying far beyond his ken. Whether or not such external systems do exist and are with our own sidereal system units in some still vaster creation we cannot know.

So vast, indeed, is this one visible universe of ours that the mind of man, accustomed to earthly standards, cannot comprehend its magnitude or the infinitesimal size of our whole solar system compared to it.

XXXI

SOME ASTRONOMICAL FACTS WORTH REMEMBERING

Kepler's Three Laws of Planetary Motion:

I. The planets move in ellipses with the sun at one focus.

II. The radius vector of a planet (line adjoining sun and planet) sweeps over equal areas in equal times.

III. The square of the time of revolution (the year) of each planet is proportional to the cube of its mean distance from the sun.

* * * * *

Sir Isaac Newton discovered that the law of gravitation extends to the stars. That is, every mass in the universe attracts every other mass with an attraction directly proportional to the product of the masses and inversely proportional to the square of the distances between them.

* * * * *

Ocean tides are caused by the difference between the attraction of the sun and moon for the main body of the earth and their attraction for different particles of the earth's surface. The tide-raising force of the disturbing body is proportional to its mass and inversely proportional to the cube of its distance. The tides produced by the sun are, therefore, only two-fifths as great as the tides produced by the moon.

* * * * *

The celestial sphere is an imaginary sphere of infinite radius, with the earth at its center, upon which the celestial bodies are considered to be projected for convenience in determining their positions with respect to fixed points of reference in the heavens.

The north and south poles of the heavens are the points on the celestial sphere directly above the north and south poles of the earth.

The celestial equator is the great circle in which the plane of the earth's equator intersects the celestial sphere. It passes through the east and west points of the horizon and through the zenith--or point directly overhead--at the earth's equator.

The ecliptic is the great circle in which the plane of the earth's orbit intersects the celestial sphere. The celestial equator and the ecliptic are inclined to each other at an angle of 23-1/2°, which is called the obliquity of the ecliptic. The two points in which the celestial equator and the ecliptic intersect are called respectively the vernal equinox and the autumnal equinox.

The vernal equinox is an important point of reference on the celestial sphere.

As the position of a point on the earth's surface is determined by its longitude and latitude so the position of an object on the celestial sphere--star, sun, planet--is determined by its Right Ascension and Declination.

The Declination of a celestial object is its distance north or south of the celestial equator, measured in degrees, minutes and seconds of arc, on a great circle of the celestial sphere passing through the object and north and south poles of the heavens. These great circles are called hour circles and they correspond to the meridians or circles of longitude on the earth's surface. The declination of an object in the heavens corresponds to the latitude of a point on the earth's surface. The Right Ascension of a point on the celestial sphere corresponds to the longitude of a point on the earth's surface. It is measured--as longitude is measured--in degrees, minutes and seconds of arc or in hours, minutes and seconds of time--eastward along the celestial equator from the hour circle passing through the vernal equinox to the foot of the hour circle passing through the object. The hour circle passing through the vernal equinox is the zero meridian for the celestial sphere just as the meridian of Greenwich is the zero meridian on the earth's surface.

* * * * *

The mean distance of the earth from the sun is 92,900,000 miles and is called the astronomical unit.

The sun with its satellites advances through the universe at the rate of 4 astronomical units in a year or approximately one million miles a day.

The parallax of a star is the angle at the star subtended by the radius of the earth's orbit, 92,900,000 miles, or the astronomical unit. It is, in other words, the angular distance between the earth and sun as viewed from the star. The larger the parallax the nearer the star. The largest known stellar parallax is that of Alpha Centauri and its value is 0".75.

The light-year is the distance that light travels in one year. It is equal to about 63,000 astronomical units or nearly six trillion (6,000,000,000,000) miles. The velocity of light is 186,000 miles per second.

The parsec is equal to 3.26 light-years. It is the distance of a star that has a parallax of one second of arc.

The apparent magnitude of a star is its apparent brightness estimated on a scale in which a difference of one magnitude corresponds to a difference in brightness of 2.51, or the fifth root of one hundred. A difference of five magnitudes corresponds to a difference one one hundredfold in brightness, of ten magnitudes to ten thousandfold in brightness. In exact measurements on this scale magnitudes are estimated to tenths.

Stars that are one magnitude brighter than stars of the standard first magnitude are of the zero magnitude and stars still brighter are of negative magnitudes.

Sirius is a star of the -1.6 magnitude. Jupiter at opposition is of -2.0 magnitude and Venus at greatest brilliancy of -4.0 magnitude. The sun on this scale of comparative brightness is of the -26.7 apparent magnitude. The faintest stars visible in the most powerful telescope in the world--the 101-inch Mt. Wilson Hooker telescope--are of the twentieth magnitude.

The _absolute_ magnitude of a star is its apparent magnitude at the standard distance of ten parsecs or 32.6 light years. The absolute magnitude of the sun is five. That is, the sun would be a fifth-magnitude star at the standard distance of 32.6 light-years. The absolute magnitudes of stars indicate how bright they would be relatively if they were all at the same standard distance. Apparent magnitudes indicate how bright the stars appear to be at their true distances.

* * * * *

The mean distance of the moon from the earth is approximately 240,000 miles or sixty times the earth's radius.

The sun is four hundred times farther away than the moon and its diameter is about four hundred times greater than the moon's diameter.

The nearest star is about 275,000 times more distant than the sun, and the most distant known object, the globular star cluster, N.G.C. 7106, is about fourteen billion times more distant than the sun.

The earth is a spheroid flattened at the poles and its polar diameter is about twenty-seven miles shorter than its equatorial diameter. An object weighs less at the poles than at the equator.

The earth's interior is as rigid as steel and probably consists of a core of magnetic iron surrounded by an outer stony shell.

Eclipses of the sun occur when the moon passes between the earth and sun. They can only occur at the time of new moon. There must be at least two solar eclipses every year separated by an interval of six months and there may be as many as five solar eclipses in a year. Eclipses of the moon occur when the earth comes between the sun and moon, and the moon passes into the earth's shadow. Eclipses of the moon can only occur at full moon. There may or may not be eclipses of the moon every year. The greatest number of eclipses than can occur in any one year, solar and lunar combined, is seven and the least number is two and in that case they are both solar eclipses.

The sun is a yellow, dwarf star of a density of one and one-fourth that of water and with a surface temperature of about 12,000° F. except in sun-spot regions where the temperature is about 6,000° F. It is probably gaseous throughout.

The sun, as well as the planets, rotates on its axis and different portions of the surface rotate at slightly different rates. The average period of the rotation of the sun on its axis is about twenty-six days.

The sun is a variable star with a twofold variation. One is of long period during the eleven-year sun-spot cycle with a range of from three to five per cent. The other is a short irregular variation with a period of a few days, weeks or months and a range of from three to ten per cent.

Sun-spots are solar cyclones and appear black only by contrast with their hotter and brighter surroundings. They come in eleven-year cycles (approximately) with periods of maximum and minimum appearance.

The brightness and blue color of the sky is due to the scattering of sunlight by the molecules of oxygen and nitrogen in the earth's upper atmosphere. If there were no atmosphere the skies would appear black except in the direction of the heavenly bodies, which would be visible by day as well as by night.

The solar corona is the rare outer envelope of the sun and it is visible only during a total eclipse of the sun. It is partly of an electrical nature and it varies in form during the sun-spot cycle. It often extends to a distance of several solar diameters on either side of the sun.

The warmth and the habitability of the earth's surface is due to the presence of water-vapor and carbon-dioxide in the atmosphere. Without these substances in the atmosphere life on the earth's surface would be impossible.

Half of the earth's atmosphere and all clouds lie within seven miles of the earth's surface, and at high elevations above the earth the temperature is many degrees below zero.

The temperature of space approaches the absolute zero of -459° F.

The only planets in the solar system with the exception of the earth that might possibly support life are Venus and Mars.

Stars shine by their own light but planets shine only by reflected light from the sun.

* * * * *

If the earth were represented by a six-inch school globe the sun would be on the same scale a globe fifty-four feet in diameter. Mercury would be a small ball two and a third inches in diameter. Venus would be another six-inch globe. Mars would be a ball about the size of a baseball, three and a fifth inches in diameter. The moon would be about the size of a golf ball, one and a half inches in diameter. The largest asteroids would be the size of marbles. Average-sized asteroids would be the size of shot and the smallest would be merely grains of sand.

Jupiter would be a huge globe standing as tall as a man five feet six inches in height. Saturn would be a smaller globe four and a half feet in diameter and its ring system would extend to a distance of five and a half feet on either side of the globe. Uranus would be represented by a globe almost exactly two feet in diameter and Neptune would be a slightly larger globe with a diameter of two feet two and a half inches.

The satellites of the outer planets would range in size from tennis and golf balls for the largest, to marbles for the smaller and grains of sand for the smallest.

On the same scale of measurement the distance of the six-inch globe of the earth from the fifty-four foot globe representing the sun would be one and one-tenth miles. The moon would be placed fifteen feet from the earth-globe and the diameter of the solar system on the same scale measured across the orbit of Neptune would be sixty-six miles. The nearest star on this scale would be three hundred thousand miles away.

* * * * *

If the distance from the earth to the sun is taken as one inch so that the scale of the universe is reduced six trillion times, the diameter of the solar system across Neptune's orbit is five feet and the distance of one light-year comes out almost exactly equal to one mile. The nearest star to the five-foot solar system would be four and a third miles away; the most distant known object would be two hundred and twenty thousand miles away, and the extent of the visible universe would be three hundred thousand miles. On the same scale the diameter of our sun would be about one hundredth of an inch and the diameters of the giant stars Antares and Betelgeuze would be four inches and two and three-fourth inches respectively. To see the earth we would need a microscope.

TABLES

TABLE I

THE PRINCIPAL ELEMENTS OF THE SOLAR SYSTEM

=============+=============+========+============+========+============+=========== |Mean Distance from Sun| |Velocity| | +-------------+--------+ | in | |Inclination | |Relative| Period | Orbit |Eccentricity| of Planet | In Miles | to | of |(Miles | of | Orbit to | |Earth's | Revolution | per | Orbit | Ecliptic | |Distance| |Second )| | ------------+-------------+--------+------------+--------+------------+----------- Mercury | 36,000,000| 0.39 | 87.97 days |23 to 35| .2056 | 7° 0' Venus | 67,200,000| 0.72 |224.70 days | 21.9 | .0068 | 3 23 Earth | 92,900,000| 1.00 |365.25 days | 18.5 | .0167 | 0 0 Mars | 141,500,000| 1.52 | 1.88 years| 15.0 | .0933 | 1 51 Asteroids[1]| ...........|2.0-5.2 | ......... | ...... | .00 to .40 | 0° to 35° Jupiter | 483,300,000| 5.20 | 11.86 years| 8.1 | .0484 | 1 18 Saturn | 886,000,000| 9.54 | 29.46 years| 6.0 | .0558 | 2 29 Uranus |1,781,900,000| 19.19 | 84.02 years| 4.2 | .0471 | 0 46 Neptune |2,971,600,000| 30.07 |164.79 years| 3.4 | .0085 | 1 47 ============+=============+========+============+========+============+===========

FOOTNOTE:

[1] About 940 have been discovered up to the present time.

THE PRINCIPAL ELEMENTS OF THE SOLAR SYSTEM (Continued)

=========+========+==========+==========+========+============+==========+==========+============+=========== | Mean | | |Density | Surface |Velocity |Reflecting| Period |Inclination Name |Diameter| Mass | Volume |Relative| Gravity |of Escape | Power | of | of | in +----------+----------+to that | (Relative |(Miles per| in | Axial | Equator | Miles | Relative to Earth's |of Water| to Earth's)| Second) | Per Cent | Rotation | to Orbit ---------+--------+----------+----------+--------+------------+----------+----------+------------+----------- Sun | 864,392| 329,390 | 1,300,000| 1.40 | 27.64 | 383 | ..... |25 d. 8 h.| 7° 15' Moon | 2,160| .012 | .02 | 3.34 | 0.16 | 1.5 | 7 |27 d. 7.7 h.| 6 41 Mercury | 3,009| .045 | .06 | 4.48? | 0.31? | 2.2 | 7 |88 d. ? | ? Venus | 7,575| .807 | .92 | 4.85? | 0.85 | 6.6 | 59 | ? | ? Earth | 7,918| 1.000 | 1.00 | 5.53 | 1.00 | 7 | 44 |23 h. 56 m. | 23 27 Mars | 4,216| .106 | .15 | 3.58 | 0.35 | 1.5 | 15 |24 37 | 23 59 Asteroids|5-485[2]|very small|very small| 3.3 |.0008 to .04|.33 to .01| 7 | ..... | ..... Jupiter | 88,392| 314.50 | 1309 | 1.25 | 2.52 | 37 | 56 | 9 55± | 3° Saturn | 74,163| 94.07 | 760 | 0.63 | 1.07 | 22 | 63 |10 14± | 27° Uranus | 30,878| 14.40 | 65 | 1.44 | 0.99 | 13 | 63 |10 45± | ? Neptune | 32,932| 16.72 | 85 | 1.09 | 0.87 | 14 | 73 | ? | ? =========+========+==========+==========+========+============+==========+==========+============+===========

FOOTNOTE:

[2] Extreme values.

TABLE II

THE SATELLITES OF THE SOLAR SYSTEM