of winter look up through the frosty air at Capella, as it stands at the apex of the starry vault, shining with a clear white light. You will be ready to admit that it is a fit jewel for the crown of the ice-king. As soon as your own eyes have marked the fact that Sirius is, in point of brightness, a seven-fold Vega, its splendid scintillations will

great nebula in Andromeda?

Have you at any time turned your operaglass upon the famous double

cluster in Perseus, or upon the Pleiades? How many stars can you see within the bowl

of the Great Dipper? Is

Nebulæ and clusters.

your eye sufficiently keen to split the double-star Ep-
silon Lyræ, which lies but three moon-breadths from
Vega? Has a telescope ever split again each of these
stars for you, so that you realized that they formed a
system of four revolving suns? Have you seen Venus
at mid-day, or can you recognize her in the evening, as moon.

Venus and the

The Milky Way.

Urania.

she glows with silvery sheen in the west, and weaves her way in and out among the stars, from night to night? Can Venus be seen at midnight? Is the full moon visible at noon? Do the horns of the crescent moon point toward the sun? Does the moon always set directly in the west? In what direction does the moon move among the stars, eastward or westward?

On some night when the sky was perfectly clear, and the moon was not in sight, have you made a study of the wonderful river of light which foams across the sky? Have you seen the dark rocks against which it dashes, the foaming eddies here and there, and the profusion of starry spray with which it besprinkles the adjoining constellations?

Must you give a negative answer to most of these questions? Then let the arm-chair control you no longer. Yield to the charms of Urania: woo her, and make her your friend. How shall this wooing proceed? This chapter and the next four shall be your guide in this matter. In them will be developed an orderly method of procedure, which will lead, by easy stages, to the attainment of the desired end.

First we mention briefly the classes of objects with which our study will be concerned.

The fixed stars, or more simply the stars, are those The fixed stars. brilliant points of light which stud the heavens, remaining in the same relative position from year to year, and from century to century, as nearly as the unaided eye can judge. Had an ancient Assyrian made a rude representation of the Great Dipper on one of his tablets of clay, we should at this day instantly recognize the configuration as one with which we are familiar. The fixed stars are suns, at such amazing distances from us that their motions seem exceedingly small.

The nebulæ are cloud-like masses of matter of vast extent, which are as far away as the stars. The great nebula in Andromeda can be seen easily with the naked eye, and the nebula in the sword-handle of Orion can be glimpsed. The vast majority of these objects, however, are visible only with powerful telescopes. Quite a number are invisible even in the largest instruments, but have imprinted themselves on photographic plates exposed for hours in the foci of special star-cameras.

The planets look like the fixed stars, when viewed with the naked eye, except that they do not twinkle. Jupiter and Venus are usually brighter than the brightest fixed stars. Mars, Mercury, Saturn, Uranus, and Neptune are less brilliant, Neptune never being visible to the unaided eye. The ancients, who were unacquainted with Uranus and Neptune, discovered that the other planets changed their apparent positions among the stars. From this circumstance arose the designation "planet," which signifies "wanderer." These bodies are all comparatively near us, the most distant being less than three thousand million miles away. The minor planets, also called asteroids, are small bodies coursing about the sun in paths which lie between those of Mars and Jupiter.

The nebulæ.

The planets.

Comets derive their name, which means 'hairy Comets. ones," from their tails or trains, which often attain to great magnificence. Some of them are to be regarded as members of the solar system, since they revolve about the sun in closed curves. Others are simply visitors, which display their beauty for a time, and then whisk off to regions unknown.

Meteors are those rash little bodies which plunge headlong into the earth, and thus end their careers in an outburst of evanescent glory.

Meteors.

earth.

The sun, moon, and earth need no particular mention, Sun, moon, and the earth being one of the sun's family of planets, and the moon being her attendant; the moon belongs to the class of bodies known as satellites, which revolve about the planets.

We are now in a position to understand any mention

The Great
Dipper.

which may be made of these celestial objects, prior to the detailed discussion of them which will come later. Our present business is to get acquainted with the fixed The Great Dipper is the first configuration to be learned (Fig. 9). Around the margin of the diagram are given dates, which will aid in finding it. To locate it on February 1 at 8 p. m., the book is to be held out in front

stars.

of the reader, with the center of the diagram on a level with his eyes, and the point marked February 1 at the uppermost part of the circle. The diagram then shows that the Dipper is at the right of Polaris, the pole-star. Two of the stars in the bowl are called the Pointers, because they point toward Polaris. The distance

between the Pointers is about five degrees, and should

be fixed in mind as a sort of yardstick with which to The yardstick. estimate distances between other stars. The distance

from Polaris to the nearest Pointer is about five times our yardstick.

S'

In order to get an accurate notion of measurement by degrees, imagine that the stars are fastened upon the inner surface of a huge celestial sphere, the distance from the earth to the surface of the sphere being so great as to be beyond adequate comprehension (Fig. 10). Let E be the position of the observer on the earth, while S and S' are two stars said to be 30° apart. Through these stars a circle whose center is at E is drawn on the surface of the celestial sphere. From E two lines, ES and ES', are drawn, making the angle SES'. This angle is measured by the number of degrees in the arc SS', there being 360° in an entire circle. If the arc SS' is one twelfth of the entire circumference, the angle SES' is an angle of 30°.

FIG. 10.-MEASUREMENT OF AN ANGLE.

Now the diameter of the earth, which is less than

Angular measurement.