10. The moon invariably presents towards the earth the same hemisphere, with the exception of a trifling irregularity called libration, by which sometimes a little more or a little less than the average of the eastern and western limbs is brought into view. Now, as she revolves round the earth, she would, if she had no revolution on her axis, bring successively every portion of her surface into view: since then as much of the surface as is brought into view by the revolution in her orbit, is carried out of view by her rotation on her axis, it is plain that the time of rotation must be equal to the time of a sidereal revolution, that is to a month.

11. The best time for watching the physical peculiarities of the surface of the moon is from the time of new moon to the end of the first quarter. Then the sun's light falls obliquely on the visible surface, and the boundary of the illuminated surface presents admirable views of the mountains and craters. The rocky sides of the craters are seen boldly projected, while the bottoms of them lie intensely black in shadow, and frequently an intensely-bright point in the centre reveals the existence of an elevation or mountainous heaving up of the crust at the bottom.

12. The science of photography promises to be of material service to astronomy, by enabling us to make exact copies of the appearances of the heavenly bodies, and especially of the moon, under different circumstances. Every one is familiar with photographic and daguerreotype likenesses of individuals, and with pictures of scenery and buildings, and with the minute accuracy with which the slightest peculiarities of costume or feature in a portrait, or of architectural style in a building, are represented. The goodness of the picture (supposing the apparatus to be perfect) depends upon the fixity of the object during the time necessary to fix the impression made by the action of the light on the paper or the silver plate, previously rendered extremely sensitive by being washed with certain chemical substances. A picture of the moon might therefore be made in a similar manner by projecting the image upon

paper or a plate properly prepared and properly protected from the action of extraneous light, if the position of the image could be kept for a short time. immoveable, relatively to the paper on which it is projected. Now this is done with tolerable accuracy by all equatorials provided with clockwork, where the mounting is sufficiently firm to prevent any sensible tremor from being communicated to the telescope, and already some good specimens of photographic pictures of the moon have been produced. One beautiful image of the moon in her first quarter, taken by the great refracting telescope of the observatory of Cambridge, near Boston, in the United States, was exhibited in June 1853, and since that time other specimens have been exhibited by Professor Phillips and Mr. De la Rue.

13. The great advantage of such pictures is that they not only delineate the geometrical positions of all the parts of the object with absolute truth, but that every separate part requiring examination will bear a high magnifying power, and may be scrutinized at leisure.

LESSON V.

TELESCOPIC VIEW OF THE PLANETS.

"Thou, even thou art Lord alone: thou hast made heaven, the heaven of heavens, with all their hosts."

1. THE PLANETS.-Next to the sun and moon, the most important objects to be examined with the telescope are the planets. These must be taken in their order of distance from the sun, which, for the present, we shall assume without proof. Modern discoveries have, during the present century, added wonderfully to the number of those known to exist. In the order of distance from the sun, their names are, MERCURY and VENUS (revolving in orbits interior to the earth); the EARTH and MARS; a group of small planets included within a small range of distances from the sun, of which thirty-six are already known; and finally JUPITER, SATURN, URANUS, and NEPTUNE.

2. MERCURY.-Of the surface of Mercury very little is known, since it is always so near to the sun as to

His

render any accurate examination of it impossible. apparent diameter, corresponding to his distances from the earth at inferior and superior conjunctions, varies from 12" to 5", and this corresponds to a real diameter of about 3000 miles. The German astronomer, Schröter, at the beginning of the present century, thought he discovered mountainous irregularities on the surface, and, by observation of these, he assigned a time of revolution to the planet on its axis, about equal to that of the earth, that is to 24h. Both the inclination of his orbit to the plane of the ecliptic, and its excentricity are large, so that the variations of heat and cold must be very great; and if the heat from the sun act on the surface in the same degree as it does on that of the earth, its mean intensity must, from its proximity to that body, be so great, that mercury would be constantly kept in a boiling state.

Heat, however, is so modified by the media through which it passes that, without a knowledge of the atmo sphere which surrounds this planet, we can say nothing positively respecting the actual heat which it experiences.

Mercury, being an inferior planet, of course exhibits phases like those of the moon, which can be calculated from a knowledge of his situation relatively to the sun and the earth.

3. VENUS.-The next planet, Venus, is well known to everybody, from the brilliancy with which, for long periods, she occasionally shines as one of the chief ornaments of the sky. Her very brightness, however, is a hindrance to any very accurate observations of her surface, since few telescopes are so well corrected for colour, or are made so achromatic, as not to exhibit a highly-coloured image, prejudicial to exact definition of the boundaries of her surface. Nevertheless Schröter, the astronomer mentioned above, by very attentive observation of her, was enabled to see some spots on her surface, which enabled him to deduce a time of revolution on her axis, rather less than that of the earth. Her variation of apparent magnitude, as seen from the earth, is much greater than that of Mercury, her diameter sometimes exceeding 1', and sometimes not being greater than 10".

Her real diameter is nearly 8000 miles, or of about the same size as the earth.

Some traces of an atmosphere have been detected, and this may probably, by its density, protect the surface of the planet, and the inhabitants, if there are any, from the intense heat and glare of the sun's rays.

She of course exhibits phases, and is, when viewed. with a telescope, a very beautiful miniature image of the moon.

4. If the planes of the orbits of Mercury and Venus coincided with the ecliptic, it is plain that every time either planet came to inferior conjunction, that is every time it came between the earth and the sun, it would be projected upon the sun's disk. On account of the inclinations of the orbits, this does not happen often ; but when it does take place in the case of Venus, this phenomenon (called the Transit of Venus) is of extreme importance, and is observed with the greatest care at different stations selected on the earth's surface, for the determination of the distance of the earth from the sun, or the solar parallax. This phenomenon, which is analogous to the eclipses of the sun and moon, will be treated of specifically in the next book.

His

5. MARS. Omitting, for obvious reasons, the consideration of the Earth, the next planet on which our telescope is to be employed is Mars. This planet is easily distinguishable with the naked eye, by the fiery redness of his colour; but with the telescope, many peculiarities of his surface are discernible, on account of the apparent magnitude of his surface when he is nearest to us. orbit has considerable excentricity, and therefore at some oppositions he comes much nearer to us than he does at others; and such opportunities are diligently made use of by astronomers for measuring his physical peculiarities. At the last opposition he did not come near to us, and the labour of a minute examination would have been in a great measure thrown away.

His diameter is about 4000 miles, and, by measures very carefully made under favourable circumstances, his shape appears to be slightly elliptical, the equatorial exceeding the polar diameter by about 1-50th part.

6. He is evidently surrounded by a very dense atmosphere, and the spots on his surface are so easily seen that a tolerable map might be made by collecting the pictures given by different observers. The poles are generally capped with white, indicating the existence of snow in large quantities, and the mottled appearance of the remainder of the surface would seem to denote the existence of continents and seas similar to those of our own globe. In fact, there is no planet whose appearance presents so many points of similarity to the earth; and the time of revolution on his axis, as found by the rotation of the spots, is only a trifle greater. His orbit also is very slightly inclined to the ecliptic, and the inclination of his equator nearly the same as that of the earth; so that his seasons must be very similar, and the length of his year being 687 days, or rather less than two of ours, the seasons are brought round with tolerable rapidity.

Mars being a superior planet never appears horned like Venus, but is always gibbous whenever he is not in opposition to or in conjunction with the sun. On account of his time of revolution being about two years, his oppositions occur after the same interval of time, and when he is in perihelion also at this time he approaches the nearest to the earth.

7. SMALL PLANETS.-The next planet visible to the naked eye is Jupiter; but between this body and Mars liea group of minute objects moving in planetary orbits, of which the telescope alone has revealed the existence.

The history of the original discovery of the first known of these bodies is curious, and shows the value of an empirical law, even though it may prove afterwards to have no foundation in fact. The distances from the sun, of the planets known at the beginning of the present century, were connected by a numerical law, which they obeyed with as much accuracy as could be expected. Thus, calling the distance of Mercury 4, that of Venus is 4+3=7; that of the Earth is 4+3×2 = 10; that of Mars is 4+3x2x2 = 16; that of Jupiter is 4+3×2×2×2×2=4+3× 24 = 52; that of Saturn is 4+3x25 = 100; and that of Uranus is 4+3x2 = 196.