The tides of the sea are due to the pull of the moon, and, in lesser degree, of the sun. The whole earth is pulled by the moon, but the loose and mobile water is more free to obey this pull than is the solid earth, although small tides are also caused in the earth's solid crust. The effect which the tides have on slowing down the rotation of the earth is explained in the text.
(Drawn approximately to scale)
The isolation of the Solar System is very great. On the above scale the nearest star (at a distance of 25 trillions of miles) would be over one half mile away. The hours, days, and years are the measures of time as we use them; that is: Jupiter's "Day" (one rotation of the planet) is made in ten of our hours; Mercury's "Year" (one revolution of the planet around the Sun) is eighty-eight of our days. Mercury's "Day" and "Year" are the same. This planet turns always the same side to the Sun.
(Drawn approximately to scale)
On this scale the Sun would be 17½ inches in diameter; it is far greater than all the planets put together. Jupiter, in turn, is greater than all the other planets put together.
Diagram Showing the Main Layers of the Sun
The plains were originally supposed to be seas: hence the name "Mare."
A Diagram of a Stream of Meteors Showing the Earth Passing Through Them
Zenith Telescope constructed for the International Stations at Mizusawa, Carloforte, Gaithersburg and Ukiah, by Hermann Wanschaff, Berlin.
The figure represents an altazimuth theodolite of an improved pattern used on the Ordnance Survey. The horizontal circle of 14-in. diameter is read by three micrometer microscopes; the vertical circle has a diameter of 12 in., and is read by two microscopes. In the great trigonometrical survey of India the theodolites used in the more important parts of the work have been of 2 and 3 ft. diameter—the circle read by five equidistant microscopes. Every angle is measured twice in each position of the zero of the horizontal circle, of which there are generally ten; the entire 610number of measures of an angle is never less than 20. An examination of 1407 angles showed that the probable error of an observed angle is on the average ±0″.28
The Sun, the most important of the celestial bodies so far as we are concerned, occupies the central position; not, however, in the whole universe, but only in that limited portion which is known as the Solar System. Around it, in the following order outwards, circle the planets Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. At an immense distance beyond the solar system, and scattered irregularly through the depth of space, lie the stars. The two first-mentioned members of the solar system, Mercury and Venus, are known as the Inferior Planets; and in their courses about the sun, they always keep well inside the path along which our earth moves. The remaining members (exclusive of the earth) are called Superior Planets, and their paths lie all outside that of the earth.
By the second century of the Christian era, the ideas of the early philosophers had become hardened into a definite theory, which, though it appears very incorrect to us to-day, nevertheless demands exceptional notice from the fact that it was everywhere accepted as the true explanation until so late as some four centuries ago. This theory of the universe is known by the name of the Ptolemaic System, because it was first set forth in definite terms by one of the most famous of the astronomers of antiquity, Claudius Ptolemæus Pelusinensis (100–170 a.d.), better known as Ptolemy of Alexandria.
In his system the Earth occupied the centre; while around it circled in order outwards the Moon, the planets Mercury and Venus, the Sun, and then the planets Mars, Jupiter, and Saturn. Beyond these again revolved the background of the heaven, upon which it was believed that the stars were fixed—
"Stellis ardentibus aptum,"
as Virgil puts it.
The 'Ring with Wings.' - Assyrian Form
It should, however, be here mentioned that Mr. E.W. Maunder has pointed out the probability that we have a very ancient symbolic representation of the corona [of an eclipse] in the "winged circle," "winged disc," or "ring with wings," as it is variously called, which appears so often upon Assyrian and Egyptian monuments, as the symbol of the Deity.
hese mysterious visitors to our skies come up into view out of the immensities beyond, move towards the sun at a rapidly increasing speed, and, having gone around it, dash away again into the depths of space. As a comet approaches the sun, its body appears to grow smaller and smaller, while, at the same time, it gradually throws out behind it an appendage like a tail. As the comet moves round the central orb this tail is always directed away from the sun; and when it departs again into space the tail goes in advance. As the comet's distance from the sun increases, the tail gradually shrinks away and the head once more grows in size (see Fig. 18). In consequence of these changes, and of the fact that we lose sight of comets comparatively quickly, one[Pg 249] is much inclined to wonder what further changes may take place after the bodies have passed beyond our ken.
In this picture the South will be found at the top of the picture; such being the view given by the ordinary astronomical telescope, in which all objects are seen inverted.
This instrument, 150 feet in length, with a skeleton tube, was constructed by the celebrated seventeenth century astronomer, Hevelius of Danzig. From an illustration in the Machina Celestis.
The attempts to construct large telescopes of the Galilean type met in course of time with a great difficulty. The magnified image of the object observed was not quite pure; its edges, indeed, were fringed with rainbow-like colours. This defect was found to be aggravated with increase in the size of object-glasses. A method was, however, discovered of diminishing this colouration, or chromatic aberration as it is called from the Greek word χρῶμα (chroma), which means colour, viz. by making telescopes of great length and only a few inches in width. But the remedy was, in a way, worse than the disease; for telescopes thus became of such huge proportions as to be too unwieldy for use. Attempts were made to evade this unwieldiness by constructing them with skeleton tubes.
Corresponding views of the same situations of an Inferior Planet as seen from the Earth, showing consequent phases and alterations in apparent size.
From an illustration in the Opera Varia of Christian Huyghens.
Attempts were made to evade this unwieldiness by constructing them with skeleton tubes. or , indeed, even without tubes at all; the object-glass in the tubeless or "aerial" telescope being fixed at the top of a high post, and the eye-piece, that small lens or combination of lenses, which the eye looks directly into, being kept in line with it by means of a string and manœuvred about near the ground. The idea of a telescope without a tube may appear a contradiction in terms; but it is not really so, for the tube adds nothing to the magnifying power of the instrument, and is, in fact, no more than a mere device for keeping the object-glass and eye-piece in a straight line, and for preventing the observer from being hindered by stray lights in his neighbourhood. It goes without saying, of course, that the image of a celestial object will be more clear and defined when examined in the darkness of a tube.
Great telescope at the Yerkes Observatory of the University of Chicago, Williams Bay, Wisconsin, U.S.A. It was erected in 1896–7, and is the largest refracting telescope in the world. Diameter of object-glass, 40 inches; length of telescope, about 60 feet. The object-glass was made by the firm of Alvan Clark and Sons, of Cambridge, Massachusetts; the other portions of the instrument by the Warner and Swasey Co., of Cleveland, Ohio.
We have mentioned Halley's Comet of 1682, and how it revisits the neighbourhood of the earth at intervals of seventy-six years. The comet of 1066 has for many years been supposed to be Halley's Comet on one of its visits. The identity of these two, however, was only quite recently placed beyond all doubt by the investigations of Messrs Cowell and Crommelin. This comet appeared also in 1456, when John Huniades was defending Belgrade against the Turks led by Mahomet II., the conqueror of Constantinople, and is said to have paralysed both armies with fear.
1. The Sun, 2. Mercury, 3. Venus, 4. The Earth, 5. The Moon, 6. The Sydereal Dial plate, 7. The Hour Circle, 8. ye Circle for ye. Moon’s Age, 9. The Moon’s Orbit, 10. ye Pointer, Shewing the Sun’s Place & Day of the Month, 11. The Ecliptic, 12. The Handle for turning ye whole machine
The Motions and Phases of Mercury and Venus explained
The Moon’s surface mountainous
The Method of finding the Distances of the Sun, Moon, and Planets
The Geometrical Construction of Solar and Lunar Eclipses
This Piece of Mechanism exhibits the Time, Quantity, Duration, and Progress of solar Eclipses, at all Parts of the Earth.
Why the weather is coldest when the Earth is nearest the Sun.
The cause of the Tides was discovered by Kepler, who, in his Introduction to the Physics of the Heavens, thus explains it: “The Orb of the attracting power, which is in the Moon, is extended as far as the Earth; and draws the waters under the torrid Zone, acting upon places where it is vertical, insensibly on confined seas and bays, but sensibly on the ocean whose beds are large, and the waters have the liberty of reciprocation; that is, of rising and falling.”
The seasons shewn in another view of the Earth, and it’s Orbit
The superstitious notions of the antients with regard to Eclipses
By Astronomy we discover that the Earth is at so great a distance from the Sun, that if seen from thence it would appear no bigger than a point; although it’s circumference is known to be 25,020 miles. Yet that distance is so small, compared with the distance of the Fixed Stars, that if the Orbit in which the Earth moves round the Sun were solid, and seen from the nearest Star, it would likewise appear no bigger than a point, although it is at least 162 millions of miles in diameter.
The primary Planets never eclipse one another
The apparent magnitudes of the Planets continually change as seen from the Earth, which demonstrates that they approach nearer to it, and recede farther from it by turns. From these Phenomena, and their apparent motions among the Stars, they seem to describe looped curves which never return into themselves, Venus’s path excepted. And if we were to trace out all their apparent paths, and put the figures of them together in one diagram, they would appear so anomalous and confused, that no man in his senses could believe them to be representations of their real paths; but would immediately conclude, that such apparent irregularities must be owing to some Optic illusions.