Mars is the first planet whose orbit is exterior to that of the Earth, and when in opposition to the Sun, is a very conspicuous object, in the heavens, and of a fiery red colour; this, however, only takes place once in two years, the planet's synodic period being 780 days. When in perigee and in perihelion at the same time, as it is in opposition, Mars shines with a brilliancy rivalling Jupiter. Such was the case in August, 1719, when the planet was only 2½° from perihelion; its brightness was then so considerable as to give rise to a sort of panic. In order to be seen to the greatest advantage it must not only be in opposition, but also in perihelion at the same time; this occurs only once in 8 years.

Telescopically examined, Mars is found to be covered with numerous dusky patches; these are supposed to be the outlines of continents and seas analogous to those in our own globe: near the poles there are brilliant white spots, considered by many to be masses of snow; this idea is somewhat strengthened by the fact that they have been observed to diminish when brought under the Sun's influence at the commencement of the Martial summer; and increase again on the approach of winter.

By far the most important of what may be termed the miscellaneous astronomical instruments is the Transit, or Transitcircle, the smaller and less perfect kinds being chiefly used for taking the time, and the larger for measuring the positions of stars, &c, for forming catalogues.

We shall only describe the small, or portable, transit. The instrument consists of three principal parts, the telescope, the stand, and the circle: a b, is a telescope of a large field and low power, the tube of which is in 2 parts, connected by a cubical centre-piece, into which, at right angles to the optical axis, are fitted the larger ends of 2 cones, c, c, which form the horizontal axis of the telescope, the smaller ends of each cone are accurately ground to z perfectly equal cylinders, or pivots. These pivots rest on Y's, angular bearings, which surmount the 2 side standards, e and w, of which e may be called the eastern, and w the western. One of the Y's is fixed in a horizontal groove, so that, by means of a screw, a small azimuthal motion may be imparted to the instrument; in like manner a small motion in altitude may be obtained by turning the foot screw g. On one end of the axis is fixed, so that it may revolve with it, a decimation circle, d, divided to degrees and read by verniers to minutes, &c.

Of the other instruments used for astronomical purposes the following are the chief:—

1. The Altazimuth.

2. The Mural Circle.

3. Borda's Repeating Circle.

4. The Zenith Sector.

5. The Reflex Zenith Tube.

6. The Sextant.

7. The Box Sextant.

8. Troughton's Reflecting Circle.

9. The Dip Sector.

10. The Floating Collimator.

11. The Orbit Sweeper.

The instruments comprised in the above list are, for the most part, not required for the purposes of the amateur astronomer. We shall therefore content ourselves with a very brief mention of each, adding, however, references for the use of those who may desire to know more about them.

The Altazimuth, as its name implies, is for the measurement of altitudes and azimuths. It may be considered as a modification of the ordinary transit instrument, the telescope, circle, and stand of which are capable of motion round a vertical pivot. The altazimuth may therefore be used for meridional or extrameridional observations indifferently, and when of a portable size, may be looked upon as a theodolite of a superior construction.

The Mural Circle consists of a graduated circle furnished with a suitable telescope, and very firmly affixed to a wall (murus) in the plane of the meridian. It is used for determining, with great accuracy, meridian altitudes and zenith distances, from which may be found declinations and polar distances.

The discovery of this planet is justly esteemed one of the greatest triumphs ever recorded in the annals of astronomy. The following is a brief résumé of the circumstances that led to it:— From a long and attentive course of observation on the planet Uranus, it was ascertained that it was subject to some disturbing agency, which frequently gave rise to a considerable discordance between the calculated and the observed path of the planet amongst the stars, and which could only be accounted for on the supposition that there was another, though as yet unseen, planet revolving round the Sun in an orbit exterior to that of Uranus. To discover the. suspected planet, MM. Adams and Le Verrier, the former in 1843, the latter in 1845, commenced independently, an examination of the theory of Uranus. From their investigations, they were both led to infer the existence of the unseen planet in the constellation Aquarius. M. Le Verrier having informed Dr. Galle, of the Royal Observatory, Berlin, of the result of his labours, that gentleman was fortunate enough to discover the planet on the night of Sept. 23, 1846, very near the position assigned by theory.

The Sun as the centre of the system will first occupy our attention. The distance of the Earth from the Sun, which is usually employed by astronomers as a unit of measurement, has been ascertained with great accuracy, from the transit of Venus over the disc of the latter in 1769, to be 95,298,260 miles, a distance which every successive transit will render more and more exactly known. Having ascertained the true mean distance of the Earth from the Sun, it is not difficult to determine, by trigonometry, the true diameter of the latter body, its apparent diameter being known from observation; and as the most reliable results prove that the Sun, in the above position, subtends an angle of about 32', it follows that its true diameter is about 887,000 miles; the volume of this enormous globe, therefore, exceeds that of the Earth 1,400,000 times; in other words, it would take 1,400,000 Earths to make up a globe of the same size as the Sun. The Sun's mass, or attractive power, exceeds that of the Earth 355,000 times, and is 476 times greater than the masses of all the planets put together.

The earliest eclipse on record is one given in the Chinese history the Chou-king, and which is supposed to refer to the solar eclipse of Oct. 13, 2127 B.C.

One of the most celebrated eclipses of the Sun recorded in history is that which occurred in the year 584 B.C. It is so, not only on account of its having been predicted by Thales, who was the first ancient astronomer who gave the true explanation of the phenomena of eclipses, but because it seems to fix the precise date of an important event in ancient history. It appears that a war had been carried on for some years between the Lydians and Medes; and we are indebted to Herodotus for an account of the circumstances that led to its premature termination.

“In the sixth year, when they were carrying on the war with nearly equal success, on the occasion of an engagement, it happened that, in the heat of the battle, day wag suddenly turned into night (συνήν∊ικ∊ στ∊ τήs μάχηs συν∊στώσηs τήν ήμέΡην έαΠίνηs νύκτα Υ∊νέσθαι). This change of the day, Thales, the Milesian, had foretold to the Ionians, fixing beforehand, this year as the very period in which the change actually took place. The Lydians and Medes, seeing night succeeding in the place of day, desisted from fighting, and both showed a great anxiety to make peace.” A peace was accordingly made and cemented by a marriage. “For, without strong necessity, agreements are not wont to remain firm.”

Venus, when in a favourable position, is to us the most conspicuous member of the planetary system. Its apparent diameter differs considerably at different times: this is owing to the greater extent of variation of distance than occurs in the case of Mercury. Its phases are similar to those of Mercury, to which the reader is referred. The discovery of the phases of Venus is due to Galileo, who announced the fact to his friend Kepler in the following logograph or anagram: –

These things not ripe, as yet concealed from others, are read by me.

Which, when transposed, becomes —

Little is known of the appearance of this planet's surface, owing to its dazzling brilliancy when viewed in a telescope; notwithstanding this, however, spots are occasionally visible. These spots are not supposed to be connected with the planet's surface, but rather to belong to its atmosphere. Schröter has suspected the existence of numerous high mountains on its surface. The rotation of Venus on its axis was discovered by D. Cassini in the year 1667.

TheDominical Letter, called also the Sunday Letter, is an expedient by means of which we can readily find out the day of the week on which any day of the year falls, knowing the day of the week on which New Year's Day falls. To the first seven days of January are affixed the first seven letters of the alphabet—A, B, C, D, E, F, G; and the one of these which denotes Sunday is the Dominical letter. Thus, if Sunday is New Year's Day, then A is the Dominical letter; if Monday, that letter is G; and so on. If there were 364 days, or 52 weeks exactly in the year, then the Dominical letter would always be the same; but as the year contains aboixt 365¼ days, or 1¼ more than 364., this excess has to be taken into account every year, and the ¼ makes a day in every 4 years; so that the Dominical letter falls backward one letter every common year, and two letters every Bissextile or Leap year. Knowing the Dominical letter, we can ascertain all the Sundays, all the Mondays, &c, in the year. The reason why Leap years have two letters may be thus explained: —Take, for example, the year 1860.

Shooting stars, although noticed in former times, have only within the last half century attracted any particular attention. This branch of the science may therefore be considered to be, comparatively, in its infancy. We must possess a long and carefully made series of observations before we are likely to be acquainted, with any degree of precision, with the physical nature of these objects. They were formerly considered to be merely atmospheric meteors, caused by the combustion of inflammable gases. This opinion has, however, now lost much, if not all, of its force, and they are now recognised as bodies which, although they become inflamed on coming in contact with the Earth's atmosphere, yet have their origin far beyond it.

It is now an established fact, that there is no night throughout the year on which shooting stars may be not seen; and that, on an average, from 5 to 7 may be noticed on a clear night every hour. These occasional meteors may be termed sporadic, in contradistinction to those swarms which appear at certain times of the year, and which are periodic.

On examining the heavens on a clear evening, when the Moon is not shining, we find here and there groups of stars which seem to be compressed together in such a manner as to present a dull cloud-like appearance: these are termed clusters and nebulæ, and are usually classed as follows: —

1. Irregular groups, visible more or less to the naked eye.

2. Clusters resolvable into separate stars with the aid of a telescope.

3. Nebulæ, for the most part irresolvable.

The latter are subdivided into: —

1. i. Annular Nebulæ.

2. ii. Elliptic Nebulæ.

3. iii. Spiral Nebulæ.

4. iv. Planetary Nebulæ.

5. v. Nebulous Stars.

Of the 1st class, there are several examples to be found, with all of which the reader is probably more or less familiar. The cluster of the Pleiades in Taurus is probably best known. When examined directly, few persons can see more than 6 stars, but by turning the eye sideways, we discover there are many more, and between 50 and 60 are visible in a telescope. The most brilliant star in the group is Alcyone, or η Tauri, of the 3rd magnitude ; next in order come Electra and Atlas of the 4th; Maia and Taygeta of the 5th; Pleione and Celeno, which are between the 6th and 7th; Asterope between the 7th and 8th; and finally, a great number of smaller stars.

Time is, strictly speaking, of infinite duration; we are, therefore, obliged to choose some arbitrary unit by means of which a measurement of time may be effected. For short intervals, the diurnal rotation on its axis of the globe we inhabit; for longer intervals, the annual revolution of the Earth around the Sun, are the standards of measurement we employ; but any event which takes place at equal intervals of time may serve the purpose of a chronometrical register. Thus, the ages of certain trees may be ascertained by counting the number of concentric rings in the trunk, so many being formed annually; the ages of certain cattle, by the number of rings on the horns; the ages of horses may in like manner be ascertained by the successive disappearance of marks from their teeth; so also the pulsations of the heart, the flowing of a certain quantity of water from one vessel to another, the oscillations of a pendulum, may all be used as measurers of time: but, in practice, the length of a day is a natural interval of time, which the domestic habits of man force upon him; and accordingly we find that amongst all nations this unit of measurement is, under some form or other, the one adopted.

Between the orbits of Mars and Jupiter there is a wide interval, which, until the present century, was not known to be occupied by any planet. The researches of late years, as previously intimated in Chapter II., have led to the discovery of a numerous group of small bodies revolving round the Sun, which are known as the Minor Planets, and which have received names taken chiefly from the mythology of ancient Greece and Rome.

The planets differ in some respects from the other members of the system, especially in point of size, the largest being probably not more than 200 or 300 miles m diameter. Their orbits are also much more inclined, as a general rule, than the orbits of the older planets, whence they are sometimes termed the ultra-zodiacal planets.

The following is a list of these planets, together with the chief elements of their orbits:—

It is needless to give any detailed account of each, but a short summary may not be out of place.

The nearest to the Sun is Flora, which revolves round that luminary in 1193 days or 3¼ years, at a mean distance of 209,819,000 miles.

The most distant is Maximiliana, whose period is 2343 days or 6.4 years, and whose mean distance is 329,000,000 miles.