Page images
PDF
EPUB

:

the reign of Ptolemy Philadelphus, on the night of the 17th of the Egyptian month Messori, when Timocharis saw the planet eclipse a star at the extremity of the wing of Virgo. This date answers to 271 B.C., Oct. 12 A.M. " As this was not a telescopic observation, it and all others recorded before telescopes came into use, are open to this uncertainty, that the two objects may merely have been in juxta-positon so as to have appeared as one without actual super-position taking place. The recorded occultation of Mercury by Venus on May 17, 1737, was no doubt an occultation in the strict sense of the word.

The interesting discovery of the phases of Venus is due to Galileo”, who announced the fact to his friend Kepler in the following logogriphe or anagram :

“ Hæc immatura, a me, jam frustra, leguntur.-oy.” “These things not ripe [for disclosure) are read, as yet in vain, by me.” Or, as another interpretation has it“ These things not ripe; at present (read) in vain [by others) are read by me."

The “me” in the former case being the ordinary reader; in the latter, Galileo. This, when transposed, becomes

“Cynthiæ figuras æmulatur Mater Amorum.” “ The Mother of the Loves (Venus) imitates the phases of Cynthia (the Moon)."

The letters ‘o y'are, it will be observed, redundant, so far that they cannot be made use of in the transposition.

To the mariner, owing to its rapid motion, Venus is a useful auxiliary for taking lunar distances when continuous bad weather may have prevented observations of the Sun.

In computing the places of Venus the tables of Baron De Lindenau, published in 1810, were long in use, but they have now

y Hind, Sol. Syst., p. 32.

* It was one of the objections urged to Copernicus against his theory of the solar system that if it were true then the inferior planets ought to exhibit phases. He is said to have answered that if ever men obtained the power of seeing them

more distinctly, they would be found to do so. Prof. De Morgan believes the anecdote to be apocryphal. (Month. Not., vol. vii. p. 290. June 1847.) But “se non è vero, è ben trovato.”

a Opere di Galileo, vol. ii. p. 42. Ed. Padova, 1744.

been superseded by those of Le Verrier, for amongst other causes of error there existed a long inequality (first suspected by Sir G. B. Airy about 1828, and fully expounded in 1831 b) affecting the heliocentric places of the Earth and the planet to a very sensible amount. This inequality goes through all its changes in about 239), and when at a maximum displaces Venus by 3" and the Earth by 2", as viewed from the Sun.

o Phil. Trans., vol. cxviii. p. 23, 1828 ; vol. cxxii. p. 67, 1832.

CHAPTER VI.

THE EARTH. 0

O let the Earth bless the Lord: yea, let it praise Him, and magnify Him

for ever.”_Benedicite.

Period, &c.Figure of the Eurth. The Ecliptic. The Equinoxes. - The Solstices.

Diminution of the obliquity of the ecliptic.The eccentricity of the Earth's orbit.-Motion of the Line of Apsides.- Familiar proofs and illustrations of the sphericity of the Earth.--- Foucault's Pendulum Experiment.Mädler's tables of the duration of day and night on the Earth.Opinions of uncient philosophers. -English mediæral synonyms.The Zodiac.Mass of the Earth.

THE Earth is a planet which may perhaps be said to be in

- all essential respects similar to Venus and Mars, its nearest neighbours; but as we are on it, it is needless to point out the impossibility of treating of it in the same way as we treat of the other planets. It revolves round the Sun in 3650 6h 9m 9•68, at a mean distance of 92,890,000 miles. The eccentricity of its orbit amounting to o'01679, this distance may either extend to 94,450,000 miles or diminish to 91,330,000 miles ; and these differences involve variations in the light and heat reaching the Earth which will be represented by the figures 966 and 1033, the mean amount being 1000.

The Earth is not a sphere, but an oblate spheroid ; that is to say, it is somewhat flattened at the poles and protuberant at the equator; as is the case with probably all of the planets. The following table gives the latest authentic measurements.

[ocr errors][merged small][merged small][merged small][merged small][ocr errors][ocr errors]

The close coincidence between these results affords a good guarantee of the accuracy of both, and is noticeable as an illustration of the precision arrived at in the working out of such problems, the difference between the two values of the equatorial diameter being only 77 yards. If we represent the Earth by a sphere I yard in diameter, that diameter will make the polar diameter } inch too long.

Further, it has been suspected by General Schubert and Colonel A. R. Clarke that the equatorial section of the Earth is not circular, but elliptical. Colonel Clarke's conclusion is that the equatorial diameter, which pierces the Earth through the meridians 13° 58' and 193° 58' E. of Greenwich, is i mile longer than the equatorial diameter at right angles to it.

A consideration of the method in which such investigations are conducted does not fall within the scope of the present sketch, but in Airy's Popular Astronomy the subject of the Figure of the Earth is handled with much clearness d.

The great circle of the heavens apparently described by the Sun every year (owing to our revolution round that body) is called the Ecliptice, and its plane is usually employed by astronoiners as a fixed plane of reference. The plane of the Earth's equator, extended towards the stars, marks out the equator of the heavens, the plane of which is inclined to the ecliptic at an

* Encycl Metrop., art. Fig. of Earth, © Mem. R.A.S., vol. xxix. p. 39. 1861. vol. y. p. 220.

d See p. 242 et seq. Ast. Nach., vol. xiv. Nos. 333-5; vol. “The line of eclipses." xix. No. 438.

angle which, on Jan. 1, 1880, amounted to 23° 27' 17.55"; this angle is known as the Obliquity of the Ecliptic. It is this inclination which gives rise to the vicissitudes of the seasons during our annual journey round the Sun. The two points where the celestial equator intersects the ecliptic are called the Equinoxes? ; the points midway between these being the Solstices8. It is from the vernal (or spring) equinox that Right Ascensions are measured along the equator, and Longitudes along the ecliptic. The obliquity of the ecliptic is now slowly decreasing at the rate of about 46" in 100 years. “It will not always however, be on the decrease ; for before it can have altered to the cause which produces this diminution must act in a contrary direction, and thus tend to increase the obliquity. Consequently the change of obliquity is a phenomenon in which we are concerned only as astronomers, since it can never become sufficiently great to produce any sensible alteration of climate on the Earth's surface. A consideration of this remarkable astronomical fact cannot but remind us of the promise made to man after the Deluge, that 'while the earth remaineth, seedtime and harvest, and cold and heat, and summer and winter, and day and night shall not cease.' The perturbation of obliquity, consisting merely of an oscillatory motion of the plane of the ecliptic, which will not permit of its [the inclination] ever becoming very great or very small, is an astronomical discovery in perfect unison with the declaration made to Noah, and explains how effectually the Creator had ordained the means for carrying out His promise, though the way it was to be accomplished remained a hidden secret until the great discoveries of modern science placed it within human comprehension h.”

It is stated by Pliny that the discovery of the obliquity of the ecliptic is due to Anaximander, a disciple of Thales, who was

! From æquus equal, and nox a night; because when the Sun is at these points, day and night are theoretically equal throughout the world. In 1890 this oc. curs on March 20 at 4", and Sept. 22 at 145, G.M.T.

8 From sol the Sun, and stare to stand

still; because the Sun when it has reached these neutral points has attained its greatest declination N. or S. as the case may be. In 1890 this occurs on June 21 at on, and Dec. 21 at 9", G.M.T.

+ Hind, Sol. Syst., p. 33.

« PreviousContinue »