equator; as is the case with probably all of the planets. The following table gives the latest authentic measurements. 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 1 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 astronomers 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, vol. v. p. 220. b Ast. Nach., vol. xiv. Nos. 333-5; vol. xix. No. 438. c Mem. R.A.S., vol. xxix. p. 39. 1861. d See p. 242 et seq. е "The line of eclipses." 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 Solstices". 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 11° 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 ." 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 nor a night; because when the Sun is at these points, day and night are theoretically equal throughout the world. In 1890 this occurs on March 20 at 4h, and Sept. 22 at 14h, G.M.T. From sol the Sun, and stare to stand still; because the Sun when it has reached born in 610 B.C. Other authorities ascribe it to Pythagoras or the Egyptians, while Laplace believed that observations for the determination of this angle were made by Tcheou-Kong in China not less than 1100 years before the Christian erai. The accord between the various determinations ancient and modern is very remarkable, and indicates the great care bestowed by the astronomers of antiquity on their investigations. The eccentricity of the Earth's orbit amounts (to be more precise than above) to 00167917, and it is subject to a very small diminution, not exceeding 0.000041 in the course of 100 years. Supposing the change to go on continuously, the Earth's orbit must eventually become circular; but we learn from the Theory of Attraction that this progressive diminution is only to proceed for a certain time. Le Verrier has shown that this diminution cannot continue beyond 24,000 years, when the eccentricity will be at its minimum of 0033: it will then begin to increase again; so that unless some external cause of perturbation arise, these variations may continue throughout all ages, within certain not very wide limits. They are due to the attractive influence of the Planets. The above value of the eccentricity is for 18co'o A.D. The line of apsides is subject to an annual direct change of 1177", independent of the effects of precession (to be described hereafter); so that, allowing for the latter cause of disturbance, the annual movement of the apsides may be taken at rather more than 1'. One important consequence of this motion of the major axis of the Earth's orbit is the variation in the lengths of the seasons at different periods of time. In the year 3958 B.C., or, singularly enough, near the epoch of the Creation of Adam, the longitude of the Sun's perigee coincided with the autumnal equinox; so that the summer and autumn quarters were of equal length, but shorter than the winter and spring quarters, which were also equal. In the year 1267 A.D. the perigee coincided with the winter solstice; the spring quarter was therefore equal to the summer one, and the autumn quarter to the winter one, Conn. des Temps. 1811, p. 429. the former being the longest. In the year 6493 A.D. the perigee will have completed half a revolution, and will then coincide with the vernal equinox; summer will then be equal to autumn, and winter to spring; the former seasons, however, being the longest. In the year 11719 A.D. the perigee will have completed three-fourths of a revolution, and will then coincide with the summer solstice; autumn will then be equal to winter, but longer than spring and summer, which will also be equal. And finally in the year 16945 A.D. the cycle will be completed by the coincidence of the solar perigee with the autumnal equinox. This motion of the apsides of the Earth's orbit, in connection with the inclination of its axis to the plane of it, must quite obviously have been the cause of very remarkable vicissitudes of climate in pre-Adamite times. One result of this position of things we may readily grasp at this moment. As a matter of fact, in consequence of our seasons being now of unequal length, the spring and summer quarters jointly extend to 186d, whilst the autumn and winter quarters comprise only 1789. The Sun is therefore a longer time in the Northern hemisphere than in the Southern hemisphere: hence the Northern is the warmer of the two hemispheres. Probably it may be taken as one result of this fact, that the North Polar regions of the Earth are easier of access than the South Polar regions. In the Northern hemisphere navigators have reached to 81° of latitude, whereas 71° is the highest attained in the Southern hemisphere. It is not a very easy matter in treating of the Earth to determine where astronomy ends and geography begins; but a brief allusion to the means available for deciding the form of the Earth seems all that it is now necessary to add here. We learn that the Earth is a sphere (or something of the sort) by the appearance presented by a ship in receding from the spectator: first the hull disappears, then the lower parts of the rigging, and finally the top-masts. The shadow cast on the Moon during a * See Papers by Croll, Phil. Mag., 4th Ser., vol. xxxv. p. 363, May 1868; vol. xxxvi. pp. 141 and 362, Aug. and Nov. 1868; Geikie's Great Ice Age, &c. lunar eclipse, and the varying appearances of the constellations as we proceed northwards or southwards, are amongst the other more obvious indications of the Earth's globular form. Fig. 56, Plate VI, represents an experimental proof of the Earth's rotation on its axis. This particular form of proof excited no small interest in scientific (and unscientific) circles when it was first promulgated by the French savant Foucault in the year 1851. If a pendulum, or its equivalent, a heavy weight suspended by a long wire, could be erected at either pole of the Earth, and be set swinging in any direction and a note of the direction taken, it is evident that if the plane of oscillation were observed to be perpetually shifting with regard to the terrestrial point noted at the beginning of the experiment, it would be a proof that either the terrestrial station was shifting with respect to the pendulum or the pendulum was shifting with respect to the station. The latter idea being contrary to reason the former alternative must be adopted. It is evident that both poles of the Earth being inaccessible to us, the experiment cannot be carried out in the theoretically simple fashion suggested above; but in a modified form it can be tried and will yield an intelligible result at a station on the Earth's surface between the Pole and the Equator, provided it be not very near the Equator. The rationale of the experiment is simply this, that the weight being made to oscillate in a straight line (and starting it by burning the thread which holds it should secure this) it will swing backwards and forwards in an invariable plane. If the building in which the experiment is tried were at rest, the plane of oscillation would be constantly parallel to a line joining any 2 points in the building if the plane of oscillation had been parallel to that line when the start was made. But if the building moves in consequence of an axial rotation of the Earth, the angle between the plane of oscillation and the line parallel thereto at the start will be continually varying and in the course of some hours will vary through an angular space of many degrees. Could the experiment be tried at the Pole the 1 See Proc. Roy. Inst., vol. i. p. 70: Arago, Pop. Ast., Eng. ed., vol. ii. p. 27. |