ANNUAL MOTION OF THE EARTH.] 



ASTRONOMY. 



923 



momentum, will move faster through the whole of its 

 descent than the base, and reach the ground a little in 

 advance, or easterly, of the foot of the perpendicular, 

 the direction of the earth's rotation. Owing to the 

 small height of buildings suitable for the purpose, this is 

 a very difficult matter to test ; but experiments have 

 been conducted with this result, in 1804, in St. Michael's 

 Tower, at Hamburg, and, in 1805, in a coal-pit at Schle- 

 Fig. 10. busch, in the county of Mark. Let 



the circle E (Fig. 10) be the equa- 

 torial circumference of the earth, 

 the line T a tower perpendicular to 

 C the centre, the circle M will then 

 be the circumference described by 

 the summit of the tower S, in the 

 course of one rotation of the earth 

 upon its axis. If we suppose the 

 base of tower b to pass to c, the summit S will in the 

 game time pass to a ; and this being the larger arc, it 

 follows that the summit must travel faster than the base. 

 If then the earth rotate eastward, a ball dropped from 

 the summit will leave it with its momentum, and move 

 faster eastward through the whole of its descent than 

 the base. The result will be that it will deviate a little 

 from the plumb-line, and fall a little to the east of e. 



Again : a pendulum of a given length, which makes 

 86,535 vibrations in a day at London, will make only 

 86,400 in the same time if transported to the equator. 

 This shows that the force of gravity which produces 

 oscillations must be least where the movement is the 

 slowest, or less at the equator than at London. Now 

 assuming the earth's rotation, its equatorial regions, 

 where the circle of the circumference is the greatest, 

 must revolve with greater velocity than those which are 

 situated towards the poles ; consequently, the tendency 

 to fly off from the centre is greater there, which propor- 

 tionally neutralises the force of gravity, and accounts 

 for the unequal action of the pendulum. 



At the equator, the rate of the rotation is about 1,042 

 miles an hour, or 17 miles a minute ; at 30 of north 

 latitude it is 14 miles a minute ; at 45, or about the 

 centre of France, it is 11. 



ANNDAL MOTION OF THE EAKTH. 

 The annual motion of the earth, 

 or its orbital movement round the 

 sun, occupies a period of 365 days, 

 6 hours, 48 minutes, 497 secouds. 

 This forms the solar year, or the 

 period which the sun appears to 

 take, through the actual procession 

 of our planet, in passing from a 

 particular point of the ecliptic, 

 say the first point of Aries, where 

 the ecliptic and the equator inter- 

 sect, to the same point again. It 

 is also called the tropical year, 

 because it is the interval occupied 

 by the sun, in visiting the tropics 

 and returning to the equator. The 

 sidereal year, or the space of time 

 which the sun takes in apparently 

 passing from any fixed star till it 

 returns to it again, is 305 days, 

 6 hours, 9 minutes, 9 P 7 seconds; 

 rather more than 20 minutes 

 longer than the tropical year, 

 which is due to a slow annual dis- 

 placement of the equinoctial 

 points. 



If the mean distance of the 

 earth from the sun i 95,000,(XK) 

 of miles, the diameter of the orbit 

 is 100,000,000 of miles^ and its 

 600,000,000 of miles. 



an hour. This results from the second law of Kepler, 

 to which we shall afterwards return. 



Hipparchus, two thousand years ago, was the first who 

 closely approximated to the true length of the solar or 

 tropical year. His determination of 3G5 days, 5 hours, 

 55 minutes, 12 seconds, exhibits a value but slightly iu 

 excess o the truth. 



The earth's motion of translation in space, like that of 

 its rotation, is imperceptible by us ; resembling that we 

 experience in calmly floating down a stream, when sur- 

 rounding stationary objects appear to be in movement, 

 and our senses are lulled into complete forgetfulness of 

 our own progression. It appeals not to the eye, as in 

 the case of the other planets, which are seen to be con- 

 stantly changing their place ; but, besides the evidence 

 of strong probability in its favour, it has received sensible 

 confirmation from the discovery of the aberration of the 

 stars. 



In its annual revolution round the sun, the axis of the 

 earth, inclined 23| from a line perpendicular to the plane 

 of the orbit, maintains invariably the same positioa, caus- 

 ing the phenomena of the seasons. An attempt is made 

 in the following engraving to delineate the annual revo- 

 lutions of the earth about the sun, and the other pheno- 

 mena attending it. (See Fig. 11). 



By measuring the distances of the earth from the sun, 

 at different times of the year, the shape of its orbit has 

 been ascertained. These distances, as they were unequal, 

 could not, of course, be semi-diameters of a circle, but 

 they corresponded, taken together, to the radii vectores 

 of an ellipse. The straight line connecting the perihelion 

 and aphelion, passing through the centre of the sun, is 

 the line of apsides. The inclination of the earth's orbit to 

 its equator, or the so-called obliquity of tlie ecliptic, 

 amounts to 23 27'. The velocity of the earth is greatest 

 at the perihelion, and least at the aphelion. It is further 

 to be observed, that the mean distance of the earth from 

 the sun is equal to half the major axis of the earth's 

 orbit, and the line of apsides is itself the major axis. 

 There are four noteworthy points in the earth's orbit in the 

 engraving, viz., those which mark the beginning of the 

 four seasons. Two of these points are called the solstices 

 fig. 11. 



linear extent near 

 This enormous distance is tra- 

 versed at the rate of 68,000 miles an hour, or 19 miles 

 in a second. This is the mean velocity for the year ; 

 but in January the earth travels at the rate of 69,600 

 miles an hour ; which is more than 3,000 miles an hour 

 iU rate of motion in July, when it is only 00,400 miles 



they mark the beginning of winter and summer. Th') 

 straight line uniting them, passing through the centre of 

 the sun, is called the solstitial colure. The two other 

 points are the equinoxes, vernal and autumnal, marking 

 the commencement of spring and autumn. The straight 

 line cutting them at right angles, and passing through 

 the centre of the sun, is the equiiwctial colure. This 



