14 VARIATIONS IN THE EARTH'S TEMPERATURE. 



ellipse, because the larger a circle is the more nearly any part of its 

 circumference approaches to a straight line. This polar compression 

 or flattening makes the polar diameter of the earth almost 26^ miles 

 shorter than it would be if the earth were a sphere. According to 

 the most recent estimate, the measurement of the earth from pole to 

 pole is 7899.2 miles. But the equatorial circumference may also 

 be slightly elliptical, though the compression is estimated to amount 

 to less than 2 miles. It is well known that large masses of land rise 

 above the general level to a height of a mile or two, and it is quite 

 possible that the equatorial irregularity of outline, if it really exists, 

 is a consequence of the contractions of the earth's crust which have 

 changed the form of the globe by elevating the great continents. 



The Earth's Orbit as Influencing the Temperature of its Sur- 

 face. The earth's orbit is an ellipse ; its distance from the sun varies 

 from 89,860,000 miles to 92,950,000 miles. The earth is nearest the 

 sun in our winter about the beginning of January, when it is in the 

 position called perihelion ; its distance then is about three millions 

 of miles less than in summer, when it is in the position called aphe- 

 lion. The amount of heat received from the sun varies inversely as 

 the square of the earth's distance from it, so that at first sight the 

 summer at our antipodes would seem to be warmer than our own 

 summer ; but when the earth is nearest to the sun its rate of motion 

 is most rapid, as may be seen from the circumstance that there are 

 fewer days in winter than in summer, and this rapidity of motion 

 compensates for the reduced distance, and the earth receives about 

 the same amount of heat in each of the seasons. A more remarkable 

 circumstance about the earth's orbit is the fact that the ellipse itself 

 rotates, going forward i in 308 years. And this throws the positions 

 of the equinoctial points backward, so that the seasons change their 

 times with regard to the earth's surface, and of course our summer 

 comes round eventually to happen when the earth is nearest to the 

 sun. It is also calculated that owing to the attractions of the planets 

 the eccentricity of the earth's orbit varies both by the ellipse becom- 

 ing longer and shorter ; this change would probably greatly influence 

 climate, by modifying the amount of heat that the earth would 

 receive from the sun ; because the more elongated the form of tho 

 orbit, the less is the distance of the earth from the sun in perihelion 

 and the greater its distance in aphelion ; though Professor Dove found 

 that the mean temperature of the whole of the earth's surface in June 

 is much higher than in December, owing to the land being chiefly in 

 the Northern Hemisphere. When the eccentricity of the earth's orbit 

 is greatest, this part of the world may come to be eight and a half 

 millions of miles farther from the sun than it is now in winter ; and 

 though at present winter is nearly eight days shorter than summer, 

 it then might be thirty-six days longer than it is now. It is certain 

 that from this cause important changes of climate must occur every 

 10,492 years. At the last of these periods, when the earth was 

 nearest the sun in summer and farthest off in winter, the difference 

 of temperature due to that position was calculated by Sir John Her- 



