v The Earth a Planet 79 



by shrinkage, as long as the Sun remains mainly gaseous. 

 If this theory is correct, Sir William Thomson estimates 

 that twenty million years ago the substance of the Sun was 

 so diffused and cool that it had not begun to give out light 

 such as we now enjoy, and that five or six million years 

 hence the sphere will have grown solid, cold, and dark. 



1 1 9. The Earth's Share of Sun-heat. Since the Sun's 

 parallax is less than 9" it follows that, viewed from the Sun, 

 the Earth only occupies 20 ooo 1 ooooTT ^ tne s k y ' or a disc 

 1 8" in diameter. The Earth consequently receives less 

 than ^g-g.^.^-^^ of the radiant energy sent out by the Sun. 

 If the Sun were expending, instead of energy, money at the 

 rate of ; 18,000,000,000 a year, the Earth's annuity would 

 be only 9. This endowment, however, is payable con- 

 tinuously, and at the same rate throughout the year, in the 

 proportion of 6d. every day or ^d. every hour. Minute as 

 the energy which reaches the Earth appears in view of what 

 streams away into space, it is stupendous when compared 

 with the power of the greatest steam-engine ever constructed, 

 and is, indeed, the source of all the work and all the wealth 

 of the world actual and prospective. 



120. Effects of Inclined Axis. If the Earth's axis of 

 rotation were perpendicular to the plane of the ecliptic 

 the Sun's radiant energy would be dispensed for an equal 

 time each day over the whole surface every place would 

 always have 1 2 hours of daylight and 1 2 hours of darkness. 

 The Sun would always be in the zenith at noon on the 

 equator, but never elsewhere ; at the poles the Sun would 

 always be half above the horizon, and at every inter- 

 mediate point the meridian altitude would always be 

 (as in fact it is at the equinoxes) the complement of the 

 latitude, i.e. 90 minus the latitude. In consequence of the 

 inclination of the axis the distribution of radiant energy on 

 the Earth is unequal and varies at different times of the 

 year, giving rise to the difference of the seasons. 



121. Vernal Equinox. The position on 2ist March 

 (Fig. 17) is such that the equator lies in the plane of the 

 Earth's orbit as viewed from the Sun, ana!" the Sun appears 

 in the zenith at noon viewed from the equator. Sunlight 



