The wonderful precision which has adapted the position of the earth on its axis, will be apparent from the illustration (fig. 557). Here we have a table and some bottles, a candle to represent the sun, and a ball of worsted and a knitting-needle to represent the earth and its axis. Suppose we place the ball in the position at a, with its axis perpendicular to the plane of the orbit. As the earth would turn and go round the sun in this supposed case, we should find the days and nights equal, and the sun would quickly scorch up the tropics, and the other portions would have a never-changing spring or winter all the year for ever. This would not be so pleasant, for variety is the charm of nature, and the salt of life. So we may put a aside, as the earth would be scarcely habitable under the supposed conditions, and try b. Here we find the poles directed to the sun. The whole northern hemisphere would thus be illuminated one half year, and the southern similarly; such rapid changes from heat to cold and back again would not suit us. So we fall back upon c, the actual appearance of the position of the earth, and here we find all the most favourable circumstances existing for us. This inclination gives rise to all the varied phenomena of the pleasant gradations of heat and cold, summer and winter, the charming changes of season, and the wonderful results of the ever-recurring days and nights, months and years, as the earth spins round. So we see that the sun does not really rise and set upon the earth; the globe rotates, and brings us into view of the sun, and as we turn we lose his light.
Fig. 557.—Inclination of Axis.
In the foregoing brief description we have learnt some few facts concerning our earth. We have ascertained that the planet we inhabit is round; we have also seen that the earth moves around the sun and around its own axis, and also that it moves at a tremendous rate; we know that that rate is just counterbalanced by the attraction of gravitation, and the course round the sun gives us varying seasons, day and night. There are many subjects relating to the earth which will be more properly included under Physical Geography. We may here just add the diameter of the earth, and proceed to inquire concerning the moon. The polar diameter of our globe is 7,899 miles; the equatorial diameter 7,925 miles. It is distant from the moon 238,500 miles. We will close this chapter with the letters and characters of the Greek alphabet used in astronomical works to designate the stars.
- Α α = Alpha
- Β β = Beta
- Γ γ = Gamma
- Δ δ = Delta
- Ε ε = Eps[=ilon
- Ζ ζ = Zeta
- Ê ê = Eta
- Θ θ = Theta
- Ι ι = Iota
- Κ κ = Kappa
- Λ λ = Lambda
- Μ μ = Mu
- Ν ν = Nu
- Ξ ξ = Xi
- Ο ο = Omicron
- Π π = Pi
- Ρ ρ = Rho
- Σ ς = Sigma
- Τ τ = Tau
- Υ υ = Upsīlon
- Φ φ = Phi
- Χ χ = Chi
- Ψ ψ = Psi
- Ω ω = Omega
Fig. 558.—A ship disappearing below the horizon.