[3] It is useful to know the letters of the Greek Alphabet. They are easily learned, as follows:
α Alpha
β Beta
γ Gamma
δ Delta
ε Epsilon
ζ Zeta
η Eta
θ Theta
ι Iota
κ Kappa
λ Lambda
μ Mu
ν Nu
ξ Xi
ο Omicron
π Pi
ρ Rho
σ or ς Sigma
τ Tau
υ Upsilon
φ Phi
χ Chi
ψ Psi
ω Omega
[4] All the stars visible at any hour during the year can easily be found with the help of the author's Planisphere mobile.
[5] Let it be remarked in passing that the stars might be much farther off than they are, and invisible to our eyes; the Heavens would then assume the aspect of an absolutely empty space, the moon and planets alone remaining.
[6] 14″ = 14 seconds of arc. One second of the circle is an exceedingly minute quantity. It is 1 millimeter seen at a distance of 206 meters. One millimeter seen at a distance of 20 m. 62 = 10 secs. These values are invisible to the unaided eye.
[7] These fine double stars can be observed with the help of the smallest telescope.
[8] For the explanation of the angular distances of degrees, minutes, and seconds, [see Chapter XI], on Methods of Measurement.
[9] The author has endeavored on the plates to represent the aspect of the Earth in the starry sky of Mercury, Venus, and Mars; but in all representations of this kind the stars are necessarily made too large. By calculation the diameters of the Earth and Moon as seen from the planets, and their distances, are as follows:
| Diameter of the Earth. | Diameter of the Moon. | Distance Earth-Moon. | |
| Of Mercury (opposition) | 20″ | 8″ | 871″ |
| Of Venus (opposition) | 64″ | 17″ | 1,928″ |
| Of Mars (quadrature) | 15″ | 4″ | 464″ |
| Of Jupiter (quadrature) | 3.5″ | 0.1″ | 105″ |
These aspects will be appreciated if we remember that the distance of the components of ε Lyre = 207″, that of Atlas in Pleione = 301″, and that of the stars Mizar and Alcor = 708″.