Chapter XVI: Appendix: B

=Figures.=--When explaining the positions and forms of things, it is often necessary to use certain terms and figures, that is to say, lines which are either real or imaginary, but which can be drawn on paper. See Fig. 185.

(1) A _straight line_ is, of course, a line which runs uniformly in the same direction, and which is regular and without curves. A straight line is the shortest distance between two points. (2) When we say that lines are _parallel_, we mean that they lie so that every part of each is equally spaced from the other. (3) A line is _horizontal_ when it is parallel with the level surface of the Earth under it. (4) A line is _perpendicular_ to the surface of the Earth when it is _plumb_, that is, in a line with the center of the Earth. (5) A _vertical line_ generally means a plumb line. (6) A _right angle_ is formed when a vertical, or a perpendicular, line meets a horizontal line. (7) A _circle_ is a curved line, all points of which are equally distant from its center. _Circular_ means round like a circle. (8) By _diameter_ is meant a straight line drawn from one side, or half of a circle, to the opposite side through its center. (9) The _radius_ of a circle is a straight line drawn from the center of a circle, or a ball, to its circumference. (10) A _ring_ is a disk or object having a circular hole cut in its center. (11) An _arc of a circle_ is a part of a circle. (12) A _quarter circle_ is, of course, the one-fourth part of a circle. (13) A _tangent_ is a line which touches a curve but does not cut it. (14) An _ellipse_ is an oval figure, drawn on a plane surface. (15) The _equator_ is a circle which divides the Earth or other ball into equal parts, and is 90 degrees from the north and south poles. (16) The _ecliptic_ is a circle round the Sky in whose plane lies the orbit of the Earth.

APPENDIX C

THE GREEK ALPHABET

Many of the brighter stars have names, as Aldebaran, Capella, Sirius, etc., but astronomers now indicate the stars of a constellation by the letters of the Greek alphabet. The usual method is to call the brightest star of a constellation α, that is, Alpha, the next brightest β which is Beta, and so on, but sometimes the stars are lettered in order of position in the sky.

The following is the Greek alphabet:

α Alpha ν Nu
β Beta ξ Xi (Zi)
γ Gamma ο Omicron
δ Delta π Pi
ε Epsilon ρ Rho
ζ Zeta σ Sigma
η Eta τ Tau
θ Theta υ Upsilon
ι Iota φ Phi
κ Kappa χ Chi
λ Lambda ψ Psi
μ Mu ω Omega

APPENDIX D

STAR TESTS FOR EYESIGHT

There are a number of stars which are considered to be good tests for the seeing power of the eyes. The faint stars of the Pleiades are a fine test of this kind; but usually these tests are double stars and while one will be bright and easily seen its companion will be very faint. The test is to see the faint one, and if you can see it you may consider you have very good eyesight.

Eyesight tests are given on the following pages:

Page
11--Mizar and Alcor
34--Spots on the Sun
94--Grimaldi on the Moon
119--Nebula in Orion
150--Epsilon in Lyra
150--Pleiades
182--Alpha and Beta in Capricornus

APPENDIX E

MAGNITUDES OF STARS

There are not nearly as many stars in the sky as you might at first suppose. The stars are divided into magnitudes, that is, according to their brightness. Stars of the first magnitude are the brightest stars; stars of the second magnitude are second brightest, and so on. The total number of stars which can be seen with the naked eye on any one night in the United States is probably not more than 3,000. The following table gives the number of stars of the different magnitudes up to and including the sixth:

Magnitudes Number of Stars
1st 20
2nd about 65
3rd about 200
4th about 500
5th about 1,400
6th about 5,000