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Algol Alpha altitude Andromeda angle apparent diameter asteroids astronomical astronomical units axis Beta bright lines bright stars called Cassiopeia celestial equator celestial sphere centre circle comet conspicuous constellation corona dark lines Delta density determined disc distance diurnal double star earth earth's orbit eclipse Epsilon equatorial equinox Gamma globe heat heavenly bodies heavens horizon inches inferior conjunction instrument Jupiter known latitude length less light longitude lunar magnitude Mars mass mean meridian meridian circle meteors miles minutes month moon moon's motion move naked eye nearly nebula Neptune object object-glass observer parallax perihelion period photosphere planet planetary pole Pole-star probably proper motion right ascension ring rotation satellites seen shadow sidereal sidereal period Sirius spectra spectroscope spectrum sun spots sun's synodic synodic period tail tide tion transit Uranus Ursa variable velocity Venus vernal equinox visible zenith Zeta zodiacal
Сторінка 161 - The squares of the periods of the planets are proportional to the cubes of their mean distances from the sun ; that is, ti2 : k2 ,• ,• ai3 ,• (h3This is the so-called harmonic law.
Сторінка 162 - ... every particle of matter in the universe attracts every other particle with a force that varies directly as the masses of the particles, and inversely as the square of the distance between them." It would take us far beyond our limits to attempt to show how Kepler's laws follow from this, but they do. The only mystery in the case is the mystery of the " attraction " itself; for this word "attraction" is to be taken as simply describing an effect without in the least explaining it.
Сторінка 173 - On it place a globe two feet in diameter. This will represent the sun. Mercury will be represented by a grain of mustard seed on the circumference of a circle 164 feet in diameter for its orbit; Venus, a pea, on a circle of 284 feet in diameter; the Earth, also a pea, on a circle of 430 feet; Mars, a rather large pin's head, on a circle of 654 feet; the asteroids, grains of sand...
Сторінка 206 - Direct your telescope to a point on the ecliptic in the constellation of Aquarius, in longitude 826°, and you will find within a degree of that place a new planet, looking like a star of about the ninth magnitude, and having a perceptible disc.
Сторінка 77 - Force of Gravity. — For two reasons the force of gravity is less at the equator than at the poles.
Сторінка 81 - It is taken of that particular width simply because the moon and the principal planets always keep within it. It is divided into the so-called Signs, each 30° in length, having the following names...
Сторінка v - constellation-tracing," into the body of the text, and placing it near the beginning ; a change in harmony with the accepted principle that those whose minds are not mature succeed best in the study of a new subject by beginning with what is concrete, and appeals to the senses, rather than with the abstract principles.
Сторінка 54 - This is the winged horse which sprang from the blood of Medusa, after Perseus had cut off her head. He fixed his residence on Mt. Helicon, where he was the favorite of the Muses, and after being tamed by Minerva he was given to Bellerophon to aid him in conquering the Chimaera.