Letters on Astronomy

angle appear ascertain astronomers atmosphere attraction axis called celestial centre circle comet conjunction constellation Copernicus degrees diameter direction discovery disk distance diurnal motion diurnal revolution doctrine double stars earth earth’s orbit ecliptic equal equator equinox exhibited fixed stars force Galileo gravity greater greatest half Halley’s comet heavenly bodies heavens Hence Herschel horizon inferior conjunction inferior planets instrument Jupiter Kepler latitude laws length less LETTER light longitude lunar magnitude measure Mercury meridian meteors millions of miles minutes moon moon’s motion move nearer nearly nebulæ node objects observations opposite parallax passing perihelion period phenomena planetary planets pole Ptolemy respect revolve right ascension ring round satellites Saturn seen shadow side sidereal day solar system space spectator sphere sun’s superior planets supposed surface telescope thousand tides Tycho Tycho Brahe universal gravitation Uranus velocity Venus vernal equinox visible west to east