Astronomy: A Popular Handbook

actual altitude angle angular diameter angular distance annual orbit apparent motion apparent solar appear approximately ascertain astronomers atmosphere attractive force axial rotation axis bodies bright calculate calendar called celestial equator celestial meridian celestial pole celestial sphere clock comet Consequently constellation course dial difference direction disk diurnal circles earth earth's orbit ecliptic circle ecliptic plane equal exactly explained fixed stars full-moon gnomon gravitational attraction Greenwich Gregorian calendar hemisphere horizon hour-angle inferior planets instance Julian calendar Jupiter leap-year light longitude lunar Mars mass mean solar measured Mercury method miles moon moon's move nebula night observations observatory orbital motion orbital revolution parallax pendulum photograph planet planetary Plate plumb-bobs pole star position projected radius remainder result right-ascension satellite Saturn seen shadow sidereal day sidereal period simple solar day solar system space sun's synodic period telescope terrestrial latitude tion triangle velocity Venus vernal equinox zenith