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altitude amount angle angular apparent approximately astronomers atmosphere average axis body called celestial circle Class clusters comet completely conclusion Consequently considerable constellation corresponding crosses declination density depends described determined diameter direction distance double earth eclipse effects energy equal equator evidence example fact follows forces galaxy give given greater horizon important increase interval kilometers known latitude length less light light-years limits magnitude mass mean measured meridian meteors method miles month moon motion move nearly nebulae object Observatory observer orbit origin passes period photograph planets pole position present probably problem produced radiation rays received relative represent respect revolve right ascension rotation satellites seen similar solar spectral speed sphere stars Suppose surface Table telescope temperature theory thousands tion varies velocity visible