Diurnal Motion of the Stars

The stars appear to describe circular arcs parallel to one another about an axis which joins the observer to the celestial pole. Those nearest the pole -- the circumpolar stars, like A here seen at lower culmination, never sink below the horizon and may, therefore, be seen crossing the meridian below the pole. Other northerly stars, such as B, describe large arcs over the horizon and so remain above it more than 12 hours between rising and setting north of the east and west points. Stars (e.g. C) lying on the great equinoctial circle which cuts the east and west points (i.e. stars which rise due cast and set due west) remain above the horizon for half the 24 hours of the diurnal cycle. Stars (like D) which lie south of the equinoctial rise and set towards the southern horizon and are below it more than 12 out of the 24 hours. The majority of stars in a northern latitude cross the meridian south of the zenith. Hence sailors speak of the transit of a star as its "southing."

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