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The whole of this system is based upon the observation of three branches of the heavenly phænomena, the rising of the constellations, their setting, and the regular recurrence of the seasons. These risings and settings may be observed in two different ways:—The stars are either concealed, and cease to be seen at the rising of the sun, or else present themselves to our view at his setting—this last being more generally known by the name of "emersion" than of "rising," while their dis- appearance is rather an "occultation" than a "setting."— Considered, again, in another point of view, when upon certain days they begin to appear or disappear, at the setting or the rising of the sun, as the case may be, these are called their morning or their evening settings or risings, according as each of these phenomena takes place at day-break or twilight. It requires an interval of three quarters of an hour at least before the rising of the sun or after his setting, for the stars to be visible to us. In addition to this, there are certain stars which rise and set twice.1 All that we here state bears reference, it must be remembered, to the fixed stars only.

1 See c. 69, as to Arcturus and Aquila.

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