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df the Creator from our narrow conception of things; where we look for com mensuration, we find variety and infinity.
Thus ancient astronomers looked upon the motion of the sun to be sufficiently regular for the mensuration of time; but, by the accurate observations of later astronomer s, it is found, that neither the days, nor even the hours, as measured by the sun's apparent motion, are of an equal length, on two accounts.
1st, A natural or solar day of 24 hours, is that space of time the sun takes up in passing from any particular meridian to the same again; but one revolution of the earth, with respect to a fixed star, is performed in 23 hours, 56 minutes, 4 seconds; therefore the unequal progression of the earth through her elliptical orbit, (as she takes almost eight days more to run through the northern half of the ecliptic, than she does to pass through the southern) is the reason that the length of the day is not exactly equal to the time in which the earth performs its rotation about its axis.
2d. From the obliquity, of the ecliptic to the equator, on which last we measure time ; and as equal proportions of one do not correspond to equal portions of the other, the apparent motion of the sun would not be uniform; or, in other words, these points of the equator which come to the meridian, with the place of the sun on different days, would not be at equal distances from each other.
Problem Xh. To find the rigid ascension and dectU nation of any given star.
Bring the given star to the meridian, and the degree under which it lies is its declination; and the point in which the meridian intersects the equinoctial is its right ascension. Thus the right ascension of Sirius is 99°, its declination l6° 25' south; the right ascension of Arcturus is 211° 32', its declination 20° 20' north.
The declination is used to find the latitude of places; the right ascension is used to find the time at which a star or planet comes to the meridian ; to find at any given time how long it will be before any celestial body comes to the meridian; to determine in what order those bodies pass the meridian ; and to make a catalogue of the fixed stars.
Problem Xiii.' To find the latitude and longitude of a given star.
Bring the pole of the ecliptic to the meridian, over which fix the quadrant of altitude, and, holding the globe very steady, move the quadrant to lie over the given star, and the degree of the quadrant cut by the star is its latitude; the degree ofihe ecliptic cut at the same time by the quadrant is the longitude of the star.
. Thus the latitude of Arcturus is. 30° 30', N- its longitude 20° 20' of Libra: the latitude of Capella is 22° 22' north, its longitude 18° 8' of Gemini.
The latitude and longitude of stars is used to fix precisely their places on the globe, to refer planets and comets to the stars ; and, lastly, to determine whether they have any motion, whether any stars vanish, or new ones appear.
Problem xiv. The right ascension and decli • nation of a star being given, tojind its place on the globe*
Turn the globe till the meridian cuts the equinoctial in the degree of right ascension. Thus, for example, suppose the right ascension of Aldebaran to be 65° 30', and its declination to be l6° north, then turn the globe about till the meridian cuts the equinoctial in 65° 30', and under the 16' of the meridian, on the northern part, you. will observe the star Aldebaran, or the Bull's eye.
Problem xv. Tojind at what hour any known star passes the meridian at any given day.
Find the sun's place for that day in the ecliptic, and bring it to the strong brass meridian, set the horary index to XII o'clock, then turn the globe till the star comes to the meridian, and the index will mark the time. Thus, on the 15th of August, Lyra comes to the meridian at 45 minutes past VIII in the evening. On the 14th of September, the brightest
of the Pleiades will be on the meridian at IV in the morning.
This problem is useful for directing when to look for any star on the meridian, in order to find the latitude of a place, to adjust a clock, &c.
Problem xvi. To find on what day a given star will come to the meridian at any given hour.
Bring the given star to the meridian, and set the index to the proposed hour; then turn the globe till the index points to XII at noon, and observe the degree of the ecliptic then at the meridian; this is the sun's place, the day answering to which may be found on the calendar of the broad paper circle.
By knowing whether the hour be in the morning or afternoon, it will be easy to perceive which way to turn the globe, that the proper XII may be pointed to ; the globe must be turned towards the west, if the given hour be in the morning; towards the east, if it be afternoon.
Thus, Arcturus will be on the meridian at III in the morning on March the 5th, and Cor Leonis at VIII in the evening on April the 21st.
Problem xvn. To represent the face of the heavens on the globe for a given hour on any day of the year, and learn to distinguish the visible Jixed stars.
Rectify the globe to the given latitude of the place
and day of the month, setting it due north and south by the needle ; then turn the globe on its axis till the index points to the given hour of the night; then all the upper hemisphere of the globe will represent the visible face of the heavens for that time, by which it will be easily seen what constellations and stars of note are then above our horizon, and what position they have with respect to the points of the compass. In this case, supposing the eye was placed in the centre of the globe, and holes were pierced through the centres of the stars on its surface, the eye would perceive through those holes the various corresponding stars in the firmament; and hence it would be easy to know the various constellations at sight, and to be able to call all the stars by their names.
Observe some star that you know, as one of the Pointers in the Great Bear, or Sirius ; find the same on the globe, and take notice of the position of the contiguous stars in the same or an adjoining constellation ; direct your sight to,the heavens, and you will see those stars in the same situation. Thus you may proceed from one constellation to another, till you are acquainted with most of the principal stars.
"For example. The situation of the stars at London on the 9th of February, at 2 min. past IX in the evening, is as follows:
"Sirius, or the Dog-star, is on the meridian, its altitude 22°: Procyon, or the Little Dog-star, 16° towards the east, its altitude 43i : about 24° above this last, and something more towards the east, are the