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The Sun's place in the ecliptic, is called its longitude; but it is usually expressed by the degree and minute of the sign in which it is.
The elevation of the pole above the horizon, is the same as latitude on the terrestial globe. Latitude and Longitude, therefore, on the celestial globe bear the same relation to the ecliptic, as they do on the terrestrial globe to the equator.
These are the general principles of a celestial globe, which being well understood, will render the method of working problems on it equally intelligible and easy.
To rectify the Globe for any given Latitude. I would rectify the globe for latitude 40 degrees north.
RULE. Turn the pole on which the hour circle is fixed towards the north point of the horizon, slipping or moving the whole globe backwards or forwards in the notches of the horizon, till the horizon cuts the brazen meridian in 40 degrees, so the globe will be rectified for latitude 40 north.
If it were required to rectify the globe for south latitude, then you must elevate the south pole instead of the north pole.
The day of the month being given to find the sun's place in the ecliptic.
RULE.-Look on the inner calendar on the horizon of the globe, and you have the sign, and degree of the sign, that the sun is in on that day.
I would know the sun's place in the ecliptic, on May the 21st, March the 21st, June the 21st, and December the 21st. I look for these days of the month in order as they stand in the calendar, and against the day of the month, in the innermost circle of the horizon I find the sun's place among the signs.
Thus against May the 21st, I find 1 degree of Gemini, and also on March the 21st, I find the sun enters Aries, on June the 21st, he enters Cancer, and on December the 21st, he enters Capricorn.
The latitude and day of the month being given, to find the sun's place in the ecliptic and rectify the globe for use.
RULE. Find the sun's place on the horizon as before directed, and having noted what degree he is in, look upon the ecliptic on the globe and find the same sign and degree as you did on the horizon, then bring this degree of the ecliptic very carefully to the graduated edge of the brazen meridian, and holding the
globe steady, turn the index exactly to the upper twelve, which represents twelve at noon, and thus is the globe rectified for that day, and the degree of the ecliptic that lies under the meridian represents the sun's place at noon, or twelve o'clock that day.
The latitude and day of the month being given, (suppose January the 1st. at 9 o'clock at night, at Philadelphia) to set the globe so as to represent the face of the heavens at that time, and show the names and position of the most eminent fixed stars.
RULE.-Rectify the globe for the latitude, and bring the sun's place to the meridian, and the index to 12; then turn the globe to the given hour, viz. 9 at night, and there fix it. So will every star on the globe, if you set the globe north and south, correspond with, or point to the same star in the heavens; and then observing some star which you know as the Pleides (commonly called the seven stars) in Taurus or Aldebaran, 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 situations. Thus you may proceed from one constellation to another, until you have learned the most of the principal stars.
To find the rising and setting of the stars and the point of the compass a given star rises or sets upon, in any latitude, and on any day of the year.
Rectify the globe, and bring the sun's place to the meridian: then turn the globe till the given star come to the eastern verge of the horizon; and the index will show the time of rising, and the horizon will discover the point it rises upon. Turn the globe to the west, and the index will point to the time of setting, and the horizon will show the point it sets upon.
To find those stars which never rise, and those which never set, at London.
Observe what stars have more than 38 degrees south declination, for those never rise, but are always under the horizon at London: in like manner, those stars that have above 38 degrees north declination, never set at London, but are always above the horizon.
To find the cosmical rising and setting of the stars in any latitude.
Rectify the globe for the latitude of the place of observation, and bring the sun's place
to the eastern side of the horizon for the given day; then all those stars cut by the eastern verge of the horizon rise cosmically. Keep the globe in the same position, and look at the western verge of the horizon; then all those stars cut by it or lying very near it, set on that day cosmically. Turn the globe to the western edge of the horizon, and observe the degree of the ecliptic cut by the eastern edge at the same time which will answer to the day of the cosmical setting.
To find the time of the achronical rising and setting of any star.
Having rectified your globe for the latitude of the place, &c, bring the given star to the eastern verge of the horizon, and observe what point of the ecliptic is, at the same time, at the western verge of it. The latter being the place of sun-setting, by observing the month and day on which the sun is in that sign and degree, the time of the achronical rising of the given star will be found. If you bring the star to the western verge of the horizon, and observe the point of the ecliptic which is, at the same time, at the eastern verge of it, the correspondent day, or that in which the star sets achronically, will be found.