SECTION XX. 291 noon make no sensible difference in the altitude. But if you no have no clock or dial or any thing of that kind whose truth you can rely upon, then a little before noon observe and set down the altitude every four or five minutes till you find it begins to grow a little less, then review your observations, and the greatest height was the true meridian altitude. You may, by the same method, find the meridian altitude of any star above the horizon, if you make Several observations, when the star is coming near to the north or south part of the meridian. Problem VI. " How to find out the declination of the sun, or of any large or known star." If you know the latitude of the place where you are, with the meridian altitude ofthe sun any day in the year, or ifyou know the sun's place in the ecliptic, you may find thedeclination of the sun thereby geometrically, as shall be shewn afterward : But if these are not known, then in order to other astronomical oper- ations, you must seek the declination of the sun for that day, either by the globe on the brazen meridian, or in a scale of the sun's declination, which is drawn on artificial quadrants or other mathematical instruments ; or it may be found in tables of the sun's declination calculated exactly to every minute of a de- gree for every day in the year, which is the best way where it may be bad. There are also tables of declinationof several of the most noted stars. These are all the year at the same distance from the equator, and their declination does not vary as the sun's does. These tables of the sun and star's declinationare found at the end of this book, Sect XXI. But let it be noted here, that the declinationof the sun not only changes every day in the year, but it differs also some few minutes in the next year from the year foregoing, even on the same day of the month : Whence this difference arises, and how to act with respect to it, see problem XX. following, and more in Sect. XXI. Problem V11. " To find the latitude of any place by the meridian altitude and declination of the sun any day in the year." The way to find the latitude of any place (i. e. the distance of the zenith of that place from the equator) by the meridian altitude of the sun, is first to seek its colatitude, i. e. the comple- ment of its latitude, or (which is all one) the elevation of the equator above the horizon of that place. Suppose theday given be the 21st of June, or the summer solstice. This may be done by looking back to the figure iii. First, Draw the line it o for the horizon, and from the centre c raise a perpendicular c z to represent the zenith. Make the semicircle H z o for the meridian : Then suppose the meridian altitude of the sun at thesummer solstice be 62 degrees, by the use of your T 2
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