Synopsis
This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1786. Excerpt: ... LETTER XII. Of THr Equation Of Time; Or The Difference Between Mean Time And Apparent. IT isa circumstance worthy of observation, that throughout all nature, there is no one body, with which we are acquainted, whose motion is perfectly uniform and regular. The sun, in his apparent journey through the heavens, is supposed by the vulgar to surnish us with an accurate and just measure of time; but in this they: are greatly mistaken. Astronomers have found that the motion of the sun is very unequal; and therefore equal time, which flows on for ever in the fame manner, cannot be truly measured by the sun's motion, Mean, or equal time, is that which is shewn by a well regulated clock or watch; and in order that the apparent time, as shewn by a true sun-dial, may agree with this, it must be corrected by proper equations. The difference between mean and apparent time depends upon two causes, the obliquity of the ecliptic with respect to the equator, and tlfe Unequal motion of the earth in an elliptical orbit. 1 shall first explain the effects of these causes separately considered, and then the united effects effects resulting from their combination. But, before we we proceed to these particulars, it will be proper to remind you, that whenever the motion of the sun is spoken of, it is not to be understood in a positive fense, as though he actually removed from one part of space to another, but only as an appearance occasioned by the real motion of the earth in a contrary direction. The phænomena are exactly the fame; and astronomers sometimes mention one, and sometimes the other, according as they find it most convenient for their purpose. This being premised, it may be observed, that lince the earth's axis is perpendicular to the plane of the equator, any e...
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