"Upon this account it is, that every Problem in the Terrestrial Physics is very operose and perplexd, on the contrary, in the Celestial Physics, much more easy and simple; tho even the latter has its difficulties, arising from the different distances and magnitudes of the Celestial Bodies, For the Fixd Stars are so vastly distant asunder, that they have no mutual action upon each other, observable by us..."
David Gregory FRS was a Scottish mathematician and astronomer. He was professor of mathematics at the University of Edinburgh, and later Savilian Professor of Astronomy at the University of Oxford, and a proponent of Isaac Newton's Principia.
"[F]or the further improvement of natural philosophy a more advanced geometry must be found. ...[T]he reason why physical science has here been brought to a level that is the envy of foreigners is the knowledge... of some more universal geometry. Of what part of this the learned owe to this renowned university and in it to the prince of geometers I shall not speak lest I appear to be fawning, which in a mathematician would be unseemly."
"[T]he Physics, it is all taken out of the above mentiond Authors; but is here intermixd with Astronomy, in such places as seemd proper and convenient; the Geometry to be met with in it, I have either borrowed elsewhere, and quoted... or delivered it Lemmatrically."
"My design in publishing this Book, was, that the Celestial Physics, which the most sagacious Kepler had got the scent of, but the Prince of Geometers Sir Isaac Newton, brought to such a pitch as surprises all the World, might, by my... illustrating, become easier to such as are desirous of being acquainted with Philosophy and Astronomy."
"[T]hose who are less versd in the more abstruse parts of Geometry, or less concerned about the Physical parts, may pass over, and only read the Astronomy separately and distinct..."
"The Celestial Physics, or Physical Astronomy, is not only the first in dignity of all inquiries into Nature... but the first in order, because it is the easiest."
"The phenomena prove that the tail of a comet is vapour from its head which, when the comet is greatly heated in the neighbourhood of the sun in perihelion, continually rises up and moves away into the regions opposite the sun."