Herschel, John F. W. (John Frederick William), Sir, 1792-1871 · London : Longman, Brown, Green, Longmans, and Roberts · leaf 54
18 OUTLINES OF ASTRONOMY. a tangent to the surface at N, then will this line represent the visual ray along which the spectator at M will see the visible horizon; and as this tangent sweeps round M, and comes successively into the positions MOo, MPjo, MQ//, the point of contact N will mark out on… … hended within this circle is the portion of the earth’s surface visible to a spectator at M, and the angle NMQ included between the two extreme visual rays is the measure of its apparent angular diameter. Leaving, at present, out of con¬ sideration the effect of refraction in the air below M, of…
Astronomy
Internet Archive · Smithsonian Libraries · Public domain (published before 1931). Free to use for anything.
![An illustration from Nicolai Copernici Torinensis De reuolutionibus orbium coelestium, libri VI : Habes in hoc opere iam recens nato, & aedito, studiose lector, motus stellarum, tam fixarum, quàm erraticarum, cum ex ueteribus, tum etiam ex recentibus obseruationibus restitutos: & nouis insuper ac admirabilibus hypothesibus ornatos. Habes etiam tabulas expeditissimas, ex quibus eosdem ad quoduis tempus quàm facillime caculare poteris. Igitur eme, lege, fruere. [Line in Greek] (1543)](https://img.insideoutlibrary.com/img/200/ia-nicolaicopernici00cope-259-164.webp)
