PHYS 183 Lecture Notes - Lecture 2: Semi-Major And Semi-Minor Axes, Orbital Period, Deferent And Epicycle

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Basi(cid:272) idea: o(cid:271)je(cid:272)t"s lo(cid:272)atio(cid:374) (cid:272)ha(cid:374)ges relati(cid:448)e to a dista(cid:374)t, fi(cid:454)ed (cid:271)a(cid:272)kgrou(cid:374)d depe(cid:374)di(cid:374)g o(cid:374) a viewing angle. Very important in astronomy as provides unambiguous distance measurement. Location of earth every six months changes perspective. Measurement of change of angle in sky tells distance. Position has changed and thus view is different. Full circle = 360 degrees (cid:1005) degree = 6(cid:1004)" (cid:894)ar(cid:272)(cid:373)i(cid:374)utes(cid:895) (cid:1005)" = 6(cid:1004)(cid:863) (cid:894)ar(cid:272)se(cid:272)o(cid:374)ds(cid:895) Co(cid:374)sider a star for (cid:449)hi(cid:272)h the paralla(cid:454) a(cid:374)gle p = (cid:1005)(cid:863) By definition, the distance to the star is: d = 1 parsec = 200000 au, o(cid:373)ethi(cid:374)g that is (cid:1005) parse(cid:272) a(cid:449)a(cid:455) (cid:449)ill ha(cid:448)e a se(cid:373)i a(cid:374)(cid:374)ual paralla(cid:454) a(cid:374)gle of (cid:1005)(cid:863) Eg p = (cid:1006)(cid:863) d = 0. 5 parsecs: larger the parallax, the closer the object. P = 1/d: p (arcseconds) = 1/d(parsecs) Believed universe could be understood on aesthetic grounds: not a scientist. Moon, sun, planets, stars revolve around earth on crystalline spheres.

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