# ls3 - AST 3722C summary for lecture on tuesday january 22...

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AST 3722C summary for lecture on tuesday january 22, class #3. warning: this is not supposed to be a substitute for reading the textbook. Lect Part A: the cosine-declination factor. the arc between 2 lines of constant of RA changes as a function of declination. E.g., for 1 hr of RA separation, at the cel. equator that's 15 degrees, but near the NCP it'll be smaller! this is a problem when you're dealing with the small sph.triangle approximation. in fact if you're dealing with the sph. trig formulae, you don't have to worry about it. RA angle in arcsec = RA angle in sec * 15 * cos(dec) RA angle in arcmin = RA angle in min * 15 * cos(dec) RA angle in degree = RA angle in hr * 15 * cos(dec) ------------------ Lect Part. B. - textbook chapter 9 - time section 1 has some intro stuff, section 2 talks about LST, which we've covered mostly already but here are 3 more tidbits. 1st -- I was wrong last week. 1 sid sec does not equal 1 solar sec. but it's pretty close: 1 sid sec = 0.99727 regular seconds. check out table 9.1 and 9.2 on page 96. also this means that if you have calculated a sidereal-related time interval and you want to convert to "regular" or solar time, you must multiply by 0.99727. E.g., if H=-3h, and so there are 3 hours until transit, that is 3 sidereal hours. the "regular" time would be 3 h * 0.99727 = 2 h 59 m 30 s. 2nd -- Note something about how sidereal clock works. say on Night #1, you observe a star at R.A. = 10 h. And say that it transits at 9 p.m. local time. So that happens to be LST = 10 h. But because sidereal days are different from solar days: Night #1: LST = 10 h at 9 p.m. Night #2: LST = 10 h at 8:56:04 p.m. Night #3: LST = 10 h at 8:52:08 p.m. . . . Night #X: LST = 10 h at (9 p.m. - X * (3m56s)) so the solar time of transit will shift by a few minutes every day. This is important for planning an observing run. 3rd -- LST of somewhere else. LST place B = LST place A + (long of B - long of A).

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So if you want to know LST somewhere else, all you have to do is know the relative geographic longitudes. E.g.: say longitude of place A = 75 deg W = -75 deg. longitude of place B = 120 deg W = -120 deg. say LST at place A = 18 h. So: LST place B = LST place A + (long of B - long of A) = 18 h + ( -120deg - -75deg ) = 18 h + -45deg = 18 h + -3 h <---convert to hours with 15deg/hr. = 15 h ------------------ Lect Part C. -- now info about solar time the basic idea here is to have some nice stable constant clock that you can time everything else against. For the longest time, the rotation of Earth served that purpose. And it is still dictating a lot of how we live our lives. For astronomy though, in the 20th century we realized that we need better clocks. The rotation and
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ls3 - AST 3722C summary for lecture on tuesday january 22...

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