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jscott |
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SUBROUTINE ORBIT (OBLIQ,ECCN,OMEGT,DAY,SDIST,SIND,COSD,LAMBDA) 8201. |
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C**** 8202. |
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C**** ORBIT receives the orbital parameters and time of year, and 8203. |
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C**** returns the distance from the sun and its declination angle. 8204. |
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C**** The reference for the following caculations is: V.M.Blanco 8205. |
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C**** and S.W.McCuskey, 1961, "Basic Physics of the Solar System", 8206. |
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C**** pages 135 - 151. 8207. |
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C**** 8208. |
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C**** Program authors: Gary L. Russell and Robert J. Suozzo, 12/13/85 8209. |
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C**** 8210. |
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C**** All computations are in double-precision; 8211. |
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C**** but the arguments are single-precision. 8212. |
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C**** Input: OBLIQ = latitude of tropics in degrees 8213. |
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C**** ECCEN = eccentricity of the orbital ellipse 8214. |
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C**** OMEGT = angle from vernal equinox to perihelion in degrees 8215. |
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C**** DAY = day of the year in days; 0 = Jan 1, hour 0 8216. |
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C**** 8217. |
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C**** Constants: EDAYPY = Earth days per year = 365 8218. |
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C**** VERQNX = occurence of vernal equinox = day 79 = Mar 21 8219. |
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C**** 8220. |
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C**** Intermediate quantities: 8221. |
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C**** PERIHE = perihelion during the year in temporal radians 8222. |
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C**** MA = mean anomaly in temporal radians = 2J DAY/365 - PERIHE8223. |
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C**** EA = eccentric anomaly in radians 8224. |
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C**** TA = true anomaly in radians 8225. |
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C**** BSEMI = semi minor axis in units of the semi major axis 8226. |
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C**** GREENW = longitude of Greenwich in the Earth's reference frame 8227. |
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C**** 8228. |
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C**** Output: DIST = distance to the sun in units of the semi major axis8229. |
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C**** SDIST = square of DIST 8229.5 |
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C**** SIND = sine of the declination angle 8230. |
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C**** COSD = cosine of the declination angle 8231. |
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C**** LAMBDA = sun longitude in Earth's rotating reference frame 8232. |
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C**** 8233. |
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IMPLICIT REAL*8 (A-H,O-Z) 8234. |
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REAL*8 MA 8235. |
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C REAL*4 SIND,COSD,SDIST,LAMBDA,OBLIQ,ECCN,OMEGT,DAY 8236. |
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C**** 8237. |
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PI = 3.14159265358979D0 8238. |
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EDAYPY = 365. 8239. |
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VERQNX = 79. 8240. |
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OMEGA=OMEGT*(PI/180.D0) 8241. |
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DOBLIQ=OBLIQ*(PI/180.D0) 8242. |
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ECCEN=ECCN 8243. |
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C**** 8244. |
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C**** Determine time of perihelion using Kepler's equation: 8245. |
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C**** PERIHE-VERQNX = OMEGA - ECCEN sin(OMEGA) 8246. |
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C**** 8247. |
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PERIHE = OMEGA-ECCEN*SIN(OMEGA)+VERQNX*2.*PI/365. 8248. |
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C PERIHE = DMOD(PERIHE,2.*PI) 8249. |
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MA = 2.*PI*DAY/365.-PERIHE 8250. |
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MA = DMOD(MA,2.*PI) 8251. |
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C**** 8252. |
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C**** Numerically solve Kepler's equation: MA = EA - ECCEN sin(EA) 8253. |
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C**** 8254. |
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EA = MA+ECCEN*(SIN(MA)+ECCEN*SIN(2.*MA)/2.) 8255. |
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110 DEA = (MA-EA+ECCEN*SIN(MA))/(1.-ECCEN*COS(EA)) 8256. |
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EA = EA+DEA 8257. |
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IF (DABS(DEA).GT.1.D-8) GO TO 110 8258. |
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C**** 8259. |
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C**** Calculate the distance to the sun and the true anomaly 8260. |
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C**** 8261. |
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BSEMI = DSQRT(1.-ECCEN*ECCEN) 8262. |
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COSEA = COS(EA) 8263. |
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SINEA = SIN(EA) 8264. |
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SDIST = (1.-ECCEN*COSEA)*(1.-ECCEN*COSEA) 8265. |
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TA = DATAN2(SINEA*BSEMI,COSEA-ECCEN) 8266. |
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C**** 8267. |
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C**** Change the reference frame to be the Earth's equatorial plane 8268. |
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C**** with the Earth at the center and the positive x axis parallel to 8269. |
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C**** the ray from the sun to the Earth were it at vernal equinox. 8270. |
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C**** The distance from the current Earth to that ray (or x axis) is: 8271. |
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C**** DIST sin(TA+OMEGA). The sun is located at: 8272. |
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C**** 8273. |
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C**** SUN = (-DIST cos(TA+OMEGA), 8274. |
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C**** -DIST sin(TA+OMEGA) cos(OBLIQ), 8275. |
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C**** DIST sin(TA+OMEGA) sin(OBLIQ)) 8276. |
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C**** SIND = sin(TA+OMEGA) sin(OBLIQ) 8277. |
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C**** COSD = sqrt(1-SIND**2) 8278. |
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C**** LAMBDA = atan[tan(TA+OMEGA) cos(OBLIQ)] - GREENW 8279. |
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C**** GREENW = 2*3.14159 DAY (EDAYPY-1)/EDAYPY 8280. |
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C**** 8281. |
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SINDD = SIN(TA+OMEGA)*SIN(DOBLIQ) 8282. |
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COSD = DSQRT(1.-SINDD*SINDD) 8283. |
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SIND = SINDD 8284. |
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C GREENW = 2.*PI*(DAY-VERQNX)*(EDAYPY+1.)/EDAYPY 8285. |
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C SUNX = -COS(TA+OMEGA) 8286. |
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C SUNY = -SIN(TA+OMEGA)*COS(DOBLIQ) 8287. |
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C LAMBDA = DATAN2(SUNY,SUNX)-GREENW 8288. |
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C LAMBDA = DMOD(LAMBDA,2.*PI) 8289. |
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C**** 8290. |
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RETURN 8291. |
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END 8292. |
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