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Noon Sight Revisted
From: Dan Hogan
Date: 1998 Jul 29, 6:35 AM
From: Dan Hogan
Date: 1998 Jul 29, 6:35 AM
The Noon Sight is the simplest sight in celestial navigation. It can be used with pencil and paper and the Almanac data, no table required and only simple math. All the corrections made in the Noon Sight reduction are to account for the observer being on on the surface of the earth, the refraction of light from space, the difference in distance from the position of the observer to the geographical position of the body on the surface of the earth (ZD), *as if it all was observed from the center of the earth*. OBSERVING THE (all the same thing): Meridian Passage Local Apparent Noon (LAN) Noon Sight Start a series a sights before Noon. Normally use Lower Limb(LL). Each sight should be a little higher. Adjust the sextant to keep the Sun in contact with the horizon. The adjustments will get smaller and then the Sun appears to hang at the same altitude for about a minute before dropping a little. When the Sun Hs is lower LAN has passed. Keep the sextant set at the highest reading you got and record the sight Hs. Only the approximate time is put down. You can't tell LAN (Meridian Passage) time exactly. Time: HH:mm.m HH= hour mm.m= minutes NOON SIGHT REDUCTION You convert observed altitude(Ho) into Zenith Distance(ZD). The distance of the observer from the geographic position of the Sun (in arc measured at the center of the earth) is the same as zenith distance. This makes it easier to figure out whether to add or subtract the ZD and Dec. THE FORM sextant altitude(Hs) index error(+/-) dip of horizon(-) sun Correction(+) ------------------ observed altitude(Ho) Next subtract from 90d to get ZD 89d 60' (same as 90d) Ho(-) ------- zenith distance(ZD in dd mm.m) Now we know, in degrees, how far we are from the geographical position(GP) of the sun. Name all North declinations plus(+) and all South declinations minus(-), this avoids the use of rules, and then algebraically combine with the ZD. And you have your latitude. ZD declination(N+/E-) ------------------ LATITUDE Example Noon Sight Sextant Altitude Hs 71d 09.0' Index Error IE -02.0' Dip of horizon dip -02.7' Sun correction(LL) sun +16.0' ---------------------------------- Observed Altitude Ho 71d 21.3' (Same as 90d) 89d 60.0' Observed Altitude Ho -71d 21.3' ---------------------------------- Zenith distance ZD 18d 38.7' declination dec +21d 21.0' ---------------------------------- Noon Latitude lat +39d 59.7' PREDICTING THE NOON SIGHT Use your approximate longitude. divide it by 15(one hour for each 15 degrees). At the lower RH daily page of the Nautical Almanac get the GMT time of meridian passage of the sun for your date. It varies from 1144 to 1206 because of the earths eccentric orbit and speed. Take your longitude(label West longitudes plus(+) and East longitudes minus(-)) and algebraically combine with the daily page meridian passage time. The answer is your approximate time of meridian passage. Depending on the uncertainty of your longitude you can start observing a few minutes early. Your LAN = (Lon / 15) +/- GMT Mer Pass Example prediction LAN, July 30, 1998 Lon DR +04d.20' 00:16.5' = +04d.20' / 15 GMT Mer Pass. +12:06 ------------------------ Predicted LAN 12:22.5' Dan Hogan dhhogan@concentric.net =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=--=-= =-= TO UNSUBSCRIBE, send this message to majordomo@roninhouse.com: =-= =-= navigation =-= =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=--=-=