ns3012007 16 celestial application

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Information about ns3012007 16 celestial application

Published on November 12, 2007

Author: Nevada

Source: authorstream.com

Lesson 16: Celestial Applications:  Lesson 16: Celestial Applications Lesson 16: Celestial Applications:  AGENDA: Determining your position Latitude by Polaris (Theory) Latitude by LAN (Theory) Gyro Error by Polaris (Theory) Gyro Error by Azimuth of the Sun (Theory) Computing Times of Sunrise/Sunset Applicable reading: Hobbs pp. 476-491. pp. 462-475 (skim) Lesson 16: Celestial Applications YOU ARE A SHIPWRECKED SAILOR ON A LOST ISLAND:  YOU ARE A SHIPWRECKED SAILOR ON A LOST ISLAND WHAT ARE YOU GOING TO DO?? ??? The Theory::  The Theory: You can derive your unknown position from a known position. If you know where the star is on the sky, you can find your position on the earth using some simple triangulation math. Nautical Almanac or Star Finder will tell you where the star is Definitions:  Definitions Zenith Altitude Amplitude The “basics”:  The “basics” Measure the altitude of an object and write down the time it was measured Calculate the position of the object at the time of observation using the Nautical Almanac Use your assumed position to calculate what the altitude and azimuth (direction of the object) should have been Comparison of the altitude you measured with the one you calculated gives you an offset that can be plotted on a chart as a Line of Position (LOP) A “fix” is then obtained by observing where several LOPs cross Which Stars do I “shoot”?:  Which Stars do I “shoot”? A “Star Finder” will show you which stars should be visible based on where you are. Designed to determine the altitude and azimuth of the 57 navigational stars and planets listed in the Nautical Almanac Can also be used in reverse order to determine the identification of an unknown body 2102-D, “Star Finder”:  2102-D, “Star Finder” The Star Finder Consists of a Base and ten circular templates. The Base is a white opaque plastic disc, with a small pin at its center. Slide9:  One side is the North Celestial Dome THE BASE Slide10:  Each Star is Shown on the Base along with its name and indication of magnitude. Large heavy rings for the brightest stars. Decreasing to Smaller rings for dimmer stars. Graduated in half degrees of LHA of Aries and labeled at 5° intervals. Slide11:  Ten transparent templates. Nine printed in blue ink are designed for apparent altitude and azimuth determination. There is one blue template for every 10° of latitude between 5° and 85°. LATITUDE 35N Latitude Templates Slide12:  Each is printed with blue oval altitude curves at 5° intervals. A second set of curves are printed on each template showing the azimuth curves. Slide13:  The tenth printed in red ink is intended for plotting bodies other than the 57 selected stars. Slide14:  The 0° and 180° line is aligned with the LHA on the outer circumference on the base. LHA=010° Any of the 57 navigational stars inside the celestial dome may be selected. Slide15:  · Venus Altitude 10° 00 Azimuth 120° Locate planets on template and record altitude and azimuth (bearing). Now is the opportunity to select and record stars. 28° 00 @153º Mars Slide16:  TWO EASY WAYS TO GET YOUR LATITUDE Latitude By Polaris Local Apparent Noon THESE TWO WAYS ARE SPECIAL CASES OF THE NAVIGATION TRIANGLE Latitude by Polaris:  Latitude by Polaris Polaris (the “pole star”) is so named because it is nearly coincident with the celestial north pole (Pn). As a result, the celestial triangle collapses. Colatitude and coaltitude are of equal length. The observed altitude of Polaris is equivalent to the observer’s latitude. Slide18:  Pn POLARIS Observer’s Zenith Altitude Ho Observer’s latitude Celestial Equator Celestial Horizon COLATITUDE COALTITUDE Colatitude = Coaltitude 90° - lat = 90° - Ho lat = Ho See Figure 25-3B Latitude by Polaris:  Latitude by Polaris In reality, Polaris and the celestial Pn are not exactly coincident (3/4 ° offset) As a result, Polaris wanders a bit with respect to the north pole (due to precession) Normally within 2 ° deg of Polaris To account for this, a correction table is provided in the Nautical Almanac Slide20:  Less error is accrued in low altitude observations. The best use of this type of azimuth is when the ship is in low latitudes. Allows use of a telescopic alidade instead of an azimuth or bearing circle. North Star (Polaris) Latitude by LAN Sun:  Latitude by LAN Sun Because the sun completes upper transit above the observer’s celestial horizon, we can use the sun’s observed altitude and declination at LAN to determine latitude. Local Apparent Noon (LAN) - The moment at which the apparent sun transits the observer’s meridian. Slide22:  LOCAL OBSERVER’S MERIDIAN -S- -W- -N- -E- Slide23:  EQUATOR ZENITH CEL. BODY PN Slide24:  EQUATOR PN ZENITH CEL. BODY Slide25:  EQUATOR COLATITUDE COALTITUDE POLAR DISTANCE PN Slide26:  EQUATOR COALTITUDE DECLINATION LATITUDE PN Slide27:  Altitude (Ho) HORIZON SUN at LAN Observer’s meridian Slide28:  There are two methods for observing LAN. -Sighting on the sun with a sextant, constantly adjusting as needed, until the sun dips down to a lower altitude -Recording sights at intervals to be compared to discover the approximate time of LAN -In either method we must first predict the time this meridian passage will occur Plotting LAN:  Plotting LAN 31°N 119°W 120°W C-310 S-10 0800 0900 1000 1100 1200 1208 0935 1208 LAN 31° 13.3 N 0935 Slide30:  C-310 S-10 0800 0900 1000 1100 1200 0935 1208 LAN Advance the 0935 LOP to the 1208 LAN to create a Estimated Position. 0935 1208 31°N 119°W 120°W Slide31:  C-310 S-10 0800 0900 1000 1100 1200 0935 0935-1208 1208 LAN Label the 0935 LOP and the 1208 Estimated Position. 0935 1300 1400 1208 1208 EP . 31°N 119°W 120°W Circle of Equal Altitude:  Circle of Equal Altitude To illustrate the basic concept, consider a pole of known height erected vertically on level ground. The base of the pole establishes the celestial body’s GP. Guy wires are stretched taut to points on the ground equidistant from the base. Circle of Equal Altitude:  Now, let’s make two changes to our situation: make the pole infinitely tall make our surface spherical Now we have something similar to the earth and the navigational stars. Circle of Equal Altitude Slide34:  We need to relate this concept to the navigation triangle: If we know the altitude of a star and its GP, we can draw a circle of equal altitude... Circle of Equal Altitude : Slide35:  For every degree of altitude from zenith as seen through a sextant, assume a 60 mile arc of visibility Circle of Equal Altitude:  Circle of Equal Altitude Thus, if we know the altitude of a particular star and its location relative to the earth, we know that our position must lie somewhere on its circle of equal altitude. This circle of equal altitude is a celestial line of position (LOP). Slide37:  Here is a more realistic scenario, where our assumed position does not lie exactly on the circle of equal altitude… This “gap” is key !!! Circle of Equal Altitude Slide38:  If we know the altitude of two or more stars, we can cross the LOP’s and arrive at a celestial fix. Determining Gyro Error Azimuth of Polaris :  Determining Gyro Error Azimuth of Polaris Gyro Error by Polaris: Used in Northern latitudes between the equator and 65 oN. The observed true azimuth (Zn) of Polaris is compared with the tabulated azimuth of Polaris extracted from the Nautical Almanac. Determining Gyro Error Azimuth of the Sun:  Gyro Error by Azimuth of the Sun: Similar to the sun amplitude sight, but can be done any time of the day. The observed true azimuth of the sun is compared with the tabulated azimuth of the sun using a complex sight reduction form. Determining Gyro Error Azimuth of the Sun Predicting Times of Rising and Setting Phenomena:  Why are rising and setting phenomena important to the navigator? Safety Protocol Tactics Celestial Navigation Calculation requires use the Nautical Almanac and the DR plot. Predicting Times of Rising and Setting Phenomena Predicting Times of Rising and Setting Phenomena:  Terms you should be familiar with: Sun/moonrise: When the UL of the sun/moon crosses the horizon in ascent Sun/moonset: When the UL of the sun/moon dips below the horizon in descent Civil twilight: UL sun 6o below the horizon Nautical twilight: UL sun 12o below the horizon Stars are normally “shot” btwn civil and nautical twilight Predicting Times of Rising and Setting Phenomena Predicting Times of Rising and Setting Phenomena:  Given the regular rate of increase of GHA sun (15 deg per hour): The tabulated mean times of sun-associated phenomena at GMT can be used as the local mean times of the phenomena at all standard meridians. The GMT of sunrise/sunset can be found in the nautical almanac for 3-day periods of coverage. Predicting Times of Rising and Setting Phenomena REVIEW:  REVIEW QUESTIONS??:  QUESTIONS??

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