MODULE 4: IDENTIFICATION OF AN UNKNOWN CELESTIAL BODY 1. Star charts There are many star charts on the market. Even in the Nautical Almanac a star chart is introduced. But only few star charts gives the navigator the tools he needs: determination of bearing and approximate altitude. Brown’s Nautical Start Chart is one of them. The chart is a grid of declination and Right Ascention. To use the chart, you first have to find the LHAγ of the moment of observation (identification). You introduce your position as a zenith on the chart by: - your latitude on the declination scale - your longitude with LHAγ on the Right Ascension scale

latitude

Zenith (your position)

δ scale

LHAγ

RA scale

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The bearing to each star is obtained by the angle between the direction from your zenith to the star and the meridian/ hour circle. Nv bearing

To find the altitude, you need a divider. Procedure: Read the declination of the star on the star chart. Make the mean/mid latitude between your latitude and the declination of the star. Take with the divider the distance between the zenith and the star on the chart. Bring the divider to the “Scale of altitude”: place one end on the mid latitude scale at the spot corresponding with the calculated mid latitude and the other end in the horizontal plane. From that point follow the curve till the altitude scale and read the altitude. Altitude star

Distance star-zenith

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2. Star Finder Also for star finders, many models exist on the market. The most common used is “STAR-FINDER 2102-D” of Wheems & Plath, an exact duplicate of the one formerly distributed by the U.S. Naval Oceanographic Office. It portrays graphically the altitude and azimuth of the fifty-seven numbered stars in the Nautical and Air Almanacs. Additional celestial bodies may be plotted on the star base such as planets. The star finder provides the best way to identify isolated stars and planets in view, or to predict the locations of bodies not yet in view. It also helps the navigator predict and plan the best star-planet-moon combinations. The unit consists of a dual star base, nine altitude azimuth templates centred at 10 intervals of latitude, meridian angle diagram and instruction sheet.

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The basic template indicates the celestial equator. The template is printed with the stars on both sides, one side with the North Pole in the middle, the other with the South Pole. Use that side with the same name as your latitude. In the example on previous page, all stars within the celestial equator have a declination with name North, those outside the celestial equator declination South. On the South side of the template, the version is reversed. The reference is the celestial equator (declination = 0°).

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a. plotting other celestial bodies on the basic template We start to plot the planets for a certain moment of the day (moment of observation/identification). To plot the planets, we need the Right Ascension (RA) and declination. In the Nautical Almanac we enter on the day and time in UT of the observation/identification and find GHA planet and the declination. We obtain the SHA of the planet : GHA planet - GHAγ =SHA planet or at the bottom of the N.A. (SHA planet indicated for 3 days). RA = 360° - SHA planet We now use the red coloured template (picture see next page) and put it with the side having the same name as your latitude on the basic template. The red arrow of the red template has to point to the RA on the border of the basic template. Introduce the planet with a pencil as per declination on the basic template. b. determination of altitude and bearing Calculate the LHAγ for the moment of observation. Take the blue coloured template (picture on next pages) with the most approximate latitude to your latitude. Place the blue template ( side same name as the name of your latitude) on the basic template. Bring the bleu arrow in line with the LHAγ on the border of the basic template. All stars and plotted planets within the blue grid figure are visible, the others are unvisible (under the local horizon). You now can determine the bearing and altitude of each visible celestial body.

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Plotting area

RA indice

411744

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Bearing indication Altitude line

LHAγ indice

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3. HO 249 The HO 249 volume 1, selected stars, can be used to identify the selected stars for a certain moment at a certain latitude. By calculating the LHAγ of the moment of observation, you enter volume 1 and look for the page corresponding to the latitude of the observer. Introducing LHAγ on that page, you find the height (Hc) and bearing of the 7 selected stars. 4. By calculation If we have no other means than the Nautical Almanac, we can identify celestial bodies by calculation. By observation we get the altitude and bearing of the unknown celestial body. We need to calculate the declination and SHA to identify the celestial body in the Nautical Almanac. Z

Co-latitude Az

ξ P

Pn

∆

In the spherical triangle are: ξ = zenith distance = 90° - h ( observed altitude) ∆ = polar distance = 90° - δ (declination) P= polar angle Z = Az= azimuth (from observed bearing)

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Determination of SHA: cos Az x cos col = sin Az x cotg P + sin col x cotg ξ cos Az x sin l – cos l x tg h = sin Az x cotg P sin l

cos l x tg h

------ - ----------------- = cotg P tg Az sin Az

(: cos l)

tg l tg h cotg P ------ - ---------- = --------tg Az sin Az cos l We can use the ABC tables to find P: Table A: enter with latitude and AZ (LHA column) Table B: enter with altitude (declination) and Az (LHA column) Table C: with factor C and latitude you find P The sign of P = name of Az (E or W quadrant) LHA = P(west) and LHA = 360° - P(east) (LHA – g) -GHAγ = SHA celestial body g = longitude of observer GHAγ can be obtained in the Nautical Almanac with the time UT of observation. Determination of declination: Sin Az sin P -------- = ---------sin ∆ sin ξ cos δ =sin Az x cos hx cosec P The sign of the declination is the same as the sign of Az (N or S).

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