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MIT 12 215 - Nautical Almanac

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112.215 Modern NavigationThomas Herring ([email protected]),MW 11:00-12:30 Room 54-322http://geoweb.mit.edu/~tah/12.21510/05/2009 12.215 Modern Naviation L06 2Review of last Class• Motion of the Earth and Sun– Geometry of Earth/Sun system– Astronomical coordinates– Motion of the Earth around the sun– Equation of Time• Astronomical positioning– Latitude and Longitude determination usingastronomical bodies• Error contributions to latitude and longitudemeasurements.210/05/2009 12.215 Modern Naviation L06 3Todayʼs Class• Almanacs: Paper and electronics– Paper Almanacs: Nautical Almanac– Electronic: Available on many web sites10/05/2009 12.215 Modern Naviation L06 4Nautical Almanac• Probably most common for ship navigation in US.Published by the US Naval Observatory and HerMajestyʼs Nautical Almanac Office.• Contains all the necessary information for celestialnavigation• New volume published each year and containsinformation from Jan 1 to Dec 31 of that year• Largest part of book gives the RA and Declinations ofSun, Moon, Aries, Venus, Mars, Jupiter and Saturnevery hour of every day310/05/2009 12.215 Modern Naviation L06 5Cover of Nautical AlmanacCall Numbers10/05/2009 12.215 Modern Naviation L06 6Sun and Moon TablesEntries in TableGHA - Greenwich HourAngleDec - Declinationv - Rate of change ofGHA (ʻ/hr)d - Rate of change ofdeclination (ʻ/hr)HP - Horizontal parallax410/05/2009 12.215 Modern Naviation L06 7Explanation• Greenwich Hour Angle is the angle between a bodyand the Greenwich meridian measured positive west(note sign convention difference).• The Greenwich Hour Angle of the Sun is always near0 at 12:00UT (difference is equation of time).• The GHA of the first point of Aries is the negative ofGreenwich sidereal time.• v and d are computed simply by differencing valuesand make hand calculations easier10/05/2009 12.215 Modern Naviation L06 8Horizontal Parallax of Moon• Tables are given to the center of a body from thecenter of the Earth. The moon is close enough thatthe finite sizes of the Earth and Moon affectmeasurements.• HP of moon is difference in angles between edge ofEarth and edge of moon. Seehttp://www.fourmilab.ch/earthview/moon_ap_per.html510/05/2009 12.215 Modern Naviation L06 9Equation of Time• Also given in the Almanac is the equation of time foreach day of the year. From this entry you cancalculate when the meridian crossing will be aGreenwich.• The difference between the Greenwich meridian of theSun and the local crossing is the longitude.10/05/2009 12.215 Modern Naviation L06 10Phases of themoonhttp://www.usno.navy.mil/USNO/astronomical‐applications/images_aa/Moon_phases.jpg610/05/2009 12.215 Modern Naviation L06 11Comments on Nautical Almanacs• The nautical Almanac contains many other tables andexplanations. Many of these tables were used beforethe advent of calculators and computer programs.• Paper almanacs are meant to be used by ships at seawith little computational power.• Altitude (elevation angles) corrections are given forthe size of the Sun (~16ʼ) and atmospheric refraction.For atmospheric refraction an approximate formula is(accurate to 5” at 20o)€ Δε= 60"/(tanε+ 0.028)10/05/2009 12.215 Modern Naviation L06 12Atmospheric refraction0.11.010.0100.0-20.0 0.0 20.0 40.0 60.0 80.0 100.0Refraction (')Model (')ModelRefraction (')Elevation (deg)Model 1'/(tan(elev)+0.028)710/05/2009 12.215 Modern Naviation L06 13Error in simple refraction model-2.0-1.00.01.02.03.0-20.0 0.0 20.0 40.0 60.0 80.0Model Error (')Refraction (')Elevation (deg)Corresponds to 1 nautical Mile = 1.86km10/05/2009 12.215 Modern Naviation L06 14Nautical Almanac Correction• Based on Pressure and temperature zone A-N810/05/2009 12.215 Modern Naviation L06 15Corrections• From Zone and altitude additional correction applied10/05/2009 12.215 Modern Naviation L06 16On-line almanacs• If access to the internet is possible then the on-line versions ofalmanacs are much easier to use• Computer programs are also available which can be run locally.All of the values in the Almanac are now computed (observationsare no longer needed).• http://www.usno.navy.mil/USNO?has many resources including anon-line access to many astronomical data types.http://www.usno.navy.mil/USNO/astronomical‐applications/data‐services/• Other on-line sources:http://www.tecepe.com.br/nav/almanac.html-ssihttp://www.tecepe.com.br/scripts/AlmanacPagesISAPI.isaFor the remainder of the class we will examine these on-linesources and compare the results to the paper Nautical


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