Sight reduction tables

Sight Reduction Tables An Introduction Sight Reduction Tables These tables are designed to assist the navigator in two distinct operations:operations:preparation & planning of observations of stars the reduction (or calculation) of these observations The tables contain:contain:the values of altitude (Hc, to the nearest minute) the true azimuth (Zn, to the nearest degree) for seven selected stars for the complete ranges of latitude and LHA of Aries Sight Reduction Tables A total of 41 stars are used, 19 of which are of the first magnitude (brighter than a magnitude of 1.5 ) and 17 of the second magnitude The names of the 19 ‘first magnitude’ stars are given in capital letters Star Magnitude Magnitude is a measurement of a star’s brightness Brightness of stars are assigned a number starting with the brightest star having a magnitude of - Dimmer stars have a magnitude of zero or positive numbers Therefore, the larger the number the dimmer the star The brightest star, Sirius has a magnitude of - 1.4 while the sun has a magnitude of - 26.75 • This is an extract from the main body of the tables • You will see it is also a copy of your handout • You will see that each page is split into two sections so let us take a closer look:- • This is the top and bottom of the same page Sight Reduction Tables The arrangement of the tables provides:provides:the best selection of seven stars available for observation, for any position and time of an observer the necessary data for presetting the sextant altiude for the seven stars the azimuth of the seven stars accurate reduction of the sights after observation, but we not cover that section as it is not in the syllabus However, here is an example:example:- Extract from Sight Reduction Tables Sight Reduction Tables Extracting Information The tables are entered with the arguments of Latitude and LHA of Aries Latitude is tabulated to the nearest whole degree LHA Aries degree is also tabulated to the nearest whole Sight Reduction Tables Extracting Information The tables are entered with Latitude first, followed by LHA Aries From this we will obtain the altitude (Hc) and the azimuth (Zn) for seven stars As in the following example Star Charts, Diagrams & Constellations Pages 266 & 267 contain two star charts One for each of the hemispheres and also equatorial stars Extract from the Nautical Almanac Star Finder & Identifier • This is a very simple but effective means of obtaining stars available for sights and or azimuths for a given Latitude & LHA of Aries • It consists of a flat disc with selected stars overprinted on it, for both the northern & southern hemispheres • It comes with additional transparent discs, for differing Latitudes which have Altitude & Azimuth overprinted on them • These can be laid over the main disc to obtain azimuth and bearing of a star http://cgi.ebay.com/US-Navy-Star-Finder-and-Identifier-1958 Transparent discs Constellations The constellations are totally imaginary things Things that poets, farmers and astronomers have made up over the past 6,000 years (and probably even more!) The real purpose for the constellations is to help us tell which stars are which, nothing more On a really dark night, you can see about 1000 to 1500 stars Trying to tell which is which is hard The constellations help by breaking up the sky into more managable bits (http://www.astro.wisc.edu/~dolan/constellations/extra/c onstellations.html) Constellations They are used as mnemonics, or memory aids For example, if you spot three bright stars in a row in the winter evening, you might realize, "Oh! That's part of Orion!" Suddenly, the rest of the constellation falls into place and you can declare: "There's Betelgeuse in Orion's left shoulder and Rigel is his foot." And once you recognize Orion, you can remember that Orion's Hunting Dogs are always nearby Canis Major & Canis Minor (http://www.astro.wisc.edu/~dolan/constellations/extra/constellations.html) Constellation of Orion Constellation of Canis Major Constellation of Ursa Major [...]...• Lat 550 N LHA 1920 Sight Reduction Tables Why we choose certain stars You will have noticed that some of the stars have an asterisk beside them? The reason for this asterisk is to indicate that they are the three best stars which will give a three P/L fix with an optimum angle of 900 – 1200 between them This optimum angle of 900 – 1200 is what you are aiming for in star sights Consider this:this:-... 1200 Factors to be taken into account when taking star sights Always begin by shooting the stars with an easterly bearing Why do you think that should be? Factors to be taken into account when taking star sights Always use stars with an altitude greater than ~100 Why do you think that should be? Factors to be taken into account when taking star sights Always take a minimum of three stars with equidistant... charts One for each of the hemispheres and also equatorial stars Extract from the Nautical Almanac Star Finder & Identifier • This is a very simple but effective means of obtaining stars available for sights and or azimuths for a given Latitude & LHA of Aries • It consists of a flat disc with selected stars overprinted on it, for both the northern & southern hemispheres • It comes with additional transparent