Sight reduction tables

Sight Reduction Tables These tables are designed to assist the navigator in two distinct operations: distinct operations:-- preparation & planning of observations of stars the reducti

Sight Reduction Tables

An Introduction

Sight Reduction Tables

These tables are designed to assist the navigator in two

distinct operations:

distinct

operations:
preparation & planning of observations of stars

the reduction (or calculation) of these observations

the reduction (or calculation) of these observations

The tables contain: The tables

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

with the brightest star having a magnitude of 11

with the brightest star having a magnitude of

with the brightest star having a magnitude of 11

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 The brightest star, Sirius has a magnitude of 1.4 while 1.4 while the sun has a magnitude of

the sun has a magnitude of 26.7526.75

• This is an extract from the main body of the

• This is the top and bottom of the same page

Sight Reduction Tables

The arrangement of the tables provides: The arrangement of the tables

provides:
the best selection of seven stars available for observation, for any position and time of an observer

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

do not cover that section as it is not in the syllabus However, here is an example:

here is an

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  is also tabulated to the nearest whole degree

Sight Reduction Tables Extracting Information

The tables are entered with Latitude first, followed by LHA Aries

• 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

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 90 00 –– 120 120 00 between them

This optimum angle of 90 00 –– 120 120 00 is what you are aiming for in star sights

Consider this: Consider

this: Which of the following set of bearings would

provide the best fix?

Obviously the first one

120 0

120 0

120 0

Factors to be taken into account when taking star sights

with an easterly bearing

Factors to be taken into account when taking star sights

Always use stars with an altitude

Factors to be taken into account when taking star sights

with equidistant separation if possible

Star Charts, Diagrams &

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

 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

 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 ))

 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