CELESTIAL NAVIGATION.
Once out of sight of land positions have to be obtained using celestial bodies. In Navigation the following are normally used:
Planets - Venus, Mars, Jupiter, Saturn.
Stars - There are 57 used for navigation.
The moon, being close to the earth, has a complex path across the sky which involves special corrections and will not be used except for times of moonrise and moonset.
Before we can find our own position we need to know the exact position in the sky of the body we are going to use.
A simple model of the sky is used, which assumes the earth is at the centre of a sphere. All the celestial bodies move on this Celestial Sphere. The position of any body on this sphere is given in a similar way to latitude and longitude.
The Poles, Equator and Greenwich Meridian are projected on to the Celestial The Celestial Sphere
Sphere and become the The Celestial Poles, Greenwich Meridian, and Equator
Celestial Poles, the Celestial all correspond with those on earth.
Equator (or Equinoctial) and the Celestial Greenwich Meridian.
Positions on the Celestial Sphere are given from the Celestial Equator and the Celestial Greenwich Meridian.
Greenwich Hour Angle (GHA)
Greenwich Hour Angle (GHA) is measured westwards from the Celestial Greenwich Meridian. GHA goes all the way around the sphere from 00 to 3600.
Measured west from the Greenwich Meridian, 00 to 3600
Because both longitude and GHA are measured from the Greenwich Meridian it is possible to convert between longitude and GHA.
For westerly longitudes: GHA = Longitude West
Longitude = GHA [GHA less than 1800]
For easterly longitudes: GHA = 3600 - Longitude East
Longitude = 3600 - GHA [GHA over 1800]
Ex. GHA = 850 30.0’ long = 850 30.0’W
Long = 1740 13.7’W GHA = 1740 13.7’
GHA = 2310 30.0’ long = 3600 – 2310 30.0’ = 1280 30.0’E
Long = 0970 15 E GHA = 3600 – 0970 15.0’ = 2620 45.0’E
Convert GHA to longitude:
1. 0430 30.0’ 2. 1030 38.3’ 3. 1970 15.8’ 4. 2540 39.6’
Convert longitude to GHA:
5. 0230 29.9’W 6. 0050 46.8’E 7. 1790 52.4’W 8. 1790 52.4’E
GHA Longitude GHA Longitude
1) 043030.0’ = 0430 30.0’W 2) 1030 38.3’ = 1030 38.3’W
3) 1970 15.8’ – 3600 = 1620 44.2’E 4) 2540 39.6’ – 3600 = 1050 20.4’E
Longitude GHA Longitude GHA
5) 0230 29.9’W = 0230 29.9’W 6) 0050 46.8’E – 3600 = 3540 13.2’E
7) 1790 52.4’W = 1790 52.4’ W 8) 1790 52.4’E – 3600 = 1800 07.6’E
Declination (Dec).
The Declination of a body is measured North and South from the Celestial Equator. It is exactly the same as Latitude.
Measured north or south from the equator 00 to 900
Ex. Dec = S140 23.5’ lat = 140 23.5’S lat = 230 16.8’N dec = N230 16.8’
Definitions:
GHA is the distance of a body west of the Greenwich Meridian.
Declination is the distance of a body North or South of the Equator.
Both GHA and Dec are measured in degrees and minutes.
Note: GHA does not have a direction as it is always Westerly.
Declination has to have a direction (N or S) as it is measured from the Equator.
Geographical Position.
A straight line drawn from the centre of the Earth to a celestial body will pass through the earth’s surface. The position where this line cuts the earth’s surface is known as the Geographical Position of the celestial body. An observer standing at the geographical position of a celestial body would see that body directly overhead or on the observers Zenith. The geographical position is found by converting the bodies GHA and dec to lat and long.
Ex. Find the geographical position of the following body:
GHA 2650 48.1’ dec S180 26.4’ lat = dec long = 3600 – GHA
= 3600 – 2650 48.1’
180 24.1’S = 0940 11.9’E
Ex.2.
Find the geographical positions of the following bodies:
1) GHA 0630 44.5’ dec N270 13.3’ 2) GHA1970 26.8’ dec S030 46.7’
0270 13.3’ N 06 3 44.5’W 3600 – 1970 26.8’
= 1620 33.2’E
Pos = 0030 46.7’S 1620 33.2’E
3) GHA 359034.4’ dec S85049.7’ 4) GHA 2360 01.3’ dec N460 37.6’
= 3600 – 3590 34.4’ = 3600 – 2360 01.3’
= 0000 25.6’E = 1230 58.7’
Pos = 0850 49.7’S 0000 25.6’E Pos = 0460 37.6’N 1230 58.7’E
The Nautical Almanac.
The earth rotates around its axis once every twenty four hours. This rotation is the main cause of the apparent motion of the celestial bodies across the sky. The Earths rotation means that the GHA of any body is continually changing.
Declination also changes, but not as rapidly as GHA. The declination of stars remains constant from day to day. The suns declination changes between 230 30.0’N at mid summer to 230 30.0’S at mid winter. The declination of the moon and planets also change, but over a larger range than that of the sun.
Positions at sea can only be found if we know the exact position of the body we are using. This information is given in the Nautical Almanac. The Almanac is published annually. It lists the information required to obtain a bodies position in the sky, at any instant in time.
The Nautical Almanac lists the GHA and Dec of the sun, moon and four navigational planets for every hour of the day (GMT). Interpolation tables allow the GHA and Dec to be found for the minutes and seconds between each hour. In addition GHA Aries is given for each hour and a list of 57 stars is given containing their SHA and Declinations. SHA (Sidereal Hour Angle) is used to obtain the GHA of stars.
Information in the Nautical Almanac:
Inside the front cover: altitude correction tables – sun, stars, planets.
Introduction: calender, eclipse notes, planet notes.
Daily pages: detailed position information for each day of the year.
Explanation: include instructions for use.
Standard times: used by countries with difference to GMT.
Standard information: charts and monthly position information.
Pole Star tables: for latitude and azimuth.
Arc to time: conversion table.
Interpolation tables: increments and corrections (printed on yellowish paper).
Inside back cover: altitude correction tables – moon.
The parts of the Almanac most used are the altitude correction tables, daily pages (pages 10 to253), and increments and corrections table (pages ii to xxxi).
piokam2010