CEL NAV

• Declination of a celestial body is the arc of a celestial meridian or the angle at the centre of the Earth contained between the Equinoctial and the parallel of the declination through the body.
• Sidereal Hour Angle(SHA) of a celestial body is the arc of the Equinoctial or the angle at the celestial pole contained between the celestial meridian of the First Point of Aeries and that through that body, measured westward from Aeries.
• Right Ascension(RA) of a celestial body is the arc of the Equinoctial or the angle at the celestial pole contained between the celestial meridian of the First Point of Aeries and that through that body, measured eastward from Aeries.
• Greenwich Hour Angle(GHA) of a celestial body is the arc of the Equinoctial or the angle at the celestial pole contained between the celestial meridian of the Greenwich and that of the body, measured westward from Greenwich.
• Local Hour Angle(LHA) of a celestial body is the arc of the Equinoctial or the angle at the celestial pole contained between the observer’s celestial meridian and the celestial meridian through that body, measured westward from Observer.

• The Azimuth of a celestial body is the arc of the observer’s rational horizon or the angle at his zenith contained between the observer’s celestial meridian and the vertical circle through that body.
• The Amplitude of a celestial body is the arc of the observer’s rational horizon or the angle at his zenith contained between the observer’s prime vertical and the vertical circle through that body, when the body is at observer’s rational horizon i.e. at sunrise or sunset.

• The  International Date Line(IDL) is an imaginary line running between the South Pole and the North Pole defining the boundary between one day & next.
• It passes through the Pacific Ocean roughly following the 180° line of longitude and it deviates little to pass around some territories and island groups.
• It is located halfway around the world from the prime meridian (0° longitude line in Greenwich, England).

It is necessary because 

• We know that, if a ship proceeds eastward, she would have to advance her clock by one hour for every 15° of d’long & if a ship is proceeding westward, she would have to retard her clock by one hour for every 15° of d’long, to indicate the correct LMT.
• Let us consider, a ship navigating eastward making a d’long of 12° per day. She would return to her original meridian in 360°/12=30 days. During this period, she would have to advance her clock by one hour for every 15° of d’long I.e a total of 360°/15 = 24 hours, or one day. That means, by her calendar, she would return to her original meridian in 31 days.
• Similarly, a ship navigating westward making a d’long of 12° per day. She would return to her original meridian in 360°/12=30 days. During this period, she would have to retard her clock by one hour for every 15° of d’long I.e a total of 360°/15 = 24 hours, or one day. That means, by her calendar, she would return to her original meridian in 29 days.
• Thus, in compare with the date at shore station on original meridian, the date of the ship which sailed eastward is one day ahead and the date of the ship with sailed westward is one day behind.
• To avoid this anamalous situation, the Date Line has been introduced by International agreement.
• To avoid this,
  Ships crossing the IDL, on an easterly course have to retard their date by one day & ships crossing the IDL, on a westerly course, have to advance their date by one day.

Longitude is the arc of the Equator or the angle at the poles contained between the Prime meridian and the meridian through that place.

It is  measured from 0° to 180°, named East or West.

Coordinated universal time also known as Greenwich Mean time(GMT)

It is the standard time used to set all the time zones around the world.