## Introduction

Centuries ago, man started to travel by sea for various reasons. Curiosity of the world beyond, promises of riches untold as well as the desire to conquer are just some of the reasons that raised the need for a method to navigate accurately on the sea. Without a proper way to navigate, it would be virtually impossible to reach destinations which are faraway. To overcome this problem, man developed celestial navigation, the art of finding one's way with reference to the Sun, the Moon and the stars.

Why celestial navigation? Man has observed that celestial bodies obey certain behavior and the cycles repeat over a period of time. Hence they can be used as a guide, with reference to records obtained over the past few centuries. Instruments have been invented to reckon direction, time and positions on Earth. However, these inventions would be of no value without a proper co-ordinate system. Therefore, it is crucial for us to put up a fixed reference frame for our planet.

## Ordinate System for Earth

The Ordinate System for Earth is based on latitude and longitude. Latitude is the perpendicular height from the Equator with respect from the center of Earth, measured in terms of degrees. The Equator is of 0° latitude, as illustrated by the diagram below. The axis joining the poles is normal to the plane of the Equator, therefore the North Pole would be  90° North while the South Pole would be 90° South in terms of latitude. Longitude is the horizontal distance with respect to the center of Earth, measured in terms of degrees. The Prime Meridian is of 0° longitude, as illustrated by the diagram below.

Notice that position on Earth is measured in degrees instead of kilometers. First of all, it is not feasible to compute the physical distances of a location with respect to the Equator and Prime Meridian because of their large magnitude. Secondly, it is also virtually impossible for the naked eye to obtain the exact physical distance between celestial objects due to its magnitude. However, it is feasible to obtain the angular displacement. Therefore, degrees are being used instead of kilometers.

## References

#### Compass

What is the origin of the compass & its history?

Howstuffworks How Compasses Work

West Sea Co. Evolution of the Sextant

Tycho Brahe - astronomical instruments

#### Astrolabe

Martin Brunold - Astrolabienmacher

The ASTROLABE, by Gary Agranat and Dave Delligati, Fall 1980

Astronomical Instruments

#### Sextant

West Sea Co. Evolution of the Sextant

Tycho Brahe - astronomical instruments

#### Octant

From sails to satellites Williams J.E.D (1992)

A history of marine navigation W E May (1973)

#### Sundial

What is a sundial?

ROG Sundials

#### Nocturnal

Picture of nocturnal

#### Chronometer

The Invention of the Marine Chronometer

Harrison No1, No2, No3, No4

The Invention of the Marine Chronometer