Celestial Navigation is a method of orientation by using the stars to locate the traveller’s position when crossing land and oceans without a compass.
It has been in use since antiquity by the Ancient Egyptians, Greeks and Romans.

The Ancient Egyptians devised a method of telling the time at night based on the Heliacal Rising of 36 stars, known as the Decan Stars.
The Time can also be calculated by observing when a star passes overhead at the Meridian, and cross checking with the Almanac.

The Greek Astronomer Hipparchus (190-120 BCE) compiled a star catalogue of 850 stars.
Hipparchus wrote a ‘Commentary on the Phaenomena (appearances) of Eudoxus and Aratus’: This was an Astronomical Poem by Aratus, which introduced the Constellations and the Milky Way, and showed how to calculate the times of their rising and setting.

48 Constellations were listed by the Greco-Roman Astronomer Ptolemy (c.150 CE). He created an Almanac which gave the positions of all the known stars during the year.
The stars always rise at exactly the same point on the horizon, and always set at another point. What changes is the time of rising and decline, or whether they appear at all at that time of year. Knowing this the navigator can follow a particular star, and use it as a true heading.
Using the stars, a navigator can find True North, True West and True East, then adjust his heading for points inbetween.
By observing certain stars such as Sirius, rising either after Sunset or before sunrise, direction could be determined.

How to Navigate by the Stars

- Sailors followed a Parallel of Latitude by sailing either due West or due East to reach a destination or island on a known Latitude.
- This was known as ‘running down a westing or an easting’.
- By observing the number of degrees the Pole Star lies above the Horizon, this figure is equal to the Parallel of Latitude of the observer.
- For Example, if the Pole Star is observed at 52 degrees above the Horizon, then the observer’s position is on Latitude 52 degrees North.
- Sailors would reach a Parallel of Latitude by either sailing due North or South, then run due east or west until they found their intended island or landfall.

- To navigate in the Northern Hemisphere first requires knowing the position of True North.
- This is found by locating the Pole Star which is located at True North, and does not move throughout the night.
- To find the Pole Star, an observer can either use Ursa Major or Cassiopeia: A straight line through two of the stars in Ursa Major passes through the Pole Star. Or the observe can look for the ‘W’ of Cassiopeia and two of its stars also point towards the Pole Star.

- Mintaka, the lowest star in Orion’s belt rises within one degree of True East and sets within one degree of True West, between October and March.
- In the Northern Hemisphere, Orion is visible rising in the eastern sky from October becoming visible setting in the western sky in March.

- On the Equinoxes, the 21st of March and the 21st of October, the Sun rises precisely at due East and sets due west.

- Dead Reckoning is from ‘Ded Reckoning’ meaning ‘Deduced Reckoning’ which is the art of navigating from one position to another, based on an estimate of speed, heading and time, from the last known position or ‘fix’.
- Although it is subject to innacuracies, it serves as a rough guide in the absence of navigational instruments.

- He circumnavigated the globe in a westerly direction in three years using a small vessel called the ‘Spray’ between 1895-1898. He found his longitude by using Dead Reckoning. He published a book about his journey in 1900 called ‘Sailing alone around the world’.

- The Stars are all fixed in their position in the sky, and rise and set in the same direction, with the exception of the The Wandering Stars.
- The Wandering Stara are the Seven Classical Planets whose positions are not fixed, but move around in the sky.

- The Horizon is only visible at night during Sunset and Sunrise.
- Although the Stars Rise and Set 4 minutes earlier each day, they always Rise and Set in exactly the same position on the Horizon.
- Thus orientation is possible by following the position on the horizon of known rising or setting stars:

- - Rising:………..Setting:
  - 045….Capella….315
  - 090….Mintaka….270
  - 110….Sirius…….250
  - 000….The Pole Star does not rise or set but is always at Due North in the Northern Hemispere.

Telling the Time at night based on Ursa Major (The Plough (UK) or The Big Dipper (USA).

- ‘Astro Navigation Made Easy’ by Francois Meyrier.
- ‘The Barefoot Navigator’ by Jack Lagan.
- ‘Sailing alone around the world’ by Joshua Slocum (1900)