Astro Navigation

Astro Navigation (Astro), also known as Nautical Astronomy or Celestial Navigation (CelNav), is a means of navigation by the sun and stars. Used for hundreds of years astro became obsolescent with the introduction of GPS in the late 1980s.

Today a sun or star sight can be easily reduced to find latitude and longitude with a scientific calculator, spreadsheet or phone app. In the pre-electronic era it was done manually by a variety of methods and mathematical tables.

The purpose of this page is not to explain what astro is but rather to document some of its rich history, including the many sight reduction methods. This site does not provide an explanation of spherical trigonometry or the celestial sphere.

I can be contacted on this domain by email to user astro.

(Right) Cloudy Weather Johnson's slim book was first published in the 19th century. I have used the 1905 edition to check the accuracy of GPS and Google Earth.

Cloudy Weather


I believe that astro can be best understood by dealing with the method, sight taking, and sight reduction (the calculations) separately.

Decide Method

There are many ways in which the observed data can be used:

  • Noon Sight for Latitude
  • Ex-Meridian sight for Latitude
  • Equal altitudes for Longitude
  • Longitude by Chronometer (a form of Time Sight)
  • Time sights with position determined by Sumner Lines.
  • Double Chronometer for Latitude and Longitude - time sights.
  • Marc St Hilaire Intercept method - known as the New Navigation.
  • Double Altitudes. Various methods, includung mixing Sumner lines and the intercept method (aka New Navigation.)
  • Intersecting circles of equal altitude.
  • Lunar for time and longitude

Take Sight

  • Measure altitude of celestial object above the horizon.
  • Measure altitude of celestial object using an artificial horizon.
  • Measure altitude of celestial object using a bubble sextant.
  • Measure altitude of celestial object using an esoteric device such as a Bris sextant.

Reduce the Sight

There is a plethora of ways in which a sight can be reduced:

  • Longhand with mathematical tables.
  • Short methods using tables.
  • Inspection tables.
  • Slide rules, including the Bygrave and the German MH1.
  • Electronic - calculator, phone app, spreadsheet or online calculator.

Reduce the Sight

A study of sight reduction methods gives an insight into the rich history of astro.

Last Updated: Saturday 9th December 2017

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