Tag Archives: Time

Calculating Changes in Longitude and Time Along a Parallel of Latitude.

Posted on August 18, 2017 by

If two ships are both positioned exactly on the Equator but are separated by 900 nautical miles in an east/west direction, then in terms of longitude, they will be 15o apart and in terms of Greenwich Mean Time, they will be … Continue reading →

Posted in astro navigation, Astro Navigation Demystified, celestial navigation, navigation | Tagged , , , ,

Converting GMT To GHA

Posted on August 7, 2015 by

Greenwich Mean Time (GMT) is the local mean time anywhere on the meridian of Greenwich.  In other words it is the Local Hour Angle of the Mean Sun on the meridian of Greenwich. Since the Greenwich meridian is used as the … Continue reading →

Posted in astro navigation, Astro Navigation Topics, celestial navigation | Tagged , , , ,

Zone Time

Posted on November 22, 2013 by

It would be impossible for a ship at sea to keep to the time of its longitude because (unless it is travelling due north or south) the longitude will be constantly changing.  For this reason, the sea areas of the … Continue reading →

Posted in Uncategorized | Tagged , , , | Leave a comment

Lunar Distance

Posted on November 9, 2013 by

  With the invention of the chronometer in 1759, mariners were, for the first time, able to calculate longitude by finding the difference between Greenwich Mean Time and Local Time.  However, the marine chronometer did not become affordable until around … Continue reading →

Posted in Uncategorized | Tagged , , , , , | Leave a comment

How To Find Longitude

Posted on September 3, 2013 by

The longitude problem.  The measurement of longitude was a problem  until well into the 18th. century.  Determining latitude was relatively easy because it could be calculated from the altitude of the sun at noon but for longitude, early navigators had to rely on dead … Continue reading →

Posted in Uncategorized | Tagged , , , , | Leave a comment

Keeping Time By The Sun

Posted on May 24, 2013 by

The True Sun.  Life on Earth is governed by the movement of the True Sun; that is the sun we see in the sky and not by the theoretical Mean Sun.  The units of time in everyday use are defined … Continue reading →

Posted in Uncategorized | Tagged , , , , | Leave a comment

How To Make A Survival Sundial

Posted on May 8, 2013 by

Calculating Time In A Survival Situation         How To Make A Sundial Relevant  Links:   Astro Navigation in a Survival Situation, Equation of Time ,  Sidereal Time Suppose you are shipwrecked on the Skeleton Coast of Namibia or you are … Continue reading →

Posted in Uncategorized | Tagged , , , , , , | Leave a comment

What Is Sidereal Time

Posted on April 14, 2013 by

Sidereal Time.  Even though Sidereal Time is a more accurate measure of the Earth’s rotation, we normally use Solar Time for time-keeping because the Sidereal Day is not a practical unit for us to use.  On the other hand, Sidereal … Continue reading →

Posted in Uncategorized | Tagged , , , , , | 3 Comments

What Is The Equation of Time

Posted on April 6, 2013 by

The Sun as a Time-Keeper.  Because the Earth’s orbital motion is not uniform, there are corresponding variations in the apparent speed of the Sun as it travels round the earth.   For this reason, the true Sun (the Sun that we … Continue reading →

Posted in Uncategorized | Tagged , , , , , | 1 Comment