Tag Archives: Time

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

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

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Converting GMT To GHA

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

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Zone Time

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

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Lunar Distance

  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

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How To Find Longitude

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

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Keeping Time By The Sun

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

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How To Make A Survival Sundial

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

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What Is Sidereal Time

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

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What Is The Equation of Time

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

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