Cruise Training / May 8th

Now that we have an understanding of Latitude and Longitude and also, distance at sea, it is now necessary to understand the concept of  “time.”  Time is all-important to the seamen as it is how they tell where they are, how fast they are going and where celestial bodies will be in the sky.

In the early days on ships, the determination of time was as simple as using an egg timer or hour glass where sand ran at a controlled rate and gave a measure of time.  The sun also had been used for time in the early days.  When the sun was as high in the sky as it would get each day, they called it noon, or local apparent noon.  When clocks were first developed, they would reset the clocks every day at noon.  Before clocks, they would restart their hourglass at noon each day to keep the watch. 

The good part about the hourglass method was it kept the ship up with the sun.  As the ship moved around the world, it changed its time a little each day.  Remember, these ships did not usually move very far each day. 

At any one time, the sun is over one longitude at a time so if you are in London at noon, it is midnight in Guam.  Guam is half way around the world from London.  We all know that there are 24 hours in a day.  That means that there are 24 different times on the earth at any one time.  Between London at noon, there are 12 earlier times towards Guam and 12 times, each an hour later towards Guam in the other direction.  If we divided 24 times into 360 degrees (around the world), we get 15 degrees. 

Rather than using sun time, which would have every location on a different time, we keep time by zone.  We pick the middle of each 15 degrees and the entire 15-degree band keeps that time.  This is called a time zone.  At one side of the time zone, the sun is overhead almost one hour before it is overhead on the other side of the time zone. 

To find the actual solar time, one needs to apply the distance away from the central or time meridian of the zone time or the time that a celestial body is where it is calculated to be.  This is all very important in navigation and will be discussed again.