Astronomy


Astronomers have been studying the contents of the sky for at least 6000 years, or probably longer! Archaeologists have uncovered old observatories, and other evidence, which proves that people had an interest in learning about the celestial bodies and could predict their movements with good accuracy. With the instruments we have today, astronomers are able to penetrate deeper and deeper into space to discover new stars and planets.

 

Methods of obtaining one's position at sea were developed in Columbus' time. A ship's navigator could calculate the ship's position quite accurately using written tables that gave the position of a celestial body and instruments which measured the angle between a given celestial body and the horizon. Early instruments for determining the angle of a celestial body were the Cross-Staff, the Astrolabe, a Quadrant. From these early devices, eventually the Sextant was developed and is sometimes used today.

 


Position Finding At Sea


A fixed position on the trackless sea is expressed in degrees of latitude and longitude. Universal custom of mathematicians the world over has divided the circle into 360 degrees of arc. Geographers have divided the earth's surface into degrees of latitude and longitude. Latitude is measured from the equator north to the North Pole (90 degrees of arc) and from the equator to the South Pole (90 degrees of arc). Latitude is represented on a globe of the earth by circles, which are parallel to the equator. Latitude is expressed as 0 to 90 degrees north or south of the equator.

 

Longitude is measured from the prime meridian of longitude, which runs north and south through the royal observatory at Greenwich, England. It is expressed from 0 to 180 degrees of arc east of Greenwich to the international dateline, which runs north and south through the poles in the western Pacific Ocean. Longitude is also measured west from Greenwich across the Atlantic, the Americas and to the International Date Line in the Pacific Ocean.

 

Lines of the meridian are called "Meridians." They encircle the earth, converging at the North and South poles. Therefore, the Meridians are not shown as parallel lines on the globe, but rather like wedge slices of an orange.

 

Latitude is determined by calculations based upon the observed angle between a celestial body, and the visible horizon. Astronomical tables give the geographical position of these bodies. Because the earth revolves on it's axis through the poles once each day, longitude cannot be fixed without knowledge of the time of day related to the meridian at Greenwich. A great deal of voyaging and discovery was accomplished before the development of timepieces (clocks, or watches called chronometers at sea.)

 

When charts became available to navigators, a common practice was to sail to a given latitude, which could be determined at sea, and then proceed east or west until the destination was reached. This did not always provide the shortest route. The estimated track of a ship, based upon measured distance and direction traveled from the last known position at sea is known as "dead reckoning." Before the age of electronics, radar or satellite equipment, ship's navigators used their own resources to "fix" their positions. Dead reckoning was used then, and is still used today. In fact, Christopher Columbus used dead reckoning. Although he knew the general direction of the lands to which he was sailing, he did not know of the existence of the American continent, which stood between Spain and Asia. By sailing West from Spain, he could not help but bump into the New World somewhere on his way to China. He made useful charts of the Caribbean islands, which he visited. He found his way back to Spain with great accuracy of navigation, and his subsequent returns to the New World were a simple matter for him, and all those who followed.

 

To chart a trip by dead reckoning you must have and know:

 

  • A chart of the region to be traversed.
  • The point of departure. (Your starting place.)
  • Desired destination. (Where you want to end your trip.)
  • From the chart, you must determine the distance to be sailed, the direction you will sail, and any obstacles such as islands or rocks to avoid.
  • You must determine, record and plot as you proceed: your speed, the direction sailed for each speed, and the time spent on each "leg" (a change in direction constitutes a "leg").

 

The accuracy of dead reckoning is subject to human observations. Errors in determining speed, direction, or time can enter the calculations. For example, the effects of currents are undetected by dead reckoning, and since all travel errors are cumulative, so the farther you go, the greater the error. This could cause you to miss your destination by several miles. However, a navigator who is good at dead reckoning will have an estimate of their position, know what direction to sail in order to avoid hazards, and arrive at the correct destination at the estimated time of arrival.

 


Obtaining A Fix


A "fix" is a precise positioning of the vessel on a chart. The fix is more accurate than the dead reckoning position. A fix may be obtained by observing an identifiable object on the chart such as an island, a prominent rock, or a lighthouse and then determining the position of ship in relation to the object. A fix may be obtained by observing two or more celestial bodies and doing necessary calculations to reduce the sighting to latitude and longitude, which are then plotted on the chart.

 

Speed is a combination of distance and time. (Miles per hour/knots) On a modern sailing vessel, speed is measured using a speedometer. However, before the age of speedometers, speed was measured by dropping a piece of wood from the bow of the ship into the water, and timing the passage from the bow to the stern. By measuring the time it takes in minutes and then dividing the length of the ship by that time, one can determine the speed of the vessel.

 


Keeping track of speed and time at sea

 

As early as the sixteenth century, a chip log was used for measuring the speed of a ship. It consists of a flat piece of wood formed as a sector of a circle, and weighted to allow it to float upright. It is secured to a line so that it may float square. When thrown into the water, it will remain in one place and as the ship sails away, the measured line will reel off leaving enough slack in the line so that the wood left behind will remain stationary in the water. An hourglass is used to determine the fixed interval of time. The length of line that runs out in relation to the trickle of sand in the hourglass measures the speed of the ship in knots. A quick pull on the line will detach the peg in the log chip, making it float flat so that it can be hauled in.

 

Under normal conditions the ship would travel too fast to measure the speed of the wooden chip by hourglass. The glass would be used for measuring longer periods of time because the sand pouring from the top sphere through to the bottom can only be accurately measured when it has completed it's reposition. An hourglass calibrated to run all of the sand from top to bottom in one-half hour was a common shipboard item. Ship's time keeping was divided into half hour increments. The glass was turned over to start timing a new half hour each time the top emptied into the bottom. At the same time a ship's bell would be struck so that the entire ship's company could be aware of the time. In the days before clocks or watches, time was measured by the changing azimuths of the sun and a compass, or the rotation of circumpolar stars using a nocturnal.

 

Routine activities at sea were timed by a system of four hour "watches" or work periods. A day's time started at 12 midnight. The hourglass was activated. When the sand ran through, the glass was turned over and the ship's bell was struck once. Watch time was then 12:30 am. The next time the sand ran all the way through (1/2 hour) the bell was struck twice. Watch time was 1:00am. This pattern continued until eight bells were rang (4:00am). The pattern of ship's bell time is quite distinctive and easy to understand once it becomes familiar. The full hours are sounded by a series of two quick strokes, then a pause, repeating until the double strokes add up to 2,4,6, or 8. Half hours are sounded by a single stroke after the proper number of pairs and short pause.

 

When ship's bells were heard to designate four hours, the helmsman was relieved of his duties; the log was brought up to date, and any new observations were passed on to the next man on duty. This system has not changed over the years. As mentioned earlier, the hourglass was not very accurate at keeping time, as minutes are lost in just the act of turning it over to re-calibrate it. Fortunately, the early mariners had another way of correcting their time. Astronomical tables had been developed. These tables (charts) gave the time of the rising and setting of a number of stars, and planets including the sun and the moon, on each day of the year.

 

The sun is the most useful of all. In addition to rising and setting at times which vary, the sun reaches it's highest point in it's journey across the sky at apparent noon, each day. Even on overcast days, when other bodies are not visible, the disc of the sun may be observed through cloud cover. At local apparent noon each day, the early navigator was able to correct his reading of the hourglass.

 

Although modern navigators have the use of many electronics inventions, which give them greater accuracy and ease of navigation, knowledge of astronomy, dead reckoning, charting and ordinary sea sense is helpful in correctly positioning yourself on the immense and trackless seas. It is comforting to know that you can find your way to the shore of your choice with a basic knowledge of seamanship.

 


Charts


On land there are many road signs to help you find your way. A simple glance at your road map will tell you where you are and how many miles you will have to travel to get to your destination. At sea, conditions are very different; there are no road signs to tell you that you are on the right course, so you have to rely on your knowledge of navigation, and be able to read and understand your charts of the sea. Charts can be purchased in nautical bookstores and ship chandleries. Charts are published by the United States Government for world wide use. Other countries produce charts for their mariners.

 

Charts typically show significant landmarks on the shore, such as mountains, or tall buildings; plus navigational aids such as buoys, lighthouses, markers, beacons, navigational channels, or obstructions like, rocks, shoals, wrecks, islands and shorelines. The vertical scale on the side of the chart is latitude and the top and bottom scales are longitude. The compass rose points to true and magnetic directions. Charts also show the depth of the water; when it is close to the shore it is in feet, and fathoms in deep waters (a fathom equals 6 feet).

 

 

Ship to Ship Communications --- The International Morse Code


Morse code represents letters, numbers, and punctuation marks by combinations of dots and dashes. These are transmitted by telegraph, radio, visible signaling devices, or just by tapping the code on any surface. When using flashing lights, a short flash is a dot, and a long flash is a dash. People sometimes refer to the sounds as "di" for dot, and "da" for dash.

 

Morse Code Alphabet:


A .- K -.- U ..- 4 ....-
B -... L .-.. V ...- 5 .....
C -.-. M - - W . - - 6 -....
D -.. N -. X -..- 7 --...
E . O --- Y -.-- 8 ---..
F ..-. P .--. Z --.. 9 ----.
G --. Q --.- 0 ---- Comma --..--
H .... R .-. 1 .----  
I .. S ... 2 ..---  
J .--- T - 3 ...--  

 

In the days before electricity, oil lamps were used for communication. Shutters covered, then revealed the light behind them to send code. Our modern Navy and the forest service use heliographs which reflect the sun's rays from a mirror, and use shutters to produce the dots and dashes. American Indians used a blanket to make signals from a smoking fire. Long and short puffs of smoke were their version of code. No doubt their code differed from the international Morse code, but the results were just as effective.

 

Morse code has become obsolete due to the new sophisticated methods of communication, but it is important as a basic skill which can be used when the electronic instruments fail.

 

 


Signal Flags


All ships and many small yachts carry a set of signal flags which are used to communicate with other ships. If you see a flag on the stern of a vessel then this is the flag of the country of the ship's origin. There are several types of signal flags, as each flag represents a letter of the alphabet. In the laguage of the navy, flags can also convey a longer message. For instance, flag A flying over flag B could say, "Abandon Ship." Or F over P could say "Bad weather is expected." Needless to say, with our modern electronics, this sort of communication has become obsolete. However, in the unlikely event of a loss of power, it is always good to have old-fashioned communication material on board.

 


Ship and Boats Rigs

 

Why do some sailing vessels have one mast while others have two or more? Sailboats "fly" sails that catch the wind, propelling them in the direction they wish to go. The size of the sail and the number of sails they are flying determine their speed. A small boat may have only one mast and one sail, and yet move quite fast through the water because it is light in weight, and carries just one sailor. A boat with two masts can carry more sail. The boat will be heavier, and need more crew to handle the sails.

 

A boat with many masts carries many more sails. It is able to move through the water fast but must be larger and heavier, and therefore needs more crew to handle the greater amount of sail area. More crewmen, more bunks, and more food must be carried. Usually a multi-masted sailing vessel carried passengers or cargo to far away places. In the event of a storm with high winds, sails were furled or made smaller by partial furling. This helped make the vessels sail in an upright position, making it safer and more comfortable for the people on board.

 

 
If you wish to pursue the study of science and the sea, there are many books available.

 


Activities

 

 

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