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SEA Currents Blog

SEA Currents: Robert C. Seamans

April 03, 2015

Time: A Human Invention of Great Use at Sea

Arthur Davis, C Watch, Oberlin College

Oceans & Climate

Top: Conard in the doghouse (navigation center) plotting our position on the chart.Bottom Get out the sextants! The GPS in the Doghouse has been covered up, and now we will rely on the heavens!

Current Position
43° 46.8’S x 173° 36.0’W

Course & Speed
090° (East) at 9 knots!

Sail Plan
Four Lowers (Mains’l, Main Stays’l, Fore Stays’l, Jib) and Tops’l

Cloudy with strong winds

Souls on Board

Today marks the first 24 hour period that we will observe as the 3rd of April.  How is this possible? Tonight we will cross the International Date Line, which, unlike the equator, tropics, or ant/arctic circles, does not represent any change in natural phenomena. It is rather the other side of the prime meridian (itself an arbitrary line) that runs through Greenwich, England.  Although it is arbitrary, the Date Line is important because of our attention to time. 

Aboard the Robert C. Seamans, time is a vital factor in the daily workings of life.  Our watch schedules, mealtimes, and our data collection for science depend on observation and records of time.  However, time is fickle, changeable, and ultimately an invention of humanity.  We became acutely aware of this when we changed our watches forward 45 minutes in the Chatham Islands (to observe their local time) and then forward again another 15= minutes as we left Chatham Islands yesterday.  This has created a unique time zone where the sun has not reached its peak until almost 1400 (2 pm). Tonight, however, will be the most mind-boggling change to date.  At 2300 (11 pm), we will set our clocks back an hour and also set the date back one day!  There is much confusion and excitement on board about the Date Line and how it will affect the life of the ship.

As we set sail upon this next leg of our journey, this change will be of utmost importance for our navigation because all of our celestial calculations are in reference to Greenwich Mean Time, and crossing the Date Line means that our reference will shift dramatically.  Today we have covered up the GPS in the Doghouse, which is our navigational center on the ship.  Without our GPS to fix our position, we will be relying on celestial sights and dead reckoning.  This means that observing the stars, sun, planets, and moon will be our sole way to determine our exact position at any given time.  Time is a key tool for mariners in celestial navigation. For instance, every 4 seconds, the geographic position of the sun moves 1 mile!  So every second matters.  We have been practicing our celestial navigation and many of us have successfully plotted celestially-derived information on the charts, using the Sun to find a line of position or latitude at Local Apparent Noon (when the sun reaches its peak in the sky). Today, we learned how to prepare for our morning and evening twilight star sights, colloquially known as “star frenzy.”

There is something quite remarkable about holding a sextant to the sky, lowering a celestial body to the horizon, and calculating one’s exact position on the globe.  A few days ago, I was able to plot my first running fix on the chart in the Doghouse!  In the morning, I set the shades of my sextant to safely view the sun, and lowered the sun to the horizon until it was sitting right on the top of the sea.  I was then able to use the angle of the sun and the exact time (to the second) of the sight to calculate a line of position.  At Local Apparent Noon, I calculated our exact latitude because the sun and the ship at that moment were on the exact same meridian of longitude.  By advancing my morning line of position, I could determine our exact coordinates at Local Apparent Noon!  It was an empowering moment to put this observation down on the chart without the use of electronics! It is a comforting feeling to have this celestial clock ticking above us as a guide through this vast stretch of ocean.  With nothing but Antarctica to our south, and nothing but South America east of us, we are certainly in need of a dependable map to find our way, and the combination of the heavens above us and a good clock will be plenty for our purposes.  So with these tools, we sail into the open Pacific with the Chatham Islands in our wake, and our sextants and clocks to make our way forward.

May the stars shine bright, both back home and at sea,

Categories: Robert C. Seamans,Oceans & Climate, • Topics: s258  celestial navigation • (1) Comments
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#1. Posted by Roberta Chomiak on April 05, 2015

We are wishing you a very blessed and happy Easter, Leah. It won’t be the same without you here. Please know that we are continually keeping you in our thoughts and prayers and wishing you a safe journey home. Love you,
Mom, Nicholas, Emily and Jennifer.



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