Navagation and Wobble

I’m sure many people know this, but a “great circle” is a circle that goes clear around the entire globe, and whose center is at the center of the globe. A “meridian”, on the other hand, is a great circle that passes through the poles. Lines of longitude are meridians, for example, while the Equator is a great circle. And the “Prime Meridian” is the line of longitude that goes through both poles .. and right through the Greenwich Observatory.  On the other side of the planet is the Prime Meridian also called the “International Date Line”.

If you know the exact time the sun rises where you happen to be, then you know which line of longitude you are on. Easy as that (with the usual caveats, refraction, etc.) … but only if you know exactly, to the second, what time it is. And that was why lots of seamen died before John Harrison invented his chronometer … they didn’t know what time it was, so they didn’t know where they were.  The Longitude Prize was a reward offered by the British government for a simple and practical method for the precise determination of a ship's longitude. The prize, established through an Act of Parliament (the Longitude Act) in 1714, was administered by the Board of Longitude.  Harrison's invention of a clock that could keep correct time, while at sea, and the concept that time equaled longitude won the £20,000 (worth well over a million dollars in todays money) for a method that could determine longitude within 30 nautical.  

To use time to determine the longitude of a ship a map of the world is divided into 24 segments, each representing one hour.  The clock on board the ship was set to the same time at Greenwich, now called Greenwich Mean Time, then as the ship sailed east or west of Greenwich to determine their longitued they had only to find the local time using a sexton.  To do this they use the noonday sun and the horizon of the sea, when it was getting near noon the navigator would start measuring the angle between the sun and the horizon.  He would keep taking the measurement until the angle stoped increasing and started to decrease.  That point was local noon.  Then all he had to do was to count the hours, minutes, and seconds on his chart from Greenwich and that point was his longitued.  For the latitude he had to wait until dark and use either the North Star or the Southern Cross is depending which side of the equator he was.  The further north the ship went the higher went the North Star from the horizon.  So in those days a sailor could only know where he was precisely east to west in the world once a day, the rest of the day was guess work called dead reckoning.

The Greenwich Observatory back in the days before radio they needed to be able to pass an accurate time signal to anyone in the Thames. To do that, they mounted a big red ball that can slide up and down a pole, viz:  A few minutes before 1:00 they raised the ball up to the top of the mast, and then at exactly 1:00 they dropped the ball … and all of the navigators on vessels up and down the Thames could set their Chronometers to the exact second.

To close this post I would like to talk about the earth's wobble.  Throughout the year the earth wobbles on its axes as it goes around the sun, with the results that the sun appears to set way down in the south in the winter. As it goes south each day gets shorter  while each night gets longer, when it get as far south as it will go we have the longest night of the year, and that night is called the winter soloist.  After that night it starts its moves to the north for the summer with each day getting longer and each night getting shorter, when it gets as far north as it will go we have the longest day of the year which is called the summer soloist.  Then it starts on its journey back to the south.  On the 22nd of September the sun will be its halfway point on its journey south and marks the first day of Fall.  On this day the day is called the fall equinox and night and day are of equal length.  This will happen again next spring with the sun on its way back north, but it will be called the spring equinox.  For Christians the first Sunday after the first full moon after the spring equinox is Easter.

The Greek astronomer, Hipparchus of Nicea by comparing observations more than a century apart, Hipparchus proposed that the axis around which the heavens seemed to rotate shifted gradually, though very slowly.  Around the year 130 BC, Hipparchus compared ancient observations to his own and concluded that in the preceding 169 years those intersections had moved by 2 degrees. How could Hipparchus know the position of the Sun among the stars so exactly, when stars are not visible in the daytime? By using not the Sun but the shadow cast by the Earth on the moon, during an eclipse of the Moon! During an eclipse, Sun, Earth and Moon form a straight line, and therefore the center of the Earth's shadow is at the point on the celestial sphere which is exactly opposite that of the Sun.

This is what is called the pression of the equinox or Axial Pression and we go through one such complete cycle in a period of approximately 26,000 years or 1° every 72 years.  This is why I put no stock in astrology as all of the calulation is based upon the position of the constalations as they were 3-5 thousands years ago.  If we thake the 3 thousand as the date it was formalized that means the the conslations have shifted about 41.6 drgrees from where they were then. And while some, like Hipparchus, knew of the shifting stars, most had no more a clue than most of us do today.

And we think that we are smart today.