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November 25th, 2009 – At Sea, 03 56N, 046 15W
by Harry Stern and Herb McCormick
Sailors at sea often find themselves obsessing with nautical charts, and the lines of longitude and latitude that, when crossed, can supply their given position – also known as their “coordinates” – at any given time. But there’s a whole other subset of charted “lines” that they also find fascinating, some of which have very colorful names: the Tropics of Cancer and Capricorn, the Horse Latitudes, the Arctic Circle, and of course, the Equator.
Mariners obsess over these rather mysterious waypoints in the way a
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| As Ocean Watch approaches the equator, adverse ocean currents continue to pose a challenge. |
truck driver pays note of the Interstate highways and road junctions. In the very same way, they serve to illustrate the routes one takes over long journeys. Of course, the trucker has things easier; the signs on a highway are easy to follow. The modern navigator, luckily, has GPS satellite devices to help guide him on the way, but there are no billboards or medians on the world’s oceans.
Today, Ocean Watch is east of the longitude 050W, which when you look at a chart of the Atlantic Ocean reveals a couple of things. We’re now well east of the entire continent of North America, and well, well east of the island of Bermuda, a familiar oceanic outpost to many North American offshore sailors. Had we left the mid-Atlantic seaboard of the U.S. East Coast bound for England, we’d now be very much on our way.
The voyage continues to be a challenging one for the crew, as skipper Mark Schrader noted today in his personal log:
“This day before Thanksgiving happens to be the 178th day of the Around the Americas voyage and before the day is over the mileage log will record our 13,000th nautical mile. At just under 4N we’ve obviously not crossed the equator, but we’re close and very much looking forward to passing that significant milestone. Our nemesis in achieving that goal is this ever-present adverse current.
“Among other things the active watch routine includes a regular scan of the horizon and a quick check of our radar screen to see what might be creeping over the horizon and heading our way. Lately most of the on and off watch attention has been focused on the difference between our speed over the ground (SOG) and or speed through the water. The difference between those numbers equals the amount of current affecting our travel. The numbers are important because I’ve noticed a direct inverse proportion between adverse current and crew disposition. You’ve got it – the lower the number, the lighter the moment, and so forth. This might be worthy of a graph!”
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| Skipper Mark Schrader makes repairs. |
In fact, the skipper has our current chief resident scientist, Michael Reynolds, compiling relevant graphs on all sorts of data related to our voyage south: barometric pressure, true wind direction, temperature, boat speed vs. wind speed, etc. “You name it and Dr. Reynolds can display the results in a beautiful graph,” he writes.
As the skipper notes, this holiday weekend the crew of Ocean Watch will cross yet another important line signaling their ongoing progress Around the Americas: the Equator. Scientist Harry Stern of the University of Washington’s Applied Physics Lab, an important member of the Ocean Watch crew, has been giving that invisible “line in the water” a bit of thought, as we’ll see in his latest contribution, what we onboard like to call the Letter from Harry…
The Equator and the Southern Hemisphere by Harry Stern
The equator is the line around the middle of the earth that’s equidistant from the North and South Poles. It divides the earth into Northern and Southern Hemispheres. Its latitude is 0 degrees, by definition. (The North Pole is 90 degrees N latitude and the South Pole is 90 degrees S latitude).
The earth is not a perfect sphere — it bulges slightly at the equator because of the rotation about its axis. The polar radius of the earth (the distance from the center of the earth to the North or South Pole) is 6,357 kilometers (3,970 miles), while the equatorial radius (the distance from the center of the earth to the equator) is 6,378 kilometers (3,986 miles). So if you’re standing on the deck of Ocean Watch at the equator, you’re 16 miles further from the center of the earth than if you’re standing at the North Pole.
The earth’s rotation also gives rise to the Coriolis effect. Imagine you’re in space looking down at the earth from directly above the North Pole. The earth would be spinning counter-clockwise. Now look down at the earth from above the South Pole. The earth is spinning clockwise. That’s why air and water currents in the Northern Hemisphere are deflected to the right, while those in the Southern Hemisphere are deflected to the left. So in a hurricane or tropical storm in the Northern Hemisphere, air rushes in toward the low-pressure center from all directions and is deflected to the right, creating a counter-clockwise vortex. In the Southern Hemisphere, the air rushes in and is deflected to the left, forming a clockwise vortex. The equator is the dividing line, although the Coriolis effect is weak near the equator. Okay, so when the dishwater drains out of the sink in the galley of Ocean Watch at Cape Horn, will it spin clockwise? Not necessarily, because the Coriolis effect is very weak at small scales (like the diameter of a sink). It’s much stronger at large scales (like the diameter of a hurricane).
The equator is a great place to launch a rocket (and probably why the European space station mentioned in an earlier log is located in nearby French Guiana). That’s because points on the equator travel faster than any other points on earth, so rockets get the biggest boost when they’re launched from the equator toward the east (the direction the earth is spinning). For example, because of the earth’s rotation, when Ocean Watch was in Pond Inlet, above the Arctic Circle, the boat was traveling through space at about 12,000 kilometers per rotation, or 500 km/hour (310 mph). But when Ocean Watch is on the equator, it will be traveling at about 40,000 kilometers per rotation, or 1,670 km/hour (1,040 mph). Hang on dudes!
The earth’s surface is 29% land and 71% ocean, but most of the land is in the Northern Hemisphere and most of the ocean is in the Southern Hemisphere. The Northern Hemisphere is 55% ocean while the Southern is 86% ocean. Ocean Watch is about to enter the water hemisphere.
Climates of the Southern Hemisphere (except Antarctica) tend to be slightly milder than those in the Northern Hemisphere; because water heats up and cools down more slowly than land (i.e. the oceans have a large thermal inertia).
The night Ocean Watch crosses the equator they can say goodbye to Polaris and hello to the Southern Cross (though, in fact, the crew has seen the Southern Cross rise in the wee hours of the morning for about a week now). Polaris is the star directly above the North Pole, and it will disappear below the horizon when they cross the equator. The Southern Cross is a constellation more-or-less above the South Pole, and it will become even more visible as Ocean Watch makes its way further south in the Southern Hemisphere.
The crew of Ocean Watch can also say goodbye to 90% of humanity when they cross the equator into the Southern Hemisphere. Only about 10% of the human population lives south of the equator.
When the crew crosses the equator, those onboard who haven’t done so before will go through an initiation ceremony granting them access to the Ancient Order of the Deep! But they’re not there quite yet, so that’s a story for another day.
- Harry Stern and Herb McCormick with photographs by David Thoreson
This crew log submitted by Iridium OpenPort and Stratos”
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