Alan Nelson's Daily Commentary for 31 August 1999

Meck Island, Republic of Marshall Islands

Today's pictures are all about the sun. This view of the sunrise was from the middle of the lagoon as I was riding the catamaran ferry from Kwajalein to Meck. The sunrise was quite entertaining for almost 20 minutes. It was a fun way to pass a boat ride waiting for the right island features to be in front of varying degrees of red and streamers! This was my favorite. It was probably prettier for me because it was easy to pick out individual trees on the islands.

Bad news was that the equipment was down with a whole new set of symptoms. So no balloons got launched and I didn't learn anything more about its operation because it wasn't operating. Chester had fine-tuned the equipment yesterday so that the last flight had been tracked to 14 millibars--over 100,000 feet high. Then today, nothing. Frustrating. Poor Chester spent the whole day inside a small, hot trailer trying to find out what was wrong. Without any success. If the problem could be solved by conscientious effort, it would have been fully functional a long time ago.

Here is a series of pictures taken right around solar noon today on Meck. It illustrates a phenomenon only seen in the tropics: the sun was directly overhead. These pictures show the base of a 35 foot tall flag pole. When the sun was at its highest, the flag pole's shadow was barely a quarter inch long! Before I left the states, I had checked the website at the Naval Observatory and after a little bit of work learned that September 1 would be the day of straight-overhead sun at approximately this latitude. Don't know if I'll have sun at noon tomorrow, so I'll just call this THE day--a quarter inch shadow is pretty darn small!

In fact, the tropics are defined as the area of the Earth where the sun is directly overhead at some time during the year. The Tropic of Cancer is the farthest point in the northern latitudes that have the sun directly overhead, the Tropic of Capricorn the farthest point south. The spring equinox which falls on March 20th (my mother's birthday) or March 21st is the day when the sun is straight over the Tropic of Cancer. The autumnal equinox which falls on September 20th or 21st (my father's birthday) is the day when the sun is straight over the Tropic of Capricorn. We usually refer to the equinoxes as the first day of spring or the first day of autumn, but they really refer to those special days when the sun is straight overhead either the Tropic of Cancer or Capricorn. The "zenith-point", or that point where the sun is straight overhead at noon, migrates from the Tropic of Cancer to the Tropic of Capricorn and back once each year. When the sun is directly over the equator, those days are called the solstice. The summer solstice is June 20 (my parents' anniversary) or 21. The winter solstice is December 20 or 21. So there are only two days each year when a spot in the tropics has the sun directly overhead, and I was lucky enough to see one of them here at Kwajalein. Not many people noticed-- you had to look at just the right time! This whole phenomenon of where and when the sun is overhead is all caused because the Earth's axis of rotation (the line that goes through the Earth from the North Pole to the South Pole) is tipped 22.5 degrees from perpendicular to the plane of the Earth's orbit. Without that tilt, the sun would always hit every spot on the Earth the same way each day: the equator would always be the zenith-point, the poles would always have the sun on the horizon, half between would always have the sun highest at 45 degrees. If that were true, we would not have any seasons. But because of that tilt, those of us in the Northern Hemisphere get a higher sun and warmer temperatures in the summer and a lower sun and cooler temperatures in the winter. I really wish I could put a basketball on table as the sun, and stick a pencil through an orange or apple for the Earth and then walk around the basketball with the pencil tipped 22.5 degrees. It would make more sense that way. But you'll just have to do that yourself!

Alan Nelson