The Longitude Clue Why does it only work on the equinox?

What we observed:

a) In December, the line separating day from night was angled; by the March equinox the line was straight up and parallel with Earth's axis; b) In December, the South Pole was in full sunlight but the North Pole was in darkness; by the March equinox, both Poles have an equal amount of sunlight; c) In December the full Southern Hemisphere was receiving sunlight, but only a portion of the Northern Hemisphere (below the polar region) was receiving sunlight; by the March equinox, both hemispheres were receiving equal amounts of sunlight.

What we wondered and answers:

1) What causes the differences in the angle of the daylight line and the differences in which area of Earth receives more sunlight? Answer: The Earth's axis is tilted. As Earth revolves around the Sun, either the Northern Hemisphere or Southern Hemisphere will be tipped toward the sunlight, depending on the time of year. On the equinoxes, however, things are different because the Earth's axis is not tilted toward or away from the Sun, so neither hemisphere gets more sunlight than the other. Therefore, on the equinoxes, the sunlight line in the images is straight up and parallel to the Earth's axis. Read more and look at the earth tilt diagram in Modeling the Seasons >> 2) Why does the straight up position of the sunlight line on the equinox make the Longitude Clue work? December 21 In December, the sunlight line is angled so sunlight arrives at different longitude locations at the same time. Here, sunlight is arriving in the NE United States and also in Central America at the same time. March 21 On the equinox, the sunlight line is straight up and parallel to the Earth's axis. As a result, sunlight arrives at the same time all along a single longitude line, not at several different longitudes. Answer: When the sun light line runs straight up and down, sunlight happens at the same time all along a single longitude line -- from north to south. Because of this uniform arrival of sunrise along one longitude line, calculation of longitude based on the hours and minutes between sunrise at Greenwich (the zero degree longitude line) and at another location is uniform and reliable. On days that are not the equinox, however, because of the tilt of the Earth, the sunlight line is not straight up. Therefore, sunlight does not arrive at the same time all along one longitude line like it would on the equinox. Just look at the Dec 21 image, and see how sunlight is arriving in the NE United States and also in Central America at the same time, even though these two locations are 10-15 longitude degrees apart from each other. Because of this, comparing sunrise times to Greenwich to estimate longitude does not work when it is not the equinox.

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