Air Date: Week of March 17, 2000

This week, facts about the vernal equinox, including why we have more than 12 hours of sun on the day on which is supposed to be split evenly between daylight and darkness.

Transcript

CURWOOD: In Latin it's called equal night, and for those of us in the northern hemisphere the equinox that occurs toward the end of March signals the start of spring. Scientists tell us that the spring equinox is the moment when the Earth's axis lies exactly at a right angle to the sun. We lay people may better understand it as the time of year when the length of daylight and darkness is virtually the same, hence "equal night." In the northern hemisphere, the vernal equinox in March marks the point when the days start growing longer than the night, meaning more sun and more warmth for plants to grow. The autumnal equinox in September signals the opposite point in the solar cycle, when darkness starts eclipsing light, and we make the inexorable turn toward winter. If the Earth had no atmosphere, there would be exactly 12 hours of sunlight on the equinox on every point on the globe except for the poles. But because the atmosphere bends sunlight like a lens, we see slightly more than 12 hours of sunlight on the day of the equinox. As the sun rises and sets, there are a few minutes when it's visible, even though it's actually beneath the horizon. Nature's little bonus. And by the way, thanks to the light-bending powers of the atmosphere, on the equinox observers at both the South and North Poles can see the sun for the entire day circling around, just above the horizon. And for this week, that's the Living on Earth Almanac.