Unequal Equinox
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This coming Friday, those lazy days of summer will soon be nothing but a hazy memory . That's when the autumnal equinox occurs and it marks the beginning of fall.

Hi, I'm Bryan Yeaton and this is The Weather Notebook.

Each year we have two equinoxes, one in March and the other in September. This year the autumnal equinox is at 7 p.m. on September 21st. On that day every spot on the globe will get about 12 hours of sunlight and 12 hours of darkness. If you check your handy sunrise and sunset tables on Friday you may find that the daylight actually lasts a few minutes longer than 12 hours. This extra light is a gift from the atmosphere with some help from some basic geometry. Here's how.

We don't mark sunrise and sunset when the disk of the sun is centered on the horizon, but rather when it first appears at sunrise or disappears at sunset. This adds a couple of minutes to the 12-hour day. Also, when the sun's on the horizon its rays are passing through great gobs of atmosphere. This makes the rays defract, bending and dispersing, so that the sun appears roughly twice as large on the horizon as it does high in the sky. The defraction prolongs sunset even further and adds several more minutes to each day. All of these effects are most prominent at high latitudes where the sun scoots laterally across the sky as it sets.

So, those of you in Barrow, Alaska get to enjoy about 26 bonus minutes of daylight, while listeners in Miami get only about 8 minutes beyond the 12 hours. Consider it nature's compensation for those of you who have to endure those long polar nights near the winter solstice in December and January.

The Weather Notebook is funded by the National Science Foundation with support from Subaru. Thanks today to producer Margaret Landsman and Executive Engineer Sean Doucette.