Saturday, August 16, 2008

Humming in the Morning

Its 6:21 AM, the sky is opalescent, and its 63 degrees. Despite clear skies, its windy--15 mph with stiff gusts probably up to 30. So what is that hummingbird doing trying to get a sip of nectar from a wildly waving Agastache?

Here in Albuquerque we're at 35 N and 106 W. Its the 35 degrees north latitude that is making the difference. We're about 35 days from the autumnal equinox and the sun is almost 14 degrees north of the equator. That means we have 13 hours and 25 minutes of daylight today.

More importantly to our friendly hummingbird outside my window, we have 10:35 of darkness. That's down from 14:31 or more than 27% since the solstice. And that's 10:35 hours without food for a metabolism that's running 100 miles per hour.

Now of that 10+ hours of darkness, not all of it is really dark. Geoclock, the little program that does such a splendid job of charting and mapping the movement of the sun and move over the Earth, lets me set a variable for twilight. Why, do you ask, is twilight a variable?

There are three definitions of twilight: civil, nautical, and astronomical. Civil twilight is defined as the sun being less than 6 degrees below the horizon. Nautical twilight has the sun less than 12 degrees down and astronomical twilight has the sun less than 18 degrees beneath the horizon. That means there's still a good bit of light around although the sun might be well below the horizon.

The chart above shows civil twilight as a function of date and latitude. On August 16 at 35 N we have almost 15 minutes of twilight. That's an extra 5% of flight time each day for our hungry hummer, who has spent the night in a torpor to reduce metabolism.

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