Pondering the northern Equinox

It hit nine below zero this morning in the woods of east Anchorage, but the birds didn’t mind.

A raven winged over the park outside my backyard with a melodic koo-ack. The local chickadee flock fluttered from bare tree to bare tree, chattering, darting, ever in motion. One lone male pulled out from the group to perch atop a birch. Over and over, he called three descending notes. I-am-here, this-my-tree, I-will-fight, I-am-here.

It’s March 20 in southern Alaska, 61 degrees north latitude, a time when hard winter totters on the brink of spring melt and the explosion of growth that will transform the forest into green summer jungle. Any day now, they say, the cold will break and temperature will climb. Mucky puddles will inundate the black ice. The snow will rot and collapse. The willows will bud. Robins and geese will come back.

Vernal Equinox arrived in Anchorage only this minute, 3:07 p.m. local daylight time, marking the twice-yearly moment when Sun shifts its glare from one hemisphere to the other. In Anchorage, it was 25 degrees, cloudy, slight wind in the bare trees.

Many people think the equinox evenly distributes 12 hours of daylight and 12 hours of night everywhere on Earth. It’s one of those natural factoids repeated by dictionaries and newspapers. Yet it’s simply not true, according to the U.S. Naval Observatory.

Vernal Equinox — Latin for spring’s equal night — is an instant hard to fathom and probably impossible to directly perceive. It’s when the exact center of the Sun’s disk crosses the Earth’s equator, when the Sun stands directly overhead. This tick of the celestial clock struck just after midnight in London, exactly 12:07 a.m. UTC on March 21, 2007. For the entire day centered around this moment, the Sun’s belly will peek above the horizon. But that’s not the same as sunrise and sunset.

Think of the Sun as a big round ball and not some geometric point. Sunrise occurs when the leading edge of this disk slips above the horizon. Sunset returns when the last of the Sun’s trailing edge passes from view. Add in refraction by the atmosphere, and calculating the length of the local day becomes a complex exercise for the astronomers.

In Anchorage, 12-hour day overcame 12-hour night on March 18. The day containing Vernal Equinox offered Anchorage 12 hours and 11 minutes of official daylight. Add just one more notch on the calendar, March 21, and the amount of possible sunshine will leap to 12:19, a jump of seven more minutes.

In the North, day vaults forward with an urgent stride. A pair of bald eagles soared over my backyard last week at midday. Redpoles lined my fence on Anchorage’s first 12-hour day in almost six months. Someone claimed they heard robins, but no one has seen any returning ducks.

By mid-May, the sky will no longer grow dark. By mid-June, you would have to skip sleep to lose sight of the Sun. Stars are a gift brought by August, along with cool rain and ripe blueberries.