Winter solstice 2009 falls Monday, marking the shortest day in the year for the Northern Hemisphere.
Ah, another winter solstice come and gone.
At 5:47 p.m. Greenwich Mean Time (that's 12:47 p.m. Eastern Standard Time) Monday, the Northern Hemisphere marked the mid-point of another year, as measured by the sun's highest position each day above the horizon. On the winter solstice, the sun's maximum height above the horizon is at its lowest point in the sky for the year. It's the day with the fewest hours of sunlight this year.
Yes, this is showing a Northern Hemisphere bias. South of the equator, the day marks the most hours of sunlight of the year. So enjoy the austral summer, those of you below the equator. The rest of us? We'll be rooting for longer, warmer days.
The event, of course, owes its existence to the tilt of Earth's axis as the third rock from the Sun revolves around its parent star. Imagine that someone poked a suitably sharpened, oversized No. 2 pencil through the planet from the North Pole to the South Pole. Today, the eraser is pointed away from the sun by the largest angle of the year – some 23 degrees, 26 minutes off vertical, with vertical measured against the plane of the Earth's orbit.
Six months from now, that eraser will point toward the sun by the same angle, exposing the Northern Hemisphere to the longest period of sunlight in a day.
But as the hours of daylight grow between now and June 21, 2010, don't look for the increase to grow by equal amounts of time on either side of noon each day. Sunrises will still come later each day into next month, even as sunsets continue to come later in the day, as they have since early this month.
In Boston, for instance the latest sunrises will occur between Jan. 2 and Jan. 5, 2010. But the earliest sunsets came between Dec. 3 and Dec. 14, 2009. You can check similar numbers for your location at the US Naval Observatory's website.