Sunday, March 27, 2011

Where and When

Today we stopped by one of our favorite farm stands -- actually, it may now be the only one remaining in our town. Recalling that our town's Easter "sunrise" service will be held at the farm, we contemplated walking there. The service is fairly early in the morning but not right at sunrise, and though it is often chilly, the walk might do us some good. The geographic question quickly occurred to us: what time will sunrise be on Easter this year? Easter is a "movable" feast, so the first step was to confirm that it is on April 24 this year. This is nearly as late as it can be, and the latest is has been since 1943. (See the intricate details at

I found the site while trying to address the specific question of sunrise on April 24. I was pleased to see that I could find the answer based on Brockton, Massachusetts, the nearest city and one that actually used to be part of Bridgewater. Of course location is relevant, because the tilt of the earth and its rotation on its axis mean that latitude and longitude determine the time of sunrise on any given date. In the scheme of things, a distance of a few miles will not make a substantial difference in the timing of sunrise. Today, for example, the solar day has been (I'm writing this right at sundown) only 12 seconds longer in Boston than in Brockton. On Easter, therefore, we can be fairly confident that the sunrise for the purpose of our walk to the morning service will be at 5:50 -- plenty of time for a 1.5-mile walk to celebrate Easter with our friends and neighbors (especially since we have walked there and back for ice cream!)

A geographer should understand calendars and clocks better than I do; in our house, I leave any complicated discussions of dates to my favorite librarian. But I love being able to play around with the interface between time and space, and was delighted to see that the same site that showed me the available walking time before the Easter service also has a fully customizable version of the classic Geochron that my department keeps in our hallway. The Day and Night page allows a user to choose a location and a local time and date at that location, in order to display the planet as it would appear (if it were rolled out like a pie crust) at that time. The circle of illumination is shown, with dark areas "lit" according to mosaics of nighttime satellite images and daylight areas shaded according to biomes. Unlike other maps of this kind I have seen, it even shows zones of twilight. At any given moment, particular band at the edge of the circle of illumination is experiencing sunrise or sunset; the closer to the equator, the less time a particular point will spend in that band. I first realized this ("the sun sets quickly in the tropics") on a sunset "booze cruise" deep in the Amazon, at 9 degrees south latitude.

Given the opportunity to see how the earth and sun and moon were aligned at any point in time, I of course chose the most significant moment -- and one whose hour and minute I could remember: the moment I became a father. In that moment, I was not thinking about the subsolar point, the position of the moon, or the proximity to twilight (still a couple hours off). But all of that can be discerned below. When our girl was born, moon was above Madagascar, the sun above Hawaii, and we were spinning toward our first night as parents.

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