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HOW COME? Tilting at the summer solstice

Is June 21st, the summer solstice, really the longest day of the year? asks a reader.

It seems odd -- and a little unfair -- that the official first day of summer marks the high point of daylight hours for the year. Just when school is finally out and vacations are on, days begin getting shorter again.

Meteorologists usually say that when it comes to weather, the first day of spring is March 1 and that summer starts on June 1. But officially, Northern Hemisphere summer starts in late June.

Days actually begin lengthening after the winter solstice in December, the year's shortest day. But the change is dramatic by late spring. Daylight hours increase from a bit more than 12 in late March to a sunny 15 hours in early June in the northern continental United States.

Why the change? We live on a sharply tilted planet, making both seasons and day length change over the course of a year.

How it works: The Earth is tipped at a 23.5 degree angle on its axis. As it journeys around the sun, our planet's top is leaning away from the sun for half the year, and leaning in for the other half. (See for yourself by tilting a pencil, holding it still, and moving it around a lamp. Watch as the eraser leans toward the light, and then away.)

When the Northern Hemisphere happens to be tipped away from our fiery star, shorter days and colder weather prevail. But on the part of our yearlong trip when we tilt toward the sun, days lengthen and temperatures rise.

In the Northern Hemisphere the summer solstice marks the day in the year when the sun's position is farthest to the north. (Watch where the sun rises and sets on the horizon during spring months, and you'll see that its position is more and more northerly.) The solstice officially occurs this year at 6:51 a.m. EDT on June 21.

The more northerly the sun rises and sets, the longer its path through the sky -- which means more hours of sunlight. After the solstice, things begin to go backward. Day by day, the sun rises and sets a bit more to the south. And daylight hours begin to shorten again, by a few minutes a week.

While the first day of summer is the longest day of the year, it's just a second or so longer than the days just before and after. So on June 21, New York City will enjoy 15 hours, 5 minutes and 39 seconds of daylight. On June 22 the day will be only 2 seconds shorter. But by the end of the month, more than 2.5 minutes of daylight will have been lost as our planet's tilt slowly changes the season.

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