Summer Solstice: Why the days will shorten from here on out

Welcome, summer! The season officially begins Wednesday, the longest day of the year in the Northern Hemisphere.

The summer solstice of 2012 will occur at 7:09 p.m. EDT, the instant when the sun climbs to its farthest point north of the equator. For those in the Southern Hemisphere, this solstice is the astronomical marker for the beginning of winter.

Though the actual length of the 24-hour day doesn't change, daylight will last a fraction of a second longer on June 20 than on June 19 or 21. From here on out, the days will get just a bit shorter each day until the winter solstice in December.

The reason for these fluctuations has to do with the tilt of Earth's axis. Our planet's spin is skewed 23.5 degrees. At the summer solstice, the North Pole is tilted as much as possible toward the sun. The winter solstice occurs when the planet's northernmost point is directed as far as possible away from the sun. That means that those of us north of the equator get more rays in the summer months. [Gallery: Stunning Summer Solstice Photos]

The sun is always above the horizon in summertime at the North Pole, reaching its apex at the solstice. This perpetual daylight continues until the autumn equinox in September, when the sun finally sets. A months-long twilight follows until October, when the North Pole experiences round-the-clock winter darkness. 

It might seem logical that during summer's hottest days, the Earth is closest to the sun. However, when it's summer in the Northern Hemisphere, our planet is actually swinging far away from our nearest star. Earth's orbit is elliptical rather than a perfect circle. On July 4 at 10 p.m. EDT, our planet will reach its "aphelion," or the point in its orbit where it is farthest from the sun. That's a distance of 94,505,849 miles (152,092,421 kilometers). The longer days of summer still keep the temperatures plenty hot, though.

This year's solstice is expected to be a scorcher for some Americans. A heat wave in the Northeast may break records on Wednesday (June 20), according to the National Weather Service. Heat advisories are in place for Wednesday in Baltimore, Md., Washington, D.C., and parts of northern Virginia, southeastern Pennsylvania and New York and New Jersey. Temperatures may top out around 100 degrees Fahrenheit in these regions.

Nevertheless, the solstice doesn't usually correspond with the hottest days of the year, which are much more likely in July and August. That's because the oceans take time to heat up, creating a lag between peak sunshine and peak summer heat.

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