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Summer solstice 2013: Longest day, best Mercury-spotting

This year's summer solstice, Friday (June 21) at 1:04 a.m. EDT (0504 GMT), also features a rare chance to see Mercury, the planet usually obscured by the sun's glare.

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Land of the Midnight Sun: The sun sets just before 1 a.m. on June 16, 2013, in Anchorage, Alaska. Daylight in Anchorage will peak on Friday, June 21, with 19 hours, 21 minutes on the summer solstice.

Dan Joling / AP

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Don’t miss your chance to see Mercury in the night sky as the northern summer kicks off.

The last few weeks have provided an unusually fine opportunity for stargazers to spot the elusive planet Mercury because the planet has been in close proximity to brilliant Venus, and, earlier, Jupiter as well. However, the opportunity is now coming to a close as Mercury passes its maximum elongation from the sun today (June 20) and begins its rapid drop towards the horizon, passing between Earth and the sun on July 9. 

For the next few nights, Mercury will be a tiny speck just below Venus. It is closest to Venus on July 20, slightly less than two degrees away, but will also be very close one night earlier or later.

The best time to see Mercury is about half an hour after local sunset. Any earlier, and it will be lost in the sky's glare but much later and it will be too low to see. It is most easily spotted with binoculars, but once you've located it, the planet should be relatively easy to see with the naked eye.

This week also marks the summer solstice, on Friday (June 21) at 1:04 a.m. EDT (0504 GMT). The sun will reach its most northern declination, marking the middle of summer in the Northern Hemisphere and the middle of winter in the Southern Hemisphere.

Because the sun is as far north as it can get, it is above the horizon in the Northern Hemisphere as long as is possible. At local noon, it will be as high in the sky as it can get. These two factors combine to create the maximum solar heating possible in the hemisphere.

In the Southern Hemisphere, the opposite is true. The day is as short as it can get, and the sun is low in the northern sky, giving little warmth.

If June 21 is the "midsummer" or "midwinter" day, why is it that we always think of the seasons as beginning on this day? It's because it takes time for the sun to have its effect, causing the seasons to lag behind the sun, making the hottest days of summer (or the coldest days of winter) come a month or two after the solstice.

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