Notice anything odd about this photo? It was taken earlier this week in Honolulu, Hawaii. The sun is out - you can see that - but something's missing. Look around the bottom of the level ... hmm, what's happened to its shadow?
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In Hawaii they have a special name for this time of hidden shadows - Lahaina Noon - and it happens every year in late May and mid-July. Shadowless conditions only occur when the sun is directly overhead, and Hawaii is the only U.S. state where you can experience it. Vertical objects that make contact with the ground cast none at all which is why people in Honolulu walk around staring at flagpoles on May 26.
The rest of us in 49 remaining states never get to see the sun pass overhead because we're all too far north. Even Key West, Florida. Only in the tropics does the sun ever sit directly on top of your head during the noon hour. Just the thought of it makes me sweat.
Specifically, any place with a latitude between 23.5 degrees north and 23.5 degrees south will see the sun beam from the zenith twice a year - the first time when it's moving northward (higher) in the sky during spring and a second time when it's dropping southward (lower) after the summer solstice.
This zone of latitude lies between the Tropics of Cancer and Capricorn . Some of you might remember those lines from that old grade school globe of the Earth. Besides making good book titles, they mark the location around the globe where the sun is directly overhead at noon on or around the winter solstice (Capricorn) and summer solstice (Cancer). Any city within that band will see the sun cross the zenith twice a year.
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23.5 degrees is a familiar number, right? That's the angle at which Earth's axis is tilted. Seen from our tipped planet, the sun travels from 23.5 degrees south of the celestial equator (an extension of the real equator into space) on the first day of winter to 23.5 north of the equator on the first day of summer. Add it up and you get 47 degrees or about five fists held at arm's length against the sky. That's the full range of the sun's up and down movement in the sky over the course of a year.
Here in Duluth, Minn. at latitude 47 degrees north the sun never gets any closer than 26.5 degrees (47 minus 23.5 = 26.5) from the overhead point. That's why power poles hold no allure in my town on June 21. Things are better in Key West where the latitude is 24.5 degrees. Let's do the math again: 24.5 minus 23.5 = 1 degree. That's darn close to the zenith with only a thin rim of shadow coating the edges of an old man's cane.
If you live outside the tropics, you can determine how close the sun gets to your zenith by simply finding the difference between your latitude and 23.5. This works for all latitudes outside the tropics. For Honolulu, cozily situated at 21 degrees north latitude, Lahaina Noon will occur again on July 15. Cities farther north or south of the Honolulu experience the overhead sun a few days later in May and earlier in July.
At the north and south poles, even on the summer solstice, the sun never gets any closer than 66.5 degrees from overhead. Anyone wanting to celebrate Polar Noon would have to be content with the sun only 23.5 degrees above the horizon. Ah, but there's compensation for this pitiful altitude - the sun's up all night 6 months in a row.
Coming tomorrow: How to find asteroid 1998 QE2 when it passes Earth this weekend
