Shadows run and hide at Lahaina Noon

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...

Lahaina noon Bernice Pauahi Bishop Museum credit

Lahaina noon Honolulu Shadow AlexS
Alex Dzierba, Jr. figured out a great way to capture "Lahaina (lah-HI-nah) Noon" in Honolulu on May 26. A level standing upright casts no shadow. Lahaina means "cruel sun" in the old Hawaiian language. Credit: Alex Dzierba, Jr.

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?

Honolulu 1816 Louis Choris
Land of enchantment and short shadows. This painting of the port of Honolulu was made in 1816 by Louis Choris


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.

Lahaina Noon Craig Miyamoto
Craig Miyamoto of Honolulu photographs his shadow under the Lahaina Noon sun on May 28, 2011. Miyamoto explains: "I don't have a beach ball on my head. That's my stomach that's protruding out in front and my big ol' butt hanging out the rear." Click photo to go to Craig's blog

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.

Sun seasons 23_5 v2
The sun'??s position at noon on the first days of winter, spring and summer 2013. The sun climbs upward or north starting on the first day of winter and reaches its maximum height above the horizon on the first day of summer. That's when it's closest to the zenith or overhead point. From lowest to highest, the sun's position changes by 47 degrees over the year. Created with Stellarium

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.


Lahaina noon Bernice Pauahi Bishop Museum credit
Everyone gets into the fun in a Lahaina Noon event at the Bernice Pauahi Bishop Museum on May 27, 2011. Click photo to find this year's Lahaina Noon dates for locations in Hawaii. Credit: Bernice Pauahi Bishop Museum

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

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