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DIANA: This episode is supported by Squarespace.
Hi.
I'm Diana.
You're watching "Physics Girl," and I
am relieved to have gotten you this footage, courtesy
of my momma.
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This rare solar phenomenon was filmed on Kauai
in Hawaii, where my childhood home is.
This phenomenon happens twice a year in Hawaii,
if it's not cloudy, during solar noon, which is of course when
the sun is highest in the sky, but it doesn't usually
fall at noon.
Photos and videos taken during this time
seem eerie, almost Photoshopped because there are no shadows.
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This is called the sub-solar point, or lahaina noon
in Hawaii, and it happens when the sun is directly overhead.
But here's the sad thing-- it doesn't
happen for any of the other 49 states in the US.
Sorry.
And yet it happens at some point for 80% of the continent
of Africa because Africa is--
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Because 80% of Africa is between the Tropic of Cancer
and the Tropic of Capricorn.
Ever notice how the degrees of the Tropic of Cancer
and the Tropic of Capricorn match
the tilt angle of the Earth?
Its almost like they were defined that way.
Yeah.
The Tropic of Cancer is where the sun-Earth line meets Earth
during the summer solstice, which
is when the sun is directly overhead
at the Tropic of Cancer.
Directly overhead means that line makes exactly
a right angle with that spot.
So in this setup, the sun would not be directly overhead
for the North or South Pole.
In fact, the sun can never be directly overhead for anything
north of the Tropic of Cancer or anything
south of the Tropic of Capricorn because the solstice is already
when you get the maximum tilt of the northern hemisphere
toward the sun.
And it has this weird property-- the Tropic of Cancer
and the Tropic of Capricorn are the only places on the planet
where this can happen--
when it's the longest day of the year and you
can have the sun directly overhead.
Here's a fun aside to think about.
Why doesn't the longest day of the year
match the hottest day of the year?
Thanks to Alex in Hawaii who snail-mailed me that question.
And why doesn't lahaina noon match the longest day
of the year in most places?
Carry on.
OK, so 23.4 degrees north gets lahaina noon
during the summer solstice, and 23.4 degrees south gets
lahaina noon exactly six months later on the winter solstice.
On the dates exactly between the solstices,
lahaina noon happens at the equator.
So the equator gets lahaina noon twice a year.
And between those dates, lahaina noon
happens somewhere between the Tropic of Cancer
and the Tropic of Capricorn, and it happens twice a year.
So if you're planning your Hawaii research trip,
try to do it sometime near the end of May
or the middle of July, and you can maybe see lahaina noon
if it's not cloudy.
Of course, you could also go to Mexico,
parts of Central America, South America, Southeast Asia,
Africa, Yemen, Philippines, Australia.
But hey, thanks for watching.
Happy physicsing.
And if you ever see lahaina noon, tweet me photos of it
@thephysicsgirl.
Thanks to Squarespace for supporting this episode.
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