Solstice Shadows
He certainly looks unstoppable. The NBA All-Star weighs 315 lbs and stands 7 feet 1 inch tall. But later today, around lunchtime, Shaq's shadow will measure barely 1 foot 4 inches. It's been shrinking all week.
Good news for the rest of the league?
No. It's just the summer solstice.
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On June 21st--the first day of northern summer, a.k.a. "the summer solstice"--the noontime Sun soars high in the sky. Observers north of the tropics will see it reach its highest elevation of the year. Everyone's shadows will be short--not just Shaq's.
This happens because our planet's spin axis is tilted 23.5 degrees. On June 21st, the north pole is tipped toward the Sun, and mid-day shadows north of the tropics are short. The situation is reversed in the southern hemisphere. The south pole is tipped away from the Sun, so mid-day shadows south of the tropics are longer than usual. If Shaq flew to Buenos Aires for lunch today, his shadow would stretch an impressive 11 feet 4 inches--16 inches taller than a regulation basketball hoop.
Scary.
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Above: The ratio between your height and the length of your shadow in selected cities at mid-day on the two solstices: June 21st and Dec. 22nd. For example, if you want to know how long your shadow would be on the first day of summer in New York, multiply your height by 0.31 -- the ratio listed for New York under June 21.
The tilt of Earth's spin axis not only causes shadows to expand and shrink. It's also the reason for seasons. In June, the Sun is high in the sky. Days are long, it gets hot, and we call it summer. In December, the Sun hangs low. Days are brief and cold, and we call that winter. (Seasons are reversed south of the equator.)
No one knows why Earth's axis is tipped by 23.5 degrees. Probably it's just good luck. Some astronomers believe that, about 5 billion years ago when Earth was very young, our planet was struck by a Mars-sized protoworld. The collision created the Moon and, perhaps, knocked Earth a bit on its side.
If Earth had no tilt we would experience no seasons: no crisp winter days, no autumn leaves, no spring cleaning, no summer vacation. Too much tilt would cause seasons to be extreme. The planet Uranus, for example, is tilted 97 degrees. If Earth were like that, most of our planet would experience nights lasting weeks or months (depending on latitude). The north pole would be hotter than the Sahara desert during summer, while tropical oceans would freeze. And, incidentally, Shaq's shadow in Los Angeles on June 21st would be 10 feet 6 inches long--all day long!
Mild seasons, 24 hour days, and short summers--it's much better this way.
So step outside today around lunchtime. Feel the warmth. Glance around. Most northerners won't notice many shadows at all. They are hiding underneath cars, beneath trees and underfoot. If you can find a tree-lined road, look down it. The asphalt, normally criss-crossed with the shadows of trees, will be bare and hot--all sunlight and no shade.
Look again in six months and you'll see a big difference, something like this: Shaq's summer shadow: 1 foot 4 inches. Shaq's winter shadow: 11 feet 4 inches. That's unstoppable proof that our planet is tilted.
Seasons: The Movie -- (NASAKids.com) a too-cool movie explaining the seasons.
The Seasonal Merry-Go-Round -- (Windows to the Universe) the basic geometry of seasons.
Earth's Seasons -- (US Naval Observatory) dates and times of equinoxes, solstices, perihelia, and aphelia
Seasons of the Year -- (NASA/GSFC) a nice introduction to terrestrial seasons.
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