Length of day on Earth isn't constant

Question: Were days longer or shorter back in dino times?

Answer: Because Earth's rotation is slowing, due largely to the tidal pull of the moon, about 16 seconds are added to the day every million years, or an hour every 200 million years, says University of Illinois professor of aerospace engineering Bruce A. Conway.

As Earth's spin rate lessens, there is a related recession of the moon, by about 4 cm/year (1.5 inches) — a mighty slow distancing — confirmed by using lunar laser retroreflectors placed by the Apollo astronauts in the 1970s.

So dinosaur days were more like 23-hour days, and with a moon that was a little closer than it is now. In a marvelous concordance of astronomical and biological data, a 22-hour pre-dinosaur day has been spotted in the daily growth marks in annual rings of fossilized corals from the Devonian period some 370 million years ago. The corals show that an Earth-year had about 400 days, says Conway, so each day must have been a little shorter, about 22 hours, confirming the astronomical result. Time marches on but not always at the same speed.

Question: True or false: When it snows, there's a little bit of you falling amid the flakes.

Answer: True. Right at this moment water is evaporating from your skin, water vapor is escaping your body with every breath you take, etc. "In fact, you personally put so much water into the air that some of your water molecules almost certainly made it into the snowflakes pictured in this book," says Kenneth Libbrecht in "The Snowflake: Winter's Secret Beauty."

You exhale about a liter of water per day, and most of this gets rained or snowed back down within a week. Sure, maybe that's only about one-quadrillionth of the total in the atmosphere, but that's still about 1,000 molecules per snowflake.

And once the stuff gets really mixed up, it doesn't matter if the flake is falling in Washington state or Siberia.

Footnote on flakes: They do not form from frozen rain at all — that's sleet — but from water vapor in the air condensing directly into solid ice, says Libbrecht. Then as more vapor condenses, the crystal grows picturesquely.

Question: The breathtaking 630-foot-high 44,000-ton concrete and steel St. Louis Gateway Arch has a nifty curve. Is it (a) a semicircle, (b) a parabola, (c) a catenary or (d) part of an ellipse?

Answer: C, but make that an inverted catenary — like the droop of a rope hung loosely from pole to pole, then frozen and turned upside down. If you picked parabola you're in good company, for Galileo himself mistakenly believed this. But it's a more special type of curve. Key fact about an inverted catenary like the Gateway Arch is that every point on it is in compression, making for an extremely strong and stable architectural structure.

---

Send STRANGE questions to brothers Bill and Rich at strangetrue@compuserve.com.