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Double Tides

Q.  Why are there two high tides and two low tides each day?

A.  We have all been taught that the tides are chiefly the result of the gravitational attraction of the moon on the waters of the earth's oceans, but there are actually two rotational systems involved, and two bulges of water.

First, the moon goes around the earth, or so it appears.  Actually, both the earth and moon are rotating around the common center of mass, the center of gravity of the earth-moon system, which is a point inside the earth.  About once a month, a circle around this common center is completed.

There is a slight excess of gravity force on the side of the earth toward the moon, producing a giant bulge in the water on that side.

On the other side, the side away from the moon, the centrifugal force of the earth going around the center of gravity is slightly larger than the gravitational force, so it pushes the water out, away from the moon, making another bulge.

Meanwhile, every day the earth rotates on its axis, so the earth is moving under those two bulges twice a day.  This accounts for the pair of high tides. The pair of low tides occurs because the water has to come from somewhere.

The centrifugal force involved can be visualized by thinking of a pan of water on a phonograph turntable.  There would be a low tide in the middle and a high tide on the outside, because the force tends to push water out from the center of rotation.

The tidal forces are tremendously more complicated than this simplified explanation, because the earth is not entirely covered by water.  There are continents in the way and the bulges in the ocean vary immensely because of these different land masses.


Fly on the Wall

Q.  Why don't flies and ants fall off the ceiling?

A.  In terms of weight in comparison to volume, flies don't weigh much, so very little force is required to keep a fly from falling.  That force is exerted by structures on the tarsi, the tips of the fly's legs.

First, there is a set of claws, which can be seen with a very good hand magnifier.  There are also spongy pads that have ridges like a ruffled potato chip, providing for greater contact or adhesion.  They are cushions, not suction cups.  These body parts allow the fly to stay in place and move with confidence.  When the fly moves, two of its six legs can be out of contact with the surface at any time.

Ants tend to rely more heavily on their claws, which are relatively larger than those of flies, than their pads, which are relatively smaller.

Some are not as good at climbing as you might think.  Ants that live on the ground where the soil is rough might not be able to climb well on smooth surfaces or upside down, the way tree-dwelling ants do.

The ones that get into houses, of course, usually have that ability.  They are very small, so they have even less of a weight problem than flies.


Round Bodies

Q.  Why are heavenly bodies round?

A.  They are not perfectly round, of course.  They only seem spherical, or almost spherical.

Earth, for example, is flattened at the poles, and Jupiter and Saturn, because their extremely dense atmospheres are all we can see, appear even flatter at the poles.

The reason that stars, planets and so forth are even almost spherical, as opposed to a square or some strange shape, has to do with the law of gravitation.

Any bit of matter will attract other units of mass, and as Newton said, the force of this attraction is proportional to the inverse square of the distance between these masses.  It doesn't matter in which direction these masses are located.  A finite number of uniformly distributed uniform particles would thus tend to coalesce into a spherical clump.  Meanwhile, many other forces are at work in the formation of planets and stars.

We assume that at some time well after the big bang, we have a collection of particles that are not uniform and not uniformly distributed, an inhomogeneous cloud of matter, in which all the particles are attracting each other, but the forces of gravity do not totally balance out.  But there is also along the way some kind of perturbing force that sets the thing rotating.  In particular, you are likely to have a neighboring body, so there is gravitational interaction between the two bodies.  There are also tangled questions of electromagnetism, friction, heat, etc.

So you have gradual coalescence, under the force of gravity, and things beginning to spin, because of inhomogeneities and outside forces.  The result is a roughly but not perfectly spherical rotating body.  The shape is going to be determined by how fast the thing is spinning.  The faster it spins, the more oblate it's going to be, and it depends on the density of matter in the body, too.

Assuming a perfectly spherical billiard ball, for example, it will retain its spherical shape closely, but a rotating water balloon would become quite oblate, bulging around the equator.  In fact, with a heavenly body, you are liable to have so much matter and so high a rate of rotation that matter around the equator will spin off, leaving the body without its "spare tire." The spare tire can be dispersed, or under some circumstances can form a roundish satellite, by a similar process.


Giraffe Hypertension

Q.  Is the blood pressure of a giraffe extremely high to get blood to the head? Do giraffes get hardening of the arteries and strokes?

A.  In an adult giraffe, the systolic blood pressure, the pressure when the heart contracts to pump blood through the body, is about 200 millimeters of mercury at the level of the heart, about twice that of an adult human being.

During exercise, the blood pressure at the level of the giraffe's feet might be double what it is at heart level because of gravity.  At the head, it is comparable to the blood pressure of a human being.

Stroke and hardening of the arteries can occur in most mammal species, but is not a notable problem with giraffes.  The giraffe's circulatory system has evolved with special adaptations for both high and low pressure.

For example, because the blood pressure in the lower part of the body is so high, the artery walls in the legs are very thick, much thicker than in the neck, to prevent edema, or leakage of fluid out of the vessels.  The skin and tissue under the skin on the limbs are also very tight to prevent leaks.

In the large veins of the neck, there are valves to restrict the flow of blood so that it does not flow backward and pool in the head when the giraffe puts its head down to drink or eat.


Mammoth Meat

Q.  I heard about explorers eating frozen mammoth.  What was it like?

A.  Everyone has heard the rumor, but probably only dogs, or passing wolves, ever enjoyed a modern feast of mammoth meat.  It looks like horse meat but quickly rots when it thaws.

Several mammoths were found in permafrost, preserved since the last glacial period.  The most famous was found in 1901 near the banks of the Berezovka River in Siberia.  An Academy of Science expedition with dogsleds was quickly sent.  The expedition found a nearly complete carcass, but wolves and dogs had left the skull almost bare.

The meat was dark red, suggesting horse meat, and marbled with fat.  The dogs ate it avidly; the men could not quite steel themselves to try it too.  Members of the expedition said the stench was like that of a badly kept stable blended with that of offal.  Members of later trips said scientists never banqueted on or even sampled the meat.

Books that describe a banquet usually lack a place and date.  The rumor seems to have arisen from Siberian natives' superstitions warning against eating the meat, though the Yakut fed it to their dogs in time of famine.