Imagine how it
would be our idea of the Universe if the sky was always covered by clouds.
At a
certain hour of the morning we would begin to have a diffuse light and after a
while the dark returns. We would not have a reference that will show us the
time during day or night (Sun, Moon or stars) and would not know even the
shadows of the various objects exposed to the sun.
In addition to not know the
charm of some sunsets, sundials would not have been invented.
We would have the same
clues that could provide some element: magnetic materials that orients
according to a certain direction, the pendulums that tend to have a rotary
motion and the relation between the periods of the year where the days are
warmer, with the fact that they are also brighter and longer.
Thanks to different
position of stars at different latitudes, Greek philosophers were able to understand
curvature of the Earth.
Or, as a result, the
observation of the shadow of the Earth projected onto the Moon during a lunar
eclipse.
And, finally, on the
horizon that seems bent, with ships that disappear at a certain distance from
the shore, providing a clear indication of the curvature of the Earth, perhaps
we couldn’t observe it so frequently, since few people would venture offshore
without the help of the stars for orientation.
The French physicist
Gaspard Gustave de Coriolis in 1835 showed that an observer placed on a rotating
frame of reference will have to deal with forces (Coriolis) which depend not
only by angular velocity, but also by direction and body speed.
These fictitious forces
are the basis of cyclonic or anti-cyclonic atmosphere systems formation.
To remember the sense
of rotation of the phenomenon you can use this simple mnemonic scheme (valid
only in the northern hemisphere):
· Anticyclone (high pressure) >>> Clockwise
· Cyclone (Low pressure) >>> Counterclockwise
The effects of this
force are also manifested in atomic physics (in polyatomic molecules whose
molecular motion can be described as a rigid rotation with in addition a
vibration of the parts around the position of equilibrium) and this produces a
particular confusion, in the molecular spectrum, between the rotational and vibration
levels.
Flies (Diptera) and
some moths (Lepidoptera) exploit the Coriolis effect in flight with specialized
appendages and organs that relay information about the angular velocity of
their bodies. Coriolis forces resulting from linear motion of these appendages
are detected within the rotating frame of reference of the insects' bodies. In
the case of flies, their specialized appendages are dumbbell shaped organs
located just behind their wings called halteres. The halteres oscillate in a
plane at the same beat frequency as the main wings so that any body rotation
results in lateral deviation of the haltered from their plane of motion. In
moths, their antennae are responsible for the sensing of Coriolis forces in the
similar manner as with the halters in flies. In both flies and moths, a
collection of mechanic-sensors at the base of the appendage are sensitive to
deviations at the beat frequency, correlating to rotation in the pitch and roll
planes, and at twice the beat frequency, correlating to rotation in the yaw
plane.
The best known example
is the Foucault pendulum.
It is a pendulum free
to swing in any direction for many hours. The Foucault pendulum was first
presented to the public in 1851 and consisted of a sphere of 28 kg suspended on
the dome of the Pantheon in Paris with a 67 m long wire. In an inertial system,
it would track lines always in the same direction, but it didn’t.
At different latitudes
of the Earth, except along the equator, it’s observed that the oscillation
plane of the pendulum slowly rotates. In particular at North and South Pole the
rotation is in a sidereal day: the swing plane remains stationary while the
Earth rotates, in accordance with the law of motion of Newton.
At other latitudes the
oscillation plane rotates with an inversely period proportional to the sine of
the same latitude; at 45° the rotation occurs every 1.4 days, at 30° every 2 days
and so on.
The Coriolis force is
also used in modern gyroscopes that use solid state switches with MEMS
(Micro-Electro-Mechanical Systems).
What is certain is that
without the help of the stars the world-view of the Universe would be definitely
different.
http://en.wikipedia.org/wiki/Horizon
http://en.wikipedia.org/wiki/Spherical_Earth
http://zibalsc.blogspot.fr/2015/05/186-notti-senza-stelle.html
