Q. & A.s ‑‑‑ SCI.
& TECH. – 11
296 Q. What
causes tides? Why is their periodicity close to 12 hours?
The Moon's
--and to a lesser extent, the Sun's-- gravitational attraction at any spot on
Earth depend on the distance between that spot and the Moon --or Sun, as the
case may be. At any moment in time, the attraction on the surface of the Earth
closest to the Moon is larger than that over the center of the Earth, and that
on the surface of the Earth farthest farthest from the Moon is smaller. To a
first approximation, the attraction on the solid continents, because they're a
mostly rigid body, is the attraction on the center of mass of the Earth, which
is close to the center of the Earth. The consequent difference in the
attraction on the seas and the land are the cause of tides. At the
aforementioned points in the Earth-Moon axis, the attraction only has a
vertical component, pulling water away from the Earth (up). It is this symmetry
that causes the periodicity of tides to be close to 12 hours rather than 24. At
all other places, there are horizontal components that contribute to currents.
297Q. Why are
tides largest during New Moon and Full Moon?
During this
time, the Sun, the Moon and the Earth are collinear, and thus the solar tide is
co-aligned with the lunar tide and their effects add up. However, the
horizontal components are maximal during New Moon (Sun and Moon on the same
side of Earth) and minimal during Full Moon (Sun and Moon on opposite sides of
Earth).
298Q. Why do we
always see the same side of the Moon?
Because the
Moon's spinning has slowed down because of the loss of energy due to tidal
forces on the Earth. More specifically, tidal movement causes friction against
the rotating mass of the Moon, and slows it down. that it is only the tidal
lag, that causes an asymmetry between the bulges on both sides of Earth/Moon,
which causes the friction and thus the slowing. But the Moon has not stopped
spinning! It rotates with a period equal to the period of its orbit around the
Earth, such that the same side will always face the Earth.
299Q. Why has
the Moon stopped spinning before the Earth has?
The Earth,
with its larger mass, has much more inertia and takes a lot more energy loss to
stop spinning.
300Q. Is the
length of days and months changing? Why?
Tidal friction slows the Earth's
rotation, but the angular momentum of the Earth-Moon system remains
constant. Consequently, the Moon is slowly receding from the Earth, with
the result that the month and the day are both getting longer. Extending this
relationship back into the past, both periods must have been significantly
shorter hundreds of millions of years ago, and this hypothesis is confirmed by
measuring the diurnal and tide-related growth rings of fossil corals.
301Q. Does the
Moon have seasons?
Because the Moon's spin axis
is inclined only 1 1/2 from the normal to the ecliptic,
the Moon has no seasons. Sunlight is always nearly horizontal at the
lunar poles, resulting in permanently cold and dark environments.
302Q. Which
gravitational attraction is greater on Earth: that of the Sun or that of the
Moon?
At the surface of the Earth the
gravitational force due to the Sun is about 200 times stronger than that of the
Moon, but the gravitational gradient (tidal force) due to the Moon is
about 2.2 times greater than that of the Sun.
303Q. Does the
Earth translate around the Moon then?
The two bodies orbit each other
about their centre of mass--called the barycentre--a point inside the Earth
about 4,700 kilometres from its centre.
304Q. What is
the Coriolis force?
It is the
apparent force eastward or westwards (i.e. respectively, in favor or opposite
to the direction of rotation of the Earth) that an object traveling with any
non-zero N-S component of motion experiences as a result of the rotation of the
frame of reference, the Earth. Objects beginning motion are also subject to
eastward rotation, but the tangential velocity of a point on the Earth is a
function of latitude (the velocity is essentially zero at the poles and it
attains a maximum value at the Equator). Thus, if a cannon were fired northward
from a point on the Equator, the projectile would land to the east of its due
north path. This variation would occur because the projectile was moving
eastward faster at the Equator than was its target farther north. Similarly, if
the weapon were fired toward the Equator from the North Pole, the projectile
would again land to the right of its true path. In this case, the target area
would have moved eastward before the shell reached it because of its greater
eastward velocity. An exactly similar displacement occurs if the projectile is
fired in any direction.
The
Coriolis deflection is therefore related to the motion of the object, the
motion of the Earth, and the latitude. For this reason, the magnitude of the
effect is given by 2^(vw) sin L , in which v is the velocity of the object, w
is the angular velocity of the Earth, and L is the latitude. At the equator, L
is zero and thus there is no Coriolis force.
305Q. What
causes ocean currents?
The general
circulation of the oceans consists primarily of the wind-driven currents.
These, however, are superimposed on the much more sluggish circulation driven
by horizontal differences in temperature and salinity--namely, the thermohaline
circulation.
306Q. Why do
cyclonic and anticyclonic sinks/sources act in a whirlwind set of directions?
(i.e. clockwise & anticlockwise)
Because of
the Coriolis force, every current (air or water) with any N-S component will
turn right in the N hemisphere and L in the S hemisphere.
307Q. Does ice
in the sea have any salt?
There are
two types of ice in the seas: sea ice, which is ice formed by the freezing of
seawater, and ice that has come from land, such as icebergs and ice islands.
From an
initial stage of so-called frazil crystals (floating needles and platelets) and
sludge composed of them, sea ice grows to a compact aggregate of crystals of
pure ice with pockets of seawater entrapped between them. Because of this
composition, the salinity of sea ice is lower than that of the seawater from
which it has grown. The initial sea-ice salinity may vary between 2 and 20
parts per thousand; the more rapid the freezing, the saltier the ice, as brine
can be trapped in cavities in the forming ice and become isolated from the
seawater.
After sea
ice has formed, a process of salt removal by drainage of part of the enclosed
brine sets in, because the cells in which it is contained are not completely
isolated. Old ice has very low salinity, on the order of 1 part per thousand or
less.
308Q. How thick
is Antarctica's ice?
In the
Antarctic, perennial sea ice is found only in the Weddell Sea and a narrow
strip around the continent. Most of the Antarctic sea ice is seasonal and
reaches a thickness of about 1.5 metres by the end of October.
309Q. How old
are the oceans?
There is
little information on the early history of the Earth's waters. However, fossils
dated from the Precambrian some 3.3 billion years ago show that bacteria and
cyanobacteria (blue-green algae) existed, indicating the presence of water
during this period. Carbonate sedimentary rocks, obviously laid down in an
aquatic environment, have been dated to 1 billion years ago.
310Q. Do Northern and Southern summers
have different durations due to the differing distances to the Sun?
Planets move more slowly at aphelion than they do at perihelion (see Kepler's 2nd Law of planetary motion) and, so, seasons occurring near aphelion last longer. Northern summer on Earth is ~5 days longer than northern winter for the same reason. It's a difference that goes largely unnoticed on our planet, but it's unmistakable on Mars.
Planets move more slowly at aphelion than they do at perihelion (see Kepler's 2nd Law of planetary motion) and, so, seasons occurring near aphelion last longer. Northern summer on Earth is ~5 days longer than northern winter for the same reason. It's a difference that goes largely unnoticed on our planet, but it's unmistakable on Mars.
311Q. Given
that the Earth's orbit is elliptical, is the Earth warmer when it's closer to
the Sun?
"Averaged over the globe, sunlight falling on Earth in January [at perihelion] is about 7% more intense than it is in July [at aphelion],". "The fact that the northern hemisphere of Earth has more land, while the southern hemisphere has more water, tends to moderate the impact of differences in sunlight between perihelion and aphelion." January 4, 2001 -- This morning at 5 o'clock Eastern Standard time (0900 UT) Earth made its annual closest approach to the Sun -- an event astronomers call perihelion. Northerners shouldn't expect any relief from the cold, however. Although sunlight falling on Earth will be slightly more intense today than it is in July, winter will continue unabated. "Seasonal weather patterns are shaped primarily by the 23.5-degree tilt of our planet's spin axis, not by Earth's elliptical orbit," explains George Lebo, a professor of astronomy at the University of Florida. "During northern winter the north pole is tilted away from the Sun. Days are short and that makes it cold. The fact that we're a little closer to the Sun in January doesn't make much difference.
"Averaged over the globe, sunlight falling on Earth in January [at perihelion] is about 7% more intense than it is in July [at aphelion],". "The fact that the northern hemisphere of Earth has more land, while the southern hemisphere has more water, tends to moderate the impact of differences in sunlight between perihelion and aphelion." January 4, 2001 -- This morning at 5 o'clock Eastern Standard time (0900 UT) Earth made its annual closest approach to the Sun -- an event astronomers call perihelion. Northerners shouldn't expect any relief from the cold, however. Although sunlight falling on Earth will be slightly more intense today than it is in July, winter will continue unabated. "Seasonal weather patterns are shaped primarily by the 23.5-degree tilt of our planet's spin axis, not by Earth's elliptical orbit," explains George Lebo, a professor of astronomy at the University of Florida. "During northern winter the north pole is tilted away from the Sun. Days are short and that makes it cold. The fact that we're a little closer to the Sun in January doesn't make much difference.
312Q. Why are
planets' orbits elliptical?
Because of conservation of energy under the attraction of the Sun, which varies as the inverse square of distance.
Because of conservation of energy under the attraction of the Sun, which varies as the inverse square of distance.
313Q. Why do
the orbits sweep equal areas in equal times?
Because of conservation of angular momentum.
Because of conservation of angular momentum.
314Q. Why is
Kepler's third law, which states that the ratio of the square of a planet's
period (T^2) to the cube of the semi-major axis of its orbit (a^3) is a
constant, true for all planets of all planetary systems?
It's a consequence of gravitation, and the balance between gravitation and centrifugal "force". It also relates the period to the mass of the parent body, an equation that is used to calculate the mass of planets using the motion of satellites .
It's a consequence of gravitation, and the balance between gravitation and centrifugal "force". It also relates the period to the mass of the parent body, an equation that is used to calculate the mass of planets using the motion of satellites .
315Q. Are all orbits almost circular?
The orbits of comets are very elongated; some are long ellipses, some are nearly parabolic (see parabola), and some may be hyperbolic. Natural satellites that are close to their primaries tend to have nearly circular orbits in the same plane as that of the planet.s equator, while more distant satellites may have quite eccentric orbits with large inclinations to the planet.s equatorial plane. Because of the moon.s proximity to the earth and its large relative mass, the earth-moon system is sometimes considered a double planet. It is the center of the earth-moon system, rather than the center of the earth itself, that describes an elliptical orbit around the sun in accordance with Kepler.s laws. All of the planets and most of the satellites in the solar system move in the same direction in their orbits, counterclockwise as viewed from the north celestial pole; some satellites, probably captured asteroids, have retrograde motion, i.e., they revolve in a clockwise direction.
The orbits of comets are very elongated; some are long ellipses, some are nearly parabolic (see parabola), and some may be hyperbolic. Natural satellites that are close to their primaries tend to have nearly circular orbits in the same plane as that of the planet.s equator, while more distant satellites may have quite eccentric orbits with large inclinations to the planet.s equatorial plane. Because of the moon.s proximity to the earth and its large relative mass, the earth-moon system is sometimes considered a double planet. It is the center of the earth-moon system, rather than the center of the earth itself, that describes an elliptical orbit around the sun in accordance with Kepler.s laws. All of the planets and most of the satellites in the solar system move in the same direction in their orbits, counterclockwise as viewed from the north celestial pole; some satellites, probably captured asteroids, have retrograde motion, i.e., they revolve in a clockwise direction.
316Q. Do all
planets revolve around the Sun in the same sense?
Yes.
Yes.
317Q. Do all planets
rotate in the same direction?
All except for Venus, and, strictly speaking, Pluto and Uranus, since their axes are tilted away from the pole to the ecliptic at angles slightly larger than 90 degrees.
All except for Venus, and, strictly speaking, Pluto and Uranus, since their axes are tilted away from the pole to the ecliptic at angles slightly larger than 90 degrees.
318Q. Tell me
about Jupiter.
It accounts for more than 70% of the mass of the planets and more than 60% of the angular momentum of the solar system.
It accounts for more than 70% of the mass of the planets and more than 60% of the angular momentum of the solar system.
319Q. How were planets formed?
We don't really know.
We don't really know.
320Q. Why are the magnetic poles close to
the geographic poles?
The magnetic field is caused by movement of electric charges in the Earth's fluid outer core, which is made mainly of iron. We know the field is not magnetostatic because the Earth's magnetism is too small for that, the movement of the field could not be explained, and the Earth's core temperature is above the Curie temperature for iron, beyond which iron loses spontaneous magnetism. The Earth's core moves around the Earth's axis of rotation due to the Earth's rotation. Movement of charged particles in a magnetic field causes further movement of electric charges in a perpendicular direction--this is called the geomagnetic dynamo, but a solution of the relevant equations had not been found by 1997. Movement of electric charges in a loop causes a magnetic field whose main axis is perpendicular to the plane of rotation (planets with slower rotation do not have an appreciable magnetic field).
The magnetic field is caused by movement of electric charges in the Earth's fluid outer core, which is made mainly of iron. We know the field is not magnetostatic because the Earth's magnetism is too small for that, the movement of the field could not be explained, and the Earth's core temperature is above the Curie temperature for iron, beyond which iron loses spontaneous magnetism. The Earth's core moves around the Earth's axis of rotation due to the Earth's rotation. Movement of charged particles in a magnetic field causes further movement of electric charges in a perpendicular direction--this is called the geomagnetic dynamo, but a solution of the relevant equations had not been found by 1997. Movement of electric charges in a loop causes a magnetic field whose main axis is perpendicular to the plane of rotation (planets with slower rotation do not have an appreciable magnetic field).
