Geophysics for Beginners: Questions and answers about the Earth and Moon

A compilation by Alex Backer

Note that I am not an astronomer! Read at your own risk.


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.


Why are tides largest during New Moon and Full Moon?

During this time, the Sun, Moon and Earth are colinear, 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).


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. Lowrie.s Fundamentals of Geophysics (1999) says that it.s only the tidal lag (delay between the time when the Moon passes overhead and time of maximum bulge), 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. Why does tidal friction not slow rotation down beyond that?


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.


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.

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.

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.


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.


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.


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.


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.


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.


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.


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.


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. Source: JPL.


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]," says Roy Spencer of the Global Hydrology and Climate Center in Huntsville, AL. "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. Source: JPL.


Why are planets' orbits elliptical?
Because of conservation of energy under the attraction of the Sun, which varies as the inverse square of distance.

Why do the orbits sweep equal areas in equal times?
Because of conservation of angular momentum.

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 (Loewrie, 1997).

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.  


Do all planets revolve around the Sun in the same sense?

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.

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.

How were planets formed?
We don't really know (Loewrie, 1997).  


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).


Why aren't the magnetic poles exactly at the geographic poles?
The magnetic pole is due to magnetic fields that are generated in the Earth's core. Because it is not a perfect dipole and has many local anomalies, the magnetic poles are not exactly at the geographic poles. Furthermore, in addition to a daily fluctuation related to interaction with the ionosphere and solar radiation, these fields change slowly, moving westward and flipping from south to north on a very long time scale. For that reason, the south magnetic pole is rarely found precisely at the real South Pole of the Earth. Because each layer of the ionosphere can reflect radio waves, daily fluctuations in it make for radio waves traveling farther in the hemisphere facing the Sun, and thus shortwave reception from places half a world away is better at night.

I'm flipping: you said magnetic poles flip?! How do we know that? Why do they flip? How often? How fast?
Because lava magnetizes according to the magnetic field prevailing on Earth at the time of solidification, and the magnetization of lava of different ages has been analyzed all over the globe. This method has also been used to track continental drift. The inversions are sudden (a few thousand years) compared to the length of periods between reversals (100's of thousands of yrs to millions of years). The distribution of times between reversals is quite irregular, having speeded up and now slowing down again. At the current rate of diminution of the field's intensity, there will be none in 2000 yrs. We may be in the middle of a flip. The mechanisms that govern the inversions are poorly understood. (W. Loewrie, Fundamentals of Geophysics, 1997)

How strong is the Earth's magnetic field?
6x10-5 Tesla at its strongest, near the poles. Although this includes effects of solar wind and other aspects of external origin, Gauss concluded in 1838 that most of it comes from the field of internal origin, predominantly a dipole.





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