Vernal Equinox

The Earth mass is kg.

The Earth's average orbital distance from the Sun is m, a distance known as the astronomical unit.

Earth's only natural satellite is the Moon, sometimes called Luna.

The solar day (i.e., the time to complete a rotation taking into account that the Earth is also advancing in its orbit and so reaches the same orientation relative to the Sun in slightly less time than its rotational period in inertial space) is 24 ours, or 86,400 seconds. The sidereal day, which is the Earth's rotational period relative to the "fixed" stars, is 23 hours, 56 minutes, and 4.1 seconds (or 86164.10 seconds).

To first approximation, the Earth is an oblate spheroid, Eric Weisstein's World of Math i.e., a squashed sphere Eric Weisstein's World of Math with a bulge at the equator. More detailed measurements show it to be shaped a bit like an inflated tetrahedron, although the approximation as a spheroid is accurate enough for most purposes. The IUGG value for the equatorial radius a of the Earth (often simply referred to as "the" earth radius) is 6378.137 km (3963.19 statute miles Eric Weisstein's World of Physics), giving an equatorial circumference Eric Weisstein's World of Math of 40,075 km (24,901.5 miles Eric Weisstein's World of Physics). The flattening Eric Weisstein's World of Math of the Earth is , where c is the polar radius.
(Astronomical Almanac 1997).

Earth's atmosphere is divided into five layers: exosphere (500 km and up), thermosphere (80-500 km; which includes the ionosphere), mesosphere (50-80 km), stratosphere (10-50 km) and troposphere (0-10 km). the atmosphere has a mass and is composed of the following gases and aerosols.

gas number fraction
0.7808
0.2095
Ar 0.0093
0.3-0.04 ppm
345 pm
Ne 18 ppm
0-12 ppm
He 5 ppm
1.7 ppm
Kr 1 ppm
500 ppb
300 ppb
CO 100 ppb
CFC-12 380 ppt
CFC-11 220 ppt

Water is also present at a global average concentration of 23 precipitable mm. Water has a mean atmospheric residence time of 1 week.

Earth's average surface temperature is 287 K. It is at perihelion on January 2, at a distance 3% closer than aphelion in July. Climatic changes are closely linked to temperature variations. A lowering of temperature creates more ice which increases albedo and reflects more radiation, reinforcing the cold. Cooling, however, decreases the quantity of water vapor in the atmosphere, producing fewer clouds and a lower albedo. Eric Weisstein's World of Physics This, in turn, leads to a cancellation of the cold. The net effect of these processes is not understood.

Earth's temperature is regulated by the oceans which carry heat to the poles and limit ice buildup in the polar ice sheets. The positions of the continents affect the amount of ice than can form, however, so continental drift can affect climate. There is a small greenhouse effect present on Earth caused by the combustion of hydrocarbons and other releases of CO2 into the atmosphere by humans.

An apparent 26 My periodicity to mass extinctions on Earth could be linked to climatic changes. Possible external causes include variability of the Sun, aging of the Sun, orbital and obliquity changes, dust-releasing vulcanism, comet impacts, a companion star to the Sun ("Nemesis"), and galactic plane dust. The possibility that periodic climate variations were linked to orbital cycles was made by Milankovitch, and these cycles are call milankovitch cycles.

region radius
crust 3-20 km
mantle 3485-6370 km
outer core 1250-3485 km
inner core 0-1250 km

Earth's interior consists of a crust, mantle, outer core, and inner core. The crust has a low density and is silica-rich on the surface and silica-poor in the interior. The crustal thickness varies between 5-15 km under the oceans and 30-50 km under continental shields. The oldest continental crust is 3.8 Gy old, but the average age is about 1.5 Gy. It represents less than 0.5% the mass of the Earth. The central pressure of the Earth is Mbar. The following tables give the crustal composition broken down by element

rank element weight %
1 O 46.60
2 Si 27.72
3 Al 8.13
4 Fe 5.00
5 Ca 3.63
6 Na 2.83
7 K 2.59
8 Mg 2.09
9 Ti 0.44
99.03
(Klein and Hurlbut 1985).

The following table gives mineral composition of the crust.
group members %
feldspar Ca, K, Na feldspar 60%
chain silicates pyroxene, amphibole 10%
quartz quartz 10%
layered silicates biotite, muscovite, chlorites, clay 10%
other carbonates, limestone, metals, etc. 10%

In the Earth's interior, pressure increases at a rate. Because a 200 K temperature increase causes a 1% seismic velocity drop (the material becomes more flexible at high temperatures), seismic velocities can be used as a probe of crustal temperatures. Regions under continental shields have fast seismic velocities, indicating that they are cold and therefore old. Low seismic velocities, on the other hand, are observed under the mid-ocean ridges, indicating that they are young and hot. The boundary between the crust and mantle, as determined from seismic velocities, is called the Mohorovicic discontinuity. It separates rocks having P-wave velocities of 6.7 km s-1 from those with velocities of km s-1.

The mantle has a low silica content and siderophile enrichment with olivine and pyroxene widespread. Discontinuities produced by changes in mineral phases are also present. The dominant minerals are olivine, orthopyroxene, clinopyroxene, garnet, and to a lesser extent, jadeite. The most common mineral structure in the mantle in perovskite, which is stable between 700-2900 km. The second most is magnesiowüstite, (Mg,Fe)O. The dominant coordination number of minerals in the Earth's mantle is 6. Covalency also increases at high pressure. The major mineral phases in the upper mantle are feldspar, olivine, pyroxene, and pyrope garnets. The existence of phase changes in the mantle is driven by seismic velocity studies, which find three transition regions where the propagation speeds undergo sudden changes. The theory that the interior of the Earth is composed of perovskite is based on the mg of perovskite which has been made in the laboratory. It has been recently found that crystals become amorphous at very high pressure. This could have profound effects on the interpretation of the Earth's interior.

The following give phase changes undergone by common mantle minerals under increasing pressure.
Garnet Structure:
Olivine Structure:
Pyroxene Structure:

The mantle is separated from the core by a region known as D", which is a 250 km thick layer which may be a thermal boundary layer or a chemical and thermal boundary layer (MgSiO3/Fe). The outer core is composed of liquid nickel-iron and some sulfur. The inner core composed of solid nickel-iron and some sulfur. The Earth's center of figure is offset from its center of mass by 2.1 km. There is no correlation between gravity and topography, although geoid highs are found at subduction zones.

Earth probably had a primeval atmosphere of carbon dioxide, nitrogen, and water vapor (with small amounts of carbon monoxide, ammonia, and methane). Surface evolution was characterized by differentiation, cratering, flooding, and surface erosion and uplift. The volatile elements H, He, C, and N are extremely depleted in the Earth. The moderately volatile elements K, Na, Rb, Cs, and S are moderately depleted in the Earth. Refractory elements such as Ca, Al, Sr, Ti, Ba, U, and Th are thought to be present in nearly cosmic abundances.

Although there are a range of theories for the conditions under which Earth formed, the large amount of gravitational energy released due to the rapid accumulation of mass probably was sufficient to make the entire planet initially molten. The energy of large impacting bodies then made an additional contribution. However, trace element partitioning would cause a pattern of elemental distribution which is not observed, suggesting that the Earth was not entirely molten. This argument, however, assumes that crystals settle out from the melt. (However, if they were too small, they would be swept along by convection and would not settle.)

Asthenosphere, Atmospheric Phenomena, Aurora, Earth Mass, Earth Radius, Magnetosphere, Moon, Planet, Solar System, Sun.

References:
Arnett, W. "The Nine Planets: Earth." Nine Planets.
Klein, C. and Hurlbut, C. S. Jr. Table 4.1 in Manual of Mineralogy (After James D. Dana), 20th ed. New York: Wiley, pp. 151-152, 1985.
Jet Propulsion Laboratory. "Earth: Geodetic and Geophysical Data." http://ssd.jpl.nasa.gov/phys_props_earth.html.
Lang, K. R. "Planet Earth." Ch. 2 in Astrophysical Data: Planets and Stars. New York: Springer-Verlag, p. 30-40, 1992.

© 1996-2006 Eric W. Weisstein