orbit:    149,600,000 km (1.00 AU) from Sun
            diameter: 12,756.3 km
            mass:     5.9736e24 kg

    Earth is the only planet whose English name does not derive from Greek/Roman mythology. The name
    derives from Old English and Germanic. There are, of course, hundreds of other names for the planet in
    other languages.

    It was not until the time of Copernicus (the sixteenth century) that it was understood that the Earth is just
    another planet.

    Earth, of course, can be studied without the aid of spacecraft. Nevertheless it was not until the twentieth
    century that we had maps of the entire planet. Pictures of the planet taken from space are of considerable
    importance; for example, they are an enormous help in weather prediction and especially in tracking and
    predicting hurricanes. And they are extraordinarily beautiful.

    The Earth is divided into several layers which have distinct chemical and seismic properties (depths in km):

                   0-      40  Crust
                 10-    400  Upper mantle
               400-   650  Transition region
               650- 2700  Lower mantle
              2700-2890  D'' layer (sometimes included in the lower mantle)
              2890-5150  Outer core
              5150-6378  Inner core

    The crust varies considerably in thickness, it is thinner under the oceans, thicker under the continents. The
    inner core and crust are solid; the outer core and mantle layers are fluid.

    Most of the mass of the Earth is in the mantle, most of the rest in the core; the part we inhabit is a tiny fraction
    of the whole (values below x10^24 kilograms):

             atmosphere     = 0.0000051
             oceans         = 0.0014
             crust          = 0.026
             mantle         = 4.043
             outer core     = 1.835
             inner core     = 0.09675

    The core is probably composed mostly of iron (or nickel/iron) though it is possible that some lighter elements
    may be present, too. Temperatures at the center of the core may be as high as 7500 K, hotter than the
    surface of the Sun. The lower mantle is probably mostly silicon, magnesium and oxygen with some iron,
    calcium and aluminum. The upper mantle is mostly olivene and pyroxene (iron/magnesium silicates), calcium
    and aluminum. We know most of this only from seismic techniques; samples from the upper mantle arrive at
    the surface as lava from volcanoes but the majority of the Earth is inaccessible. The crust is primarily quartz
    (silicon dioxide) and other silicates like feldspar. Taken as a whole, the Earth's chemical composition (by
    mass) is:

            34.6%  Iron
            29.5%  Oxygen
            15.2%  Silicon
            12.7%  Magnesium
             2.4%  Nickel
             1.9%  Sulfur
             0.05% Titanium

    The Earth is the densest major body in the solar system.

    The other terrestrial planets probably have similar structures and compositions with some differences: the
    Moon has at most a small core; Mercury has an extra large core (relative to its diameter); the mantles of
    Mars and the Moon are much thicker; the Moon and Mercury may not have chemically distinct crusts; Earth
    may be the only one with distinct inner and outer cores. Note, however, that our knowledge of planetary
    interiors is mostly theoretical even for the Earth.

    Unlike the other terrestrial planets, Earth's crust is divided into several separate solid plates which float
    around independently on top of the hot mantle below. The theory that describes this is known as plate
    tectonics. It is characterized by two major processes: spreading and subduction. Spreading occurs when
    two plates move away from each other and new crust is created by upwelling magma from below.
    Subduction occurs when two plates collide and the edge of one dives beneath the other and
    ends up being destroyed in the mantle. There is also transverse motion at some plate boundaries (i.e. the
    San Andreas Fault in California) and collisions between continental plates (i.e. India/Eurasia). There are (at
    present) eight major plates:

         North American Plate - North America, western North Atlantic and Greenland
         South American Plate - South America and western South Atlantic
         Antarctic Plate - Antarctica and the "Southern Ocean"
         Eurasian Plate - eastern North Atlantic, Europe and Asia except for India
         African Plate - Africa, eastern South Atlantic and western Indian Ocean
         Indian-Australian Plate - India, Australia, New Zealand and most of Indian Ocean
         Nazca Plate - eastern Pacific Ocean adjacent to South America
         Pacific Plate - most of the Pacific Ocean (and the southern coast of California!)

    There are also twenty or more small plates such as the Arabian, Cocos, and Philippine Plates. Earthquakes
    are much more common at the plate boundaries. Plotting their locations makes it easy to see the plate
    boundariess.

    71 Percent of the Earth's surface is covered with water. Earth is the only planet on which water can exist in
    liquid form on the surface (though there may be liquid ethane or methane on Titan's surface and liquid water
    beneath the surface of Europa). Liquid water is, of course, essential for life as we know it. The heat capacity
    of the oceans is also very important in keeping the Earth's temperature relatively stable. Liquid water is also
    reponsible for most of the erosion and weathering of the Earth's continents, a process unique in the solar
    system today (though it may have occurred on Mars in the past).

    The Earth's atmosphere is 77% nitrogen, 21% oxygen, with traces of argon, carbon dioxide and water.

    The presence of free oxygen is quite remarkable from a chemical point of view. Oxygen is a very reactive
    gas and under "normal" circumstances would quickly combine with other elements. The oxygen in
    Earth's atmosphere is produced and maintained by biological processes. Without life there would be no
    free oxygen.

    The interaction of the Earth and the Moon slows the Earth's rotation by about 2 milliseconds per century.

    Earth has a modest magnetic field produced by electric currents in the core. The interaction of the solar
    wind, the Earth's magnetic field and the Earth's upper atmosphere causes the auroras.
 
 

Planet Profile

    Mass ( kg)                                                          5.9736 x (10^24)
    Volume ( km3)                                                  108.321 x (10^10)
    Equatorial radius (km)                                      6378
    Polar radius (km)                                               6356
    Volumetric mean radius (km)                          6371
    Core radius (km)                                               3485
    Ellipticity                                                            0.0034
    Mean density (kg/m^3)                                    5520
    Surface gravity (m/s2)                                      9.78
    Escape velocity (km/s)                                    11.186

    GM (km^3/s^2)                                                  0.3986 x (10^6)
    Bond albedo                                                      0.385
    Visual geometric albedo                                  0.367
    Visual magnitude V(1,0)                                 -3.86
    Solar irradiance (W/m^2)                                 1380
    Black-body temperature (K)                             247.3
    Topographic range (km)                                   20
    Moment of inertia (I/MR^2)                               0.3308
    J2                                                                       1082.63 x (10^-6)
 

Orbital parameters

                                                                                                                             
    Semimajor axis (km)                                        149.6 x (10^6)
    Sidereal orbit period (days)                             365.256
    Tropical orbit period (days)                              365.242
    Perihelion (km)                                                  147.1 x (10^6)
    Aphelion (km)                                                    152.1 x (10^6)
    Mean orbital velocity (km/s)                             29.79
    Orbit inclination (deg)                                       0.00
    Orbit eccentricity                                               0.0167
    Sidereal rotation period (hrs)                          23.9345
    Obliquity to orbit (deg)                                     23.45
 

Terrestrial Magnetosphere

    Dipole field strength:                                                                0.3076 gauss-Re^3
    Latitude/Longitude of dipoleN:                                               78.6 degrees N/70.1 degrees W
    Dipole offset (planet center to dipole center) distance:       0.0725 Re
    Latitude/Longitude of offset vector:                                       18.3 degrees N/147.8 degrees E

    Note: Re denotes Earth radii, 6,378 km
 

Terrestrial Atmosphere
 

    Surface Pressure:                   1014 mb
    Surface Density:                      1.217 kg/m^3
    Scale height:                            8.5 km
    Average temperature:             288 K
    Diurnal temperature range:    283 K to 293 K
    Wind speeds:                           0 to 100 m/s
    Mean molecular weight:         28.97 g/mole

    Atmospheric composition (by volume, dry air):

                Major      :      78.084% Nitrogen (N2), 20.946% Oxygen (O2),

                Minor (ppm): Argon (Ar) - 9340; Carbon Dioxide (CO2) - 350
                                       Neon (Ne) - 18.18; Helium (He) - 5.24; CH4 - 1.7
                                       Krypton (Kr) - 1.14; Hydrogen (H2) - 0.55
                                       Water is highly variable, typically makes up about 1%
 
 

Earth's Satellite 

 
 
    Earth has only one natural satellite, the Moon. But thousands of small artificial satellites have also been
    placed in orbit around the Earth. Asteroid 3753 (1986 TO) has a complicated orbital relationship with the
    Earth; it's not really a moon, the term "companion" is being used. It is somewhat similar to the situation with
    Saturn's moons Janus and Epimetheus. Lilith doesn't exist but it's an interesting story.

    Mass (kg)                                                 0.07349 x (10^24)
    Volume (km^3)                                          2.1973 x (10^10
    Equatorial radius (km)                            1738
    Polar radius (km)                                     1735
    Volumetric mean radius (km)                 1737.5
    Ellipticity                                                   0.002
    Mean density (kg/m^3)                           3340
    Surface gravity (m/s^2)                          1.62
    Escape velocity (km/s)                           2.38
    GM (km^3/s^2)                                        0.0049 x (10^6)
    Bond albedo                                           0.067
    Visual geometric albedo                       0.12
    Visual magnitude V(1,0)                      +0.21
    Solar irradiance (W/m^2)                      1380
    Black-body temperature (K)                  274.5
    Topographic range (km)                       16
    Moment of inertia (I/MR^2)                    0.394
 

    Orbital parameters (for orbit about the Earth)

 
        Semimajor axis (km)                           0.3844 x (10^6)
        Perigee (km)                                        0.3633 x (10^6)
        Apogee (km)                                        0.4055 x (10^6)
        Revolution period (days)                    27.322
        Synodic period (days)                        29.53
        Mean orbital velocity (km/s)               1.023
        Orbit inclination (deg)                         5.145
        Orbit eccentricity                                 0.0549
        Sidereal rotation period (hrs)            655.728
        Equatorial inclination (deg)                6.68
        Recession rate from Earth (cm/yr)    3.8