Earth

Motion of the Earth

The Earth orbits the Sun in an almost circular path. Hence, the seasons are not due to a changing distance between our planet and the central body of the Solar System. The rotation axis of Earth is pointing 23.5° off the normal of the plane of orbit. Since the direction of North is changing only very slowly (precession), the northern hemisphere is pointed toward the Sun in northern summer, and the South Pole during northern winter.

Atmosphere

Our weather takes place in the troposphere. This is the lowest layer of the atmosphere and extents to about 8000 meters. This limit is coinciding with the highest possible altitude where life is theoretically possible (permanent presence of human life is possible up to an altitude of 5500 meters).

Other planets and moons of our Solar System support an atmosphere, but none of these bodies could support life like the one of our World. Nitrogen makes more than ¾ of the composition of the atmospheric gas, but the 23% oxygen is what we need for breathing.

According to its view from space, Earth is called the 'blue planet'. This is only partially true; astronauts are fascinated by the details of the continents and the white clouds they could observe. To bring some of these feelings to you, the renderings of Earth on the following page is based on a topographical map. To enhance realism, the clouds are mathematically overlaid on the map. This process makes it possible to use the most current satellite imaginary available! The worldwide mosaic of the clouds is based on a global composite mosaic from geostationary weather satellites, e.g., Meteosat. This cloud mosaic is produced by University of Madison. We rebuild and re-project our realistic weather map every 3-6 hours (depending on the raw data).

Figure of Earth

According to recent theories of Earth science, the Earth consists of different layers. The core is assumed to have a diameter of about 2700 km and to be solid. It may be made out of nickel and iron, with a temperature of 4000°C. This inner core is surrounded by liquid layers with various chemical compositions. The lithosphere is the outermost shell, consisting of solid plates, which actually flow on top of the fluid rock, also consisting of silicates. The science of plate tectonic teaches us, that the continents are large floes, that slowly move relatively to each other. New measurement based on space borne methods achieves assessment accuracy of these drifts in the order of millimeters per year.

The crustal movement is the reason for volcanism and earthquakes. On the border of two plates, friction occurs, which are relieved from time to time. Where plates diverge, the liquid magma gets on the surface and builds a new crust. The most prominent example is the oceanic ridge in the middle of the Atlantic Ocean; it is clearly visible on the topographical map.

Already in the 19th century, it was known that Earth does not have a perfectly circular shape. Due to the daily rotation, the polar diameter is about 30 km shorter than its counterpart on the equator. Map surveys take this effect into account by assuming the Earth as an ellipsoidal body. However, this assumption is not exactly true: due to different mass distribution of the land mass with its mountain ranges, and the oceans, and various other effects, the mean figure of Earth is much more complicated. The real figure defined by a mean sea-level of the oceans is called geoid and can only be represented in a numerical manner, using huge databases. Working with positional coordinates based on the geoid is not practical. Therefore, several simpler bodies, usually ellipsoids, have been fit to the surface of the Earth and represent either a global or a local best fit of the mean zero level. When the parameters of two or more systems are known, the coordinates can be interchanged among these reference frames.

Nowadays, the World Geodetic System (WGS84), setup in 1984, is widely used in civil applications. Since it is based on a global fit, the local errors can accumulate to several decameters. As an example, the difference of WGS84 to the geoid is about 50 meters in Switzerland. However, WGS84 is not properly defined and scientists use other more sophisticated systems instead. The accuracy of WGS84 will fulfill the needs of amateur astronomer, since the assumed error is in the order of 10 meters to ITRF89. CalSky offers the possibility to accept positional coordinates of the most important systems, and does the most accurate transformation to WGS84. Hence, you can do your astronomical holiday planning right here on astro!nfo's Observer.


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