Retrograde and prograde motion
As a prograde or prograde (per from the Latin for ',' against ',' forward ', and gradus, step ' ) is called in astronomy objects that follow in a rotating system of the main direction of rotation. Oppositely rotating or rotating objects is referred to as retrograde or retrograde (from the Latin retro, back ',' backward ').
The names are used for both orbiting star systems, planets, asteroids and comets, as well as for installations in their orbit satellites. An object with a retrograde orbit is characterized by an orbital inclination between 90 ° and 270 ° to its central body, an object with retrograde rotation by an axial inclination of 90 ° to 270 ° to its orbit.
Also in the space artificial satellites orbiting in retrograde orbits the Earth, contrary to its sense of rotation. However, they are relatively rare, because, in order to bring to its course, its acceleration relative to the earth, they have to bring ( relative to a non-rotating system ), not only to the web speed, but also has to overcome the opposing rotational speed of the earth.
Importance in the solar system
In the solar system objects such as these are considered to be prograde, which, as seen from the direction of the north pole of the ecliptic, rotate counterclockwise or rotate their central body.
As a result of the formation of the solar system all the planets, Pluto and the asteroid belt on a rechtläufigen orbit. The rotation of most of the major bodies of the solar system is also carried in rechtläufigem sense.
Some well-known decline in natural celestial bodies
Retrograde Orbit
- Triton (Neptune I), orbital inclination: 157 ° Largest retrograde moon in the solar system. It is believed that there is an object captured by the gravity in this moon.
- Various Jupiter and Saturn moons, for example: Pasiphae (Jupiter VIII)
- Carme (Jupiter XI)
- Ananke (Jupiter XII), orbital inclination: 149 °
- Phoebe (Saturn IX), orbital inclination: 175 °
- Skoll (Saturn XLVII)
- Skathi (Saturn XXVII)
Retrograde Rotation
- Venus, axis tilt: 177 °
- Uranus, axial inclination: 98 ° Largest retrograde object in the solar system, but the declining character is hardly recognizable because of the almost rectangular axial inclination.
- Pluto, axis tilt: 123 °
Apparent rechtläufige and retrograde motion
This is known objects in the solar system as a prograde if you circulation along the ecliptic, as viewed from the direction of the north pole of the ecliptic, is counterclockwise, and as a decline in a clockwise rotation. Since their orbital periods are different from the orbital period of the Earth according to the third Kepler 's law, it comes during overtaking maneuvers to change the apparent circulation Mind ( of the earth-referenced position vector ), for example, the characteristic planetary grinding in opposition of the outer planets. The explanation of this change was important for the development of the geocentric and heliocentric world view. (→ epicycle )