Proximity to earth
The proximity of the earth or the perigee [ peʀiˈɡɛːʊm ] of a celestial body is the point of its orbit that is closest to the earth , or the point in time at which this point is reached, or the distance that exists at this point. The point of the orbit furthest from the earth is called the distance from the earth or apogee .
Depending on the subject, the distances are specified differently:
- In astronomy , the perigee distance describes the distance from the center of the body to the center of the earth . It is relevant for orbit calculations.
- In the space , however, is for near-Earth satellites, the orbital height above the earth's surface the decisive factor: the orbital altitude at perigee, ie the Perigäumshöhe , is frequently even as perigee referred. Accordingly, the orbital altitude at apogee, that is apogee shortly apogee called.
The terms “near the earth” and “far from the earth” are used not only for objects that orbit the earth, but also for objects in the solar system that orbit the sun . Here, too, there are points in their orbit, as well as the associated times and distances at which their distance to the earth is minimal or maximal.
Bodies circling the earth
The lunar orbit is complicated in its details, but it can be viewed as an ellipse as a first approximation . Their peri- and apogee differ by over 13 percent ( eccentricity 0.055). The major semiaxis of the orbit measures 384,405 km, the distance to the earth varies between (on average) 356,410 and 406,740 km in the rhythm of the anomalous month .
As a result, the apparent size of the lunar disk varies . If the full moon and perigee (proximity to the earth) coincide, some media speak of the super full moon because of the apparent increase in size . The distance from the earth to the moon is also decisive for the type of solar eclipse : If the eclipse date falls close to the earth, the moon is relatively large and the eclipse is complete , but only ring-shaped when it is distant from the earth .
The distance of the perigees and apogees fluctuates according to a frequency distribution .
Bodies orbiting the sun
In the case of bodies that orbit not the earth but the sun, the proximity and distance to the earth are generally different points than the apses of the orbit; The proximity and distance to the earth can also be different for each orbit.
In the case of lower planets (planets within the Earth's orbit , ie Mercury and Venus ), proximity to the earth is close to the lower conjunction and distance from the earth is close to the upper conjunction . In both cases, the planets are very close to the sun when viewed from earth: in front of and behind it. The events repeat themselves according to the synodic period about every four months (Mercury) or every one and a half years (Venus).
In the case of upper planets , proximity to earth is close to the opposition and distant to earth is close to conjunction . At around one year (outermost planets) to two years ( Mars ), the synodic period of the upper planets is significantly longer than that of the lower planets. The upper planets become retrograde near the earth ( planetary loop ).
In the case of comets , the proximity to the earth can be close to the perihelion , but in the case of very slim orbital ellipses it can be a few weeks to months before or after. The exact situation depends on the inclination of the orbit towards the ecliptic and above all on the position of the earth in relation to the perigee. These two points in time were about 20 days apart when comet C / 2004 Q2 (Machholz) was close to the sun in 2004/2005 (orbit 120,000 years), and the two distances were 0.347 and 1.205087 astronomical units ( AU ).
Asteroids with a proximity to the earth smaller than the radius of the lunar orbit are called lunar orbit cruisers . Their observation and mapping is an important project: a proximity to the earth less than one earth radius would mean an impact (impact on the earth's surface ).
- JR Wertz: Spacecraft Attitude Determination and Control . Springer Science & Business Media, 2012, ISBN 978-94-009-9907-7 (English, limited preview in Google book search).
- In this description of an ESA mission , perigee and apogee also stand for the respective train stations.
- It is important for the information on how the Earth's figure , the figure: in addition to the height above sea level ( English above mean sea level (AMSL) ), the disclosure can also refer to an idealized as a sphere Earth, as in this NASA page on Apollo 8 (quote from it: "At insertion, conditions were: apogee and perigee 99.99 by 99.57 n mi, .... The apogee and perigee were based upon a spherical Earth with a radius of 3,443,934 n mi." ) .