Everything about Aphelion totally explained
In
astronomy, an
apsis, plural
apsides is the point of greatest or least distance of the
elliptical orbit of an
astronomical object from its center of attraction, which is generally the
center of mass of the system.
The point of closest approach is called the
periapsis or
pericentre and the point of farthest excursion is called the
apoapsis (Greek από,
from, which becomes απ before a vowel, and αφ before rough breathing),
apocentre or
apapsis (the latter term, although etymologically more correct, is much less used). A straight line drawn through the periapsis and apoapsis is the
line of apsides. This is the major axis of the
ellipse, the line through the longest part of the ellipse.
Related terms are used to identify the body being orbited. The most common are
perigee and
apogee, referring to orbits around the Earth, and
perihelion and
aphelion, referring to orbits around the Sun (Greek ‘ήλιος
hēlios sun). During the
Apollo program, the terms
pericynthion and
apocynthion were used when referring to the moon.
Formula
These
formulae characterize the periapsis and apoapsis of an orbit:
- Periapsis: maximum speed , the speed corresponding to a kinetic energy which, at any position of the orbit, added to the existing kinetic energy, would allow the orbiting body to escape (the square root of the product of the two speeds is the local escape velocity).
Terminology
The words "pericentre" and "apocentre" are occasionally seen, although periapsis/apoapsis are preferred in technical usage.
Various related terms are used for other celestial objects. The '-gee', '-helion' and '-astron' and '-galacticon' forms are frequently used in the astronomical literature, while the other listed forms are occasionally used, although '-saturnium' has very rarely been used in the last 50 years. The '-gee' form is commonly (although incorrectly) used as a generic 'closest approach to planet' term instead of specifically applying to the Earth. The term peri/apomelasma (from the Greek root) was used by physicist Geoffrey A. Landis in 1998 before peri/aponigricon (from the Latin) appeared in the scientific literature in 2002 .
| Body |
Closest approach |
Farthest approach |
| Galaxy |
Perigalacticon |
Apogalacticon |
| Star |
Periastron |
Apastron |
| Black hole |
Perimelasma/Perinigricon |
Apomelasma/Aponigricon |
| Sun |
Perihelion |
Aphelion |
| Mercury |
Perihermion |
Apohermion |
| Venus |
Pericytherion/Pericytherean/Perikrition |
Apocytherion/Apocytherean/Apokrition |
| Earth |
Perigee |
Apogee |
| Moon |
Periselene/Pericynthion/Perilune |
Aposelene/Apocynthion/Apolune |
| Mars |
Periareion |
Apoareion |
| Jupiter |
Perizene/Perijove |
Apozene/Apojove |
| Saturn |
Perikrone/Perisaturnium |
Apokrone/Aposaturnium |
| Uranus |
Periuranion |
Apouranion |
| Neptune |
Periposeidion |
Apoposeidion |
| Pluto |
Perihadion |
Apohadion |
Since "peri" and "apo" are Greek, it's considered by some purists more correct to use the Greek form for the body, giving forms such as '-zene' for Jupiter and '-krone' for Saturn. The daunting prospect of having to maintain a different word for every orbitable body in the solar system (and beyond) is the main reason why the generic '-apsis' has become the almost universal norm.
- In the Moon's case, in practice all three forms are used, albeit very infrequently. The '-cynthion' form is, according to some, reserved for artificial bodies, whilst others reserve '-lune' for an object launched from the Moon and '-cynthion' for an object launched from elsewhere. The '-cynthion' form was the version used in the Apollo Project, following a NASA decision in 1964.
- For Venus, the form '-cytherion' is derived from the commonly used adjective 'cytherean'; the alternate form '-krition' (from Kritias, an older name for Aphrodite) has also been suggested.
- For Jupiter, the '-jove' form is occasionally used by astronomers whilst the '-zene' form is never used, like the other pure Greek forms ('-areion' (Mars), '-hermion' (Mercury), '-krone' (Saturn), '-uranion' (Uranus), '-poseidion' (Neptune) and '-hadion' (Pluto)).
Earth's perihelion and aphelion
The Earth is closest to the Sun in early January and farthest in early July. The relation between perihelion, aphelion and the Earth's seasons changes over a 21,000 year cycle. This
anomalistic precession contributes to periodic
climate change (see
Milankovitch cycles).
The day and hour of these events for the next few years are:
| Year |
Perihelion |
Aphelion |
| 2007 |
Jan 3 20Z |
July 7 00Z |
| 2008 |
Jan 3 00Z |
July 4 08Z |
| 2009 |
Jan 4 15Z |
July 4 02Z |
| 2010 |
Jan 3 00Z |
July 6 11Z |
| 2011 |
Jan 3 19Z |
July 4 15Z |
| 2012 |
Jan 5 00Z |
July 5 03Z |
| 2013 |
Jan 2 05Z |
July 5 15Z |
| 2014 |
Jan 4 12Z |
July 4 00Z |
| 2015 |
Jan 4 07Z |
July 6 19Z |
| 2016 |
Jan 2 23Z |
July 4 16Z |
Further Information
Get more info on 'Aphelion'.
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