اوج و حضیض دو نقطه متقابل مداری از جهت بیشترین تفاوت فاصله گرانیگاهی با هم است. به نقطهای در مدار بیضوی، در دورترین فاصله از مرکز جرم سامانهٔ مداری نظیر ستارهٔ دوتایی یا یک ستاره و قمرش، اوج یا اوج مرکزی (apocenter) یا دوراوج (apoapsis) گفته میشود. به نزدیکترین نقطه در مدار بیضوی نسبت به گرانیگاه، حَضیض گفته میشود.
اوج خورشیدی (یا زمینی) دورترین نقطه فاصله زمین در مدار بیضویش نسبت به خورشید و حضیض خورشیدی نزدیکترین این فاصله است.
در نقطه حضیض خورشیدی (به انگلیسی: Perihelion)، یک سیاره در مدار بیضی شکلش به دور خورشید، در نزدیکترین مکان به خورشید قرار میگیرد. برای یک سیاره، ستاره دنبالهدار یا دیگر اجرام آسمانی، حرکت کردن به دور خورشید در یک مدار بیضی شکل انجام میگیرد و به این ترتیب فاصله بین جسم و خورشید در سراسر مدار تغییر میکند. در این نقطه در مدار، سیاره با حداکثر سرعتش حرکت میکند (قانون دوم کپلر). حضیض خورشیدی بهطور خاص در مورد مدارهای حول خورشید به کار میرود ولی در واقع نقطه حضیض در هر مدار معمولی بیضی شکلی که در آن یک جسم کوچکتر به دور یک جسم بزرگتر میگردد وجود دارد. برای بهطور کامل مشخص کردن موقعیت یک سیاره، بحث نقطه حضیض خورشیدی به عنوان یکی از عناصر (پایههای) مدار مورد نیاز است. در یک میدان گرانشی (جاذبهای) قوی، محل نقطه حضیض خورشیدی ممکن است بهطور پیاپی روی مدارها جلوتر برود. در منظومه شمسی، این امر را میتوان در مدار سیاره تیر (عطارد) مشاهده کرد که آزمون مهمی برای قانون نسبیت عام محسوب میشود.
اوج و حضیض خورشیدی حدود ۱۳ روز بطور دقیقتر ۱۳٫۰۶۷ درجه (اوج ۱۳٫۷ روز و حضیض ۱۲٫۸۲۵ روز) پس از نقاط انقلابین است که در اینصورت بطور متوسط و میانگین اوج خورشیدی ساعت ۲۱:۲۲ روز ۱۳ تیرماه (۴ ژوئیه) و حضیض خورشیدی ساعت ۱:۵۳ روز ۱۴ دیماه (۴ ژانویه) میباشد.
اوج خورشیدی حداکثر تا ۱۵۲٬۰۹۷٬۷۰۰ کیلومتر (۱٫۰۱۶۷۱ واحد نجومی) و حضیض حداقل تا ۱۴۷٬۰۹۸٬۰۷۰ کیلومتر (۰٫۹۸۳۲۹ واحد نجومی) است. این دو نقطه با حفظ قرینگی و تقابل هم بر روی مدار زمین به تدریج جابجا میشوند، نقطه حضیض خورشیدی در سال ۲۰۰۰م بر ۲۸۲٫۸۹۵ درجه مداری بوده که تا سال ۲۰۱۲م به ۲۸۳٫۰۶۷ درجه مداری کشیده شدهاست.
The apsides refer to the farthest (1) and nearest (2) points reached by an orbiting planetary body (1 and 2) with respect to a primary, or host, body (3). For the given primary (or host), the table names the (two) apsides of an orbiting body:
_______________________ Thus, the Earth's two apsides are: the farthest point of its solar orbit, aphelion, and the nearest point, perihelion. And the Moon's apsides are its farthest point of Earth-orbit, apogee, and its nearest point, perigee.
The term apsis (Greek: ἁψίς; plural apsides/ˈæpsɪdiːz/, Greek: ἁψῖδες; "orbit") refers to an extreme point in the orbit of an object. It denotes either the points on the orbit, or the respective distance of the bodies. The word comes via Latin from Greek, there denoting a whole orbit, and is cognate with apse. Except for a theoretical possibility, there are two apsides in any elliptic orbit, (see discussion below, in article body); they are named with the prefixes peri- (from περί (peri), meaning 'near') and ap-, apo- (from ἀπ(ό) (ap(o)), meaning 'away from'), added in reference to the body being orbited. According to Newton's laws of motion all periodic orbits are ellipses: either the two individual ellipses of both bodies (see the two graphs in the second figure), with the center of mass (or barycenter) of this two-body system at the one common focus of the ellipses, or the orbital ellipses, with one body taken as fixed at one focus, and the other body orbiting this focus (see top figure). All these ellipses share a straight line, the line of apsides, that contains their major axes (the greatest diameter), the foci, and the vertices, and thus also the periapsis and the apoapsis (see both figures). The major axis of the orbital ellipse (top figure) is the distance of the apsides, when taken as points on the orbit, or their sum, when taken as distances.
The major axes of the individual ellipses around the barycenter, respectively the contributions to the major axis of the orbital ellipses are inverse proportional to the masses of the bodies, i.e., a bigger mass implies a smaller axis/contribution. Only when one mass is sufficiently larger than the other, the individual ellipse of the smaller body around the barycenter comprises the individual ellipse of the larger body as shown in the second figure. For remarkable asymmetry, the barycenter of the two bodies may lie well within the bigger body, e.g., the Earth–Moon barycenter is about 75% of the way from Earth's center to its surface. If the smaller mass is negligible compared to the larger, then the orbital parameters are independent of the smaller mass (e.g. for satellites).
For generic situations, the terms pericenter and apocenter are used for naming the extreme points of orbits where the primary is not specified, (see table, top figure); periapsis and apoapsis (or apapsis) are equivalent alternatives, but these terms, frequently, also refer to distances, i.e., the smallest and largest distances between the orbiter and its host body, (see second figure).
For a body orbiting the Sun, the point of least distance is the perihelion (/ˌpɛrɪˈhiːliən/), and the point of greatest distance is the aphelion (/æpˈhiːliən/); when discussing orbits around other stars the terms become periastron and apastron.
When discussing a satellite of Earth, including the Moon, the point of least distance is the perigee (/ˈpɛrɪdʒiː/), and of greatest distance, the apogee, (from Ancient Greek Γῆ (Gē), "land" or "earth").
There are no natural satellites of the Moon: for man-made objects in lunar orbit, the point of least distance may be called the pericynthion (/ˌpɛrɪˈsɪnθiən/) and the greatest distance the apocynthion (/ˌæpəˈsɪnθiən/); or perilune and apolune are sometimes used.
In orbital mechanics, the apsides technically refer to the distance measured between the barycenters of the central body and orbiting body. However, in the case of a spacecraft, the terms are commonly used to refer to the orbital altitude of the spacecraft above the surface of the central body (assuming a constant, standard reference radius).
which is the speed of a body in a circular orbit whose radius is .
The words "pericenter" and "apocenter" are often seen, although periapsis/apoapsis are preferred in technical usage.
Various related terms are used for other celestial objects. The '-gee', '-helion', '-astron' and '-galacticon' forms are frequently used in the astronomical literature when referring to the Earth, Sun, stars and the Galactic Center respectively. The suffix '-jove' is occasionally used for Jupiter, while '-saturnium' has very rarely been used in the last 50 years for Saturn. The '-gee' form is commonly used as a generic 'closest approach to planet' term instead of specifically applying to the Earth. During the Apollo program, the terms pericynthion and apocynthion (referencing Cynthia, an alternative name for the Greek Moon goddess Artemis) were used when referring to the Moon. Regarding black holes, the term peri/apomelasma (from a Greek root) was used by physicist and science-fiction author Geoffrey A. Landis in a 1998 story, before peri/aponigricon (from Latin) appeared in the scientific literature in 2002, as well as peri/apobothron (from Greek bothros, meaning hole or pit).
The following suffixes are added to peri- and apo- to form the terms for the nearest and farthest orbital distances from these objects. For the Solar System objects, only the suffixes for the Earth and Sun are commonly used – the other suffixes are rarely used. Instead, the generic suffix of -apsis is used[not in citation given].
gr. melos: black gr. bothros: hole lat. niger: black
Perihelion and aphelion
The words "perihelion" and "aphelion" were coined by Johannes Kepler to describe the orbital motions of the planets around the Sun.
The words are formed from the prefixes "peri-" (Greek: περί, near) and "apo-" (Greek: ἀπό, away from), affixed to the Greek word for the sun, (ἥλιος, or hēlíou).
Currently, the Earth reaches perihelion in early January, approximately 14 days after the December Solstice. At perihelion, the Earth's center is about 6999983290000000000♠0.98329astronomical units (AU) or 147,098,070 km (91,402,500 mi) from the Sun's center. In contrast, the Earth reaches aphelion currently in early July, approximately 14 days after the June Solstice. The aphelion distance between the Earth's and Sun's centers is currently about 7011152097651119397♠1.01671 AU or 152,097,700 km (94,509,100 mi). Dates change over time due to precession and other orbital factors, which follow cyclical patterns known as Milankovitch cycles. In the short term, the dates of perihelion and aphelion can vary up to 2 days from one year to another. This significant variation is due to the presence of the Moon: while the Earth–Moon barycenter is moving on a stable orbit around the Sun, the position of the Earth's center which is on average about 4,700 kilometres (2,900 mi) from the barycenter, could be shifted in any direction from it – and this affects the timing of the actual closest approach between the Sun's and the Earth's centers (which in turn defines the timing of perihelion in a given year).
Because of the increased distance at aphelion, only 93.55% of the solar radiation from the Sun falls on a given area of land as does at perihelion. However, this fluctuation does not account for the seasons, as it is summer in the northern hemisphere when it is winter in the southern hemisphere and vice versa. Instead, seasons result from the tilt of Earth's axis, which is 23.4 degrees away from perpendicular to the plane of Earth's orbit around the sun. Winter falls on the hemisphere where sunlight strikes least directly, and summer falls where sunlight strikes most directly, regardless of the Earth's distance from the Sun. In the northern hemisphere, summer occurs at the same time as aphelion. Despite this, there are larger land masses in the northern hemisphere, which are easier to heat than the seas. Consequently, summers are 2.3 °C (4 °F) warmer in the northern hemisphere than in the southern hemisphere under similar conditions. Astronomers commonly express the timing of perihelion relative to the vernal equinox not in terms of days and hours, but rather as an angle of orbital displacement, the so-called longitude of the periapsis (also called longitude of the pericenter). For the orbit of the Earth, this is called the longitude of perihelion, and in 2000 it was about 282.895°; by the year 2010, this had advanced by a small fraction of a degree to about 283.067°.
For the orbit of the Earth around the Sun, the time of apsis is often expressed in terms of a time relative to seasons, since this determines the contribution of the elliptical orbit to seasonal variations. The variation of the seasons is primarily controlled by the annual cycle of the elevation angle of the Sun, which is a result of the tilt of the axis of the Earth measured from the plane of the ecliptic. The Earth's eccentricity and other orbital elements are not constant, but vary slowly due to the perturbing effects of the planets and other objects in the solar system. See Milankovitch cycles. On a very long time scale, the dates of the perihelion and of the aphelion progress through the seasons, and they make one complete cycle in 22,000 to 26,000 years. There is a corresponding movement of the position of the stars as seen from Earth that is called the apsidal precession. (This is closely related to the precession of the axis.) The dates and times of the perihelions and aphelions for several past and future years are listed in the following table:
The following chart shows the range of distances of the planets, dwarf planets and Halley's Comet from the Sun.
Distances of selected bodies of the Solar System from the Sun. The left and right edges of each bar correspond to the perihelion and aphelion of the body, respectively, hence long bars denote high orbital eccentricity. The radius of the Sun is 0.7 million km, and the radius of Jupiter (the largest planet) is 0.07 million km, both too small to resolve on this image.
The images below show the perihelion (green dot) and aphelion (red dot) points of the inner and outer planets.
Perihelion and aphelion points
The perihelion and aphelion points of the inner planets of the Solar System
The perihelion and aphelion points of the outer planets of the Solar System
^Since the Sun, Ἥλιος in Greek, begins with a vowel (H is considered a vowel in Greek), the final o in "apo" is omitted from the prefix. =The pronunciation "Ap-helion" is given in many dictionaries , pronouncing the "p" and "h" in separate syllables. However, the pronunciation /əˈfiːliən/ is also common (e.g.,McGraw Hill Dictionary of Scientific and Technical Terms, 5th edition, 1994, p. 114), since in late Greek, 'p' from ἀπό followed by the 'h' from ἥλιος becomes phi; thus, the Greek word is αφήλιον. (see, for example, Walker, John, A Key to the Classical Pronunciation of Greek, Latin, and Scripture Proper Names, Townsend Young 1859 , page 26.) Many  dictionaries give both pronunciations
^R. Schödel, T. Ott, R. Genzel, R. Hofmann, M. Lehnert, A. Eckart, N. Mouawad, T. Alexander, M. J. Reid, R. Lenzen, M. Hartung, F. Lacombe, D. Rouan, E. Gendron, G. Rousset, A.-M. Lagrange, W. Brandner, N. Ageorges, C. Lidman, A. F. M. Moorwood, J. Spyromilio, N. Hubin, K. M. Menten (17 October 2002). "A star in a 15.2-year orbit around the supermassive black hole at the centre of the Milky Way". Nature. 419: 694–696. arXiv:astro-ph/0210426. Bibcode:2002Natur.419..694S. doi:10.1038/nature01121.CS1 maint: Uses authors parameter (link)
^Klein, Ernest, A Comprehensive Etymological Dictionary of the English Language, Elsevier, Amsterdam, 1965. (Archived version)
^Since the Sun, Ἥλιος in Greek, begins with a vowel, H is the long e vowel in Greek, the final o in "apo" is omitted from the prefix. The pronunciation "Ap-helion" is given in many dictionaries , pronouncing the "p" and "h" in separate syllables. However, the pronunciation /əˈfiːliən/ is also common (e.g.,McGraw Hill Dictionary of Scientific and Technical Terms, 5th edition, 1994, p. 114), since in late Greek, 'p' from ἀπό followed by the 'h' from ἥλιος becomes phi; thus, the Greek word is αφήλιον. (see, for example, Walker, John, A Key to the Classical Pronunciation of Greek, Latin, and Scripture Proper Names, Townsend Young 1859 , page 26.) Many  dictionaries give both pronunciations