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 the theoretical possibility of one common circular orbit for two bodies of equal mass at diametral positions (symmetric binary star), there are two apsides for any elliptic orbit, named with the prefixes peri- (from περί (peri), meaning 'near') and ap-/apo- (from ἀπ(ό) (ap(ó)), meaning 'away from') added to a reference to the body being orbited. All periodic orbits are, according to Newton's Laws of motion, 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).
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).
These formulae characterize the pericenter and apocenter of an orbit:
Note that for conversion from heights above the surface to distances between an orbit and its primary, the radius of the central body has to be added, and conversely.
The geometric mean of the two limiting speeds is
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.
of the name
|Astronomical object||Star||Galaxy||Barycenter||Black hole|
of the name
|lat. astra: stars||galaxy||gr. melos: black|
gr. bothros: hole
lat. niger: black
The words perihelion and aphelion were coined by Johannes Kepler to describe the orbital motion of the planets. 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.
Currently, the Earth reaches perihelion in early January, approximately 14 days after the December Solstice. At perihelion, the Earth's center is about 0.98329 astronomical 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 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:
|Date||Time (UT)||Date||Time (UT)|
|2007||January 3||19:43||July 6||23:53|
|2008||January 2||23:51||July 4||07:41|
|2009||January 4||15:30||July 4||01:40|
|2010||January 3||00:09||July 6||11:30|
|2011||January 3||18:32||July 4||14:54|
|2012||January 5||00:32||July 5||03:32|
|2013||January 2||04:38||July 5||14:44|
|2014||January 4||11:59||July 4||00:13|
|2015||January 4||06:36||July 6||19:40|
|2016||January 2||22:49||July 4||16:24|
|2017||January 4||14:18||July 3||20:11|
|2018||January 3||05:35||July 6||16:47|
|2019||January 3||05:20||July 4||22:11|
|2020||January 5||07:48||July 4||11:35|
|Type of body||Body||Distance from Sun at perihelion||Distance from Sun at aphelion|
|Planet||Mercury||46,001,009 km (28,583,702 mi)||69,817,445 km (43,382,549 mi)|
|Venus||107,476,170 km (66,782,600 mi)||108,942,780 km (67,693,910 mi)|
|Earth||147,098,291 km (91,402,640 mi)||152,098,233 km (94,509,460 mi)|
|Mars||206,655,215 km (128,409,597 mi)||249,232,432 km (154,865,853 mi)|
|Jupiter||740,679,835 km (460,237,112 mi)||816,001,807 km (507,040,016 mi)|
|Saturn||1,349,823,615 km (838,741,509 mi)||1,503,509,229 km (934,237,322 mi)|
|Uranus||2,734,998,229 km (1.699449110×109 mi)||3,006,318,143 km (1.868039489×109 mi)|
|Neptune||4,459,753,056 km (2.771162073×109 mi)||4,537,039,826 km (2.819185846×109 mi)|
|Dwarf planet||Ceres||380,951,528 km (236,712,305 mi)||446,428,973 km (277,398,103 mi)|
|Pluto||4,436,756,954 km (2.756872958×109 mi)||7,376,124,302 km (4.583311152×109 mi)|
|Haumea||5,157,623,774 km (3.204798834×109 mi)||7,706,399,149 km (4.788534427×109 mi)|
|Makemake||5,671,928,586 km (3.524373028×109 mi)||7,894,762,625 km (4.905578065×109 mi)|
|Eris||5,765,732,799 km (3.582660263×109 mi)||14,594,512,904 km (9.068609883×109 mi)|
The following chart shows the range of distances of the planets, dwarf planets and Halley's Comet from the Sun.
The images below show the perihelion (green dot) and aphelion (red dot) points of the inner and outer planets.
In architecture, an apse (plural apses; from Latin absis: "arch, vault" from Greek ἀψίς apsis "arch"; sometimes written apsis, plural apsides) is a semicircular recess covered with a hemispherical vault or semi-dome, also known as an exedra. In Byzantine, Romanesque, and Gothic Christian church (including cathedral and abbey) architecture, the term is applied to a semi-circular or polygonal termination of the main building at the liturgical east end (where the altar is), regardless of the shape of the roof, which may be flat, sloping, domed, or hemispherical. Smaller apses may also be in other locations, especially shrines.Argument of periapsis
The argument of periapsis (also called argument of perifocus or argument of pericenter), symbolized as ω, is one of the orbital elements of an orbiting body. Parametrically, ω is the angle from the body's ascending node to its periapsis, measured in the direction of motion.
For specific types of orbits, words such as perihelion (for heliocentric orbits), perigee (for geocentric orbits), periastron (for orbits around stars), and so on may replace the word periapsis. (See apsis for more information.)
An argument of periapsis of 0° means that the orbiting body will be at its closest approach to the central body at the same moment that it crosses the plane of reference from South to North. An argument of periapsis of 90° means that the orbiting body will reach periapsis at its northmost distance from the plane of reference.
Adding the argument of periapsis to the longitude of the ascending node gives the longitude of the periapsis. However, especially in discussions of binary stars and exoplanets, the terms "longitude of periapsis" or "longitude of periastron" are often used synonymously with "argument of periapsis".Dwarf gallery
A dwarf gallery is an architectural ornament in Romanesque architecture.
It is a natural development of the blind arcade and consists of an arcaded gallery, usually just below the roof, recessed into the thickness of the walls. Usually dwarf galleries can be found at church towers or apses but they frequently appear at other parts of buildings as well, or even go around the entire building. Although principally meant as a decorative element, some dwarf galleries can be accessed. During the septennial Pilgrimage of the Relics in Maastricht, relics were shown daily from the dwarf gallery of St Servatius' to pilgrims gathered in front of the church in Vrijthof.
Dwarf galleries mainly appear at Romanesque churches in Germany and Italy. A few examples can be found in Belgium and the Netherlands (see Mosan art). Remarkably, in France no dwarf galleries were built. The oldest church in Germany with a dwarf gallery is Trier Cathedral. The apsis with dwarf gallery at Speyer Cathedral, described as “one of the most memorable pieces of Romanesque design”, was copied in many other places in the German Rhineland. Several of Cologne's twelve Romanesque churches feature dwarf galleries, as well as important Rhineland churches like Mainz Cathedral, Worms Cathedral and Bonn Minster.
In Italy, dwarf galleries appear at churches in the central and northern regions of the country. Examples are Santa Maria della Pieve in Arezzo, Modena Cathedral, Pistoia Cathedral, San Donato in Genoa and Pisa Cathedral. The famous Leaning Tower of Pisa could be described as having six rings of dwarf galleries.
In France, simple dwarf galleries are rare. But there was a luxurious development. In some façades, sculptures were placed between the columns. Most famous are the Galleries of Kings (Galeries des Rois) on Notre-Dame de Paris and the Cathedral of Amiens.
Dwarf galleries incidentally feature in Romanesque Revival architecture, notably in Germany, but also in other parts of the world.Isaiah Church
Isaiah Church (Danish: Esajas Kirke) is a Lutheran church in the Østerbro district of Copenhagen, Denmark.Isaszeg
Isaszeg is a town in Pest county, Budapest metropolitan area, Hungary. It has a population of 10,979 (2007).Lengyeltóti
Lengyeltóti is a town in Somogy county, Hungary.
The settlement is part of the Balatonboglár wine region.List of internet service providers in Canada
This is an alphabetical list of internet service providers in Canada:
Apsis Communications Inc.
Aurora Cable Internet
Chebucto Community Net
National Capital FreeNet
Rogers Hi-Speed Internet
Rush Communications Ltd.
Xplornet CommunicationsPerigee (disambiguation)
Perigee is a type of apsis: an extreme point in an object's orbit.
Perigee may also refer to:
Perigee: Publication for the Arts, a quarterly literary journal
Perigee Books, a former imprint of Penguin Group, now part of TarcherPerigee
Perigee moon, or supermoon
Holcomb Perigee, a prototype sportsplanePrix Bertrand du Breuil
The Prix Bertrand du Breuil is a Group 3 flat horse race in France open to thoroughbreds aged four years or older. It is run at Chantilly over a distance of 1,600 metres (about 1 mile), and it is scheduled to take place each year in June.Prix Thomas Bryon
The Prix Thomas Bryon is a Group 3 flat horse race in France open to two-year-old thoroughbreds. It is run at Saint-Cloud over a distance of 1,400 metres (about 7 furlongs), and it is scheduled to take place each year in October.Rotunda (architecture)
A rotunda (from Latin rotundus) is any building with a circular ground plan, and sometimes covered by a dome. It can also refer to a round room within a building (a famous example being within the United States Capitol in Washington, D.C.). The Pantheon in Rome is a famous rotunda. A Band Rotunda is a circular bandstand, usually with a dome.Ruth's Church
Ruth's Church (Danish: Ruts Kirke) is a parish church located in the village of Rutsker near Hasle on the Danish island of Bornholm. The church was built in the early 13th century in the Romanesque style. Situated on a hilltop 130 m above sea level, it is the island's highest-standing church.SS Empire Baffin
Empire Baffin was a 6,978 ton cargo ship which was built by Lithgows Ltd, Port Glasgow in 1941 for the Ministry of War Transport (MoWT). She was commissioned in 1943 as HMS Sancroft, being converted into a cable laying ship for Operation Pluto. She was returned to the MoWT in 1946 and subsequently sold and renamed Clintonia. A final change of ownership in 1960 saw her renamed Aspis and she was scrapped in 1963.Santa Reparata, Florence
Santa Reparata is the former cathedral of Florence, Italy. Its name refers to Saint Reparata, an early virgin martyr who is the co-patron saint of Florence. The Cattedrale di Santa Maria del Fiore was constructed over it.Tarnaszentmária
Tarnaszentmária is a village at the foot of the Mátra Mountains in Hungary, in Heves.Zánka
Zánka is a village in Veszprém county, Hungary.Østermarie
Østermarie is a village on the Danish island of Bornholm, 8 km (5.0 mi) west of Svaneke. Founded ca. 1880, its old church (Østermarie Church) , now a ruin, dates back to the 12th century. The population as of 1 January 2015 is 483.