positional astronomy Spherical astronomy, or positional astronomy, is a branch of observational astronomy used to locate astronomical objects on the celestial sphere, as seen at a particular date, time, and location on Earth. It relies on the mathematical methods o ...

, two

astronomical object An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often us ...

s are said to be in opposition when they are on opposite sides of the

celestial sphere In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, ...

, as observed from a given body (usually

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

). A planet (or

asteroid An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere. ...

comet A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ar ...

) is said to be "in opposition" or "at opposition" when it is in opposition to the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

. Because most

orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...

s in the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...

are nearly

coplanar In geometry, a set of points in space are coplanar if there exists a geometric plane that contains them all. For example, three points are always coplanar, and if the points are distinct and non-collinear, the plane they determine is unique. How ...

to the

ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic again ...

, this occurs when the

, and the body are configured in an approximately straight line, or syzygy; that is, Earth and the body are in the same direction as seen from the Sun. Opposition occurs only for superior planets (see the diagram). The instant of opposition is defined as that when the apparent geocentric

celestial longitude Astronomical coordinate systems are organized arrangements for specifying positions of satellites, planets, stars, galaxies, and other celestial objects relative to physical reference points available to a situated observer (e.g. the true horizon ...

of the body differs by 180° from the apparent geocentric longitude of the

. At that time, a body is: * in apparent retrograde motion * visible almost all night – rising around sunset, culminating around midnight, and setting around sunrise * at the point in its

where it is roughly closest to Earth, making it appear larger and brighter * nearly completely sunlit; the planet shows a full phase, analogous to a

full moon The full moon is the lunar phase when the Moon appears fully illuminated from Earth's perspective. This occurs when Earth is located between the Sun and the Moon (when the ecliptic coordinate system, ecliptic longitudes of the Sun and Moon opp ...

* at the place where the

opposition effect The opposition surge (sometimes known as the opposition effect, opposition spike or Seeliger effect) is the brightening of a rough surface, or an object with many particles, when illuminated from directly behind the observer. The term is most wid ...

increases the reflected light from bodies with unobscured rough surfaces The

Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

, which orbits Earth rather than the Sun, is in approximate opposition to the Sun at full moon. Moulton (1918), p. 191 A more exact opposition occasionally occurs with mathematical regularity if the Moon is at its usual sun and Earth-aligning point so that it appears full and happens to be aligning with the ecliptic (Earth's orbital plane) during the descending or ascending phase of its 5°

inclined Incline, inclined, inclining, or inclination may refer to: *Grade (slope), the tilt, steepness, or angle from horizontal of a topographic feature (hillside, meadow, etc.) or constructed element (road, railway, field, etc.) *Slope, the tilt, steepn ...

(tilted) orbit, which is more concisely termed at a ''

node In general, a node is a localized swelling (a "knot") or a point of intersection (a vertex). Node may refer to: In mathematics * Vertex (graph theory), a vertex in a mathematical graph *Vertex (geometry), a point where two or more curves, lines ...

'' of its orbit, in which case, a

lunar eclipse A lunar eclipse occurs when the Moon moves into the Earth's shadow. Such alignment occurs during an eclipse season, approximately every six months, during the full moon phase, when the Moon's orbital plane is closest to the plane of the Earth ...

occurs. A more exact, shaded form is when a central area of the earth aligns more precisely: a

central lunar eclipse A central lunar eclipse is a lunar eclipse in which part of the Moon passes through the center of Earth's shadow. This type of lunar eclipse typically appears darker than other lunar eclipses. It is relatively rare. Central lunar eclipses are ...

, of which there were 14 in the 50 years to 2000, others being

penumbral The umbra, penumbra and antumbra are three distinct parts of a shadow, created by any light source after impinging on an opaque object. Assuming no diffraction, for a collimated beam (such as a point source) of light, only the umbra is cast. T ...

. The

astronomical symbol Astronomical symbols are abstract pictorial symbols used to represent astronomical objects, theoretical constructs and observational events in European astronomy. The earliest forms of these symbols appear in Greek papyrus texts of late antiq ...

for opposition is ☍ (U+260D). () Seen from a superior planet, an

inferior planet In the Solar System, a planet is said to be inferior or interior with respect to another planet if its orbit lies inside the other planet's orbit around the Sun. In this situation, the latter planet is said to be superior to the former. In the ref ...

on the opposite side of the Sun is in superior

conjunction Conjunction may refer to: * Conjunction (grammar), a part of speech * Logical conjunction, a mathematical operator ** Conjunction introduction, a rule of inference of propositional logic * Conjunction (astronomy), in which two astronomical bodies ...

with the Sun. An inferior conjunction occurs when the two planets align on the same side of the

. At inferior conjunction, the superior planet is "in opposition" to the Sun as seen from the inferior planet (see the diagram).

Average interval between oppositions

When two planets are on the same side of the sun then from the point of view of the interior planet the other planet is in opposition with the sun. When two planets are on opposite sides of the sun, there is an opposition from the point of view of the sun. In either case, the interval between two such occurrences involving the same two planets is not constant because the orbits are not circular and because the planets perturb one another. But the average interval between them can be calculated from the periods of the two planets. The "speed" at which a planet goes around the sun, in terms of revolutions per time, is given by the inverse of its period, and the speed difference between two planets is the difference between these. Since the time interval between two oppositions is the time it takes for 360° to be covered by that speed difference, the average interval is: :

\frac 1

The following table gives these average intervals, in years, for combinations of the eight planets.

References

External links

Asteroids around opposition
– British Astronomical Association – Computing Section. {{DEFAULTSORT:Opposition (Planets) Astrometry Observational astronomy

Average interval between oppositions

See also

References

External links