A planet is a large astronomical body that is neither a star nor a stellar remnant. At least eight planets exist in the Solar System: the terrestrial planets Mercury, Venus, Earth and Mars, and the giant planets Jupiter, Saturn, Uranus and Neptune. The word probably comes from the Greek planḗtai, meaning "wanderers", which in antiquity referred to the Sun, Moon, and five bodies visible as points of light that moved across the background of the stars. These five planets were Mercury, Venus, Mars, Jupiter and Saturn. Earth was recognized to be a planet when heliocentrism supplanted geocentrism during the sixteenth and seventeenth centuries. With the development of the telescope, the meaning of "planet" broadened to include objects not visible to the naked eye: the ice giants Uranus and Neptune; Ceres and other bodies later recognized to be part of the asteroid belt; and Pluto, later found to be the largest member of the collection of icy bodies known as the Kuiper belt. The discovery of other large objects in the Kuiper belt, particularly Eris, spurred debate about how exactly to define "planet". The International Astronomical Union adopted a standard by which the four terrestrials and four giants qualify, placing Ceres, Pluto and Eris in the category of dwarf planet, though this standard has not been universally embraced. Further advances in astronomy led to the discovery of over five thousand planets outside the Solar System, or exoplanets. These include hot Jupiters — giant planets that orbit close to their parent stars — like 51 Pegasi b, super-Earths like Gliese 581c that have masses in between that of Earth and Neptune, and planets smaller than Earth like Kepler-20e. Multiple exoplanets have been found to orbit in the habitable zones of their respective stars, but Earth remains the only planet known to support life.
Apparent retrograde motion is the apparent motion of a planet in a direction opposite to that of other bodies within its system, as observed from a particular vantage point. Direct motion or prograde motion is motion in the same direction as other bodies.
In astronomy, a conjunction occurs when two astronomical objects or spacecraft have either the same right ascension or the same ecliptic longitude, usually as observed from Earth. The astronomical symbol for conjunction is ☌ and handwritten . The conjunction symbol is not used in modern astronomy. It continues to be used in astrology.
In astronomy, a planet's elongation is the angular separation between the Sun and the planet, with Earth as the reference point. The greatest elongation of a given inferior planet occurs when this planet's position, in its orbital path around the Sun, is at tangent to the observer on Earth. Since an inferior planet is well within the area of Earth's orbit around the Sun, observation of its elongation should not pose that much a challenge. When a planet is at its greatest elongation, it appears farthest from the Sun as viewed from Earth, so its apparition is also best at that point.
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 reference frame of the Earth, in which the terms were originally used, the inferior planets are Mercury and Venus, while the superior planets are Mars, Jupiter, Saturn, Uranus and Neptune. Dwarf planets like Ceres or Pluto and most asteroids are 'superior' in the sense that they almost all orbit outside the orbit of Earth.
In astronomy, a transit is a phenomenon when a celestial body passes directly between a larger body and the observer. As viewed from a particular vantage point, the transiting body appears to move across the face of the larger body, covering a small portion of it.
In classical antiquity, the seven classical planets or seven luminaries are the seven moving astronomical objects in the sky visible to the naked eye: the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. The word planet comes from two related Greek words, πλάνης planēs and πλανήτης planētēs, both with the original meaning of "wanderer", expressing the fact that these objects move across the celestial sphere relative to the fixed stars. Greek astronomers such as Geminus and Ptolemy often divided the seven planets into the Sun, the Moon, and the five planets.
In astronomy, an extraterrestrial sky is a view of outer space from the surface of an astronomical body other than Earth.
A great conjunction is a conjunction of the planets Jupiter and Saturn, when the two planets appear closest together in the sky. Great conjunctions occur approximately every 20 years when Jupiter "overtakes" Saturn in its orbit. They are named "great" for being by far the rarest of the conjunctions between naked-eye planets.
A planet symbol is a graphical symbol used in astrology and astronomy to represent a classical planet or one of the modern planets. The symbols were also used in alchemy to represent the metals associated with the planets, and in calendars for their associated days. The use of these symbols derives from Classical Greco-Roman astronomy, though their current shapes are a development of the 16th century.
The Family Portrait, or sometimes Portrait of the Planets, is an image of the Solar System acquired by Voyager 1 on February 14, 1990 from a distance of approximately 6 billion kilometers from Earth. It features individual frames of six planets and a partial background indicating their relative positions. The picture is a mosaic of 60 frames. The frames used to compose the image were the last photographs taken by either Voyager spacecraft. The frames were also the source of the famous Pale Blue Dot image of the Earth. Astronomer Carl Sagan, who was part of the Voyager imaging team, campaigned for many years to have the pictures taken.
Planet Heroes was a line of toys from Fisher-Price depicting various heroic characters each identified with a specific planet.
Discovery and exploration of the Solar System is observation, visitation, and increase in knowledge and understanding of Earth's "cosmic neighborhood". This includes the Sun, Earth and the Moon, the major planets Mercury, Venus, Mars, Jupiter, Saturn, Uranus, and Neptune, their satellites, as well as smaller bodies including comets, asteroids, and dust.
Retrograde motion in astronomy is, in general, orbital or rotational motion of an object in the direction opposite the rotation of its primary, that is, the central object. It may also describe other motions such as precession or nutation of an object's rotational axis. Prograde or direct motion is more normal motion in the same direction as the primary rotates. However, "retrograde" and "prograde" can also refer to an object other than the primary if so described. The direction of rotation is determined by an inertial frame of reference, such as distant fixed stars.
Planets in astrology have a meaning different from the astronomical understanding of what a planet is. Before the age of telescopes, the night sky was thought to consist of two very similar components: fixed stars, which remained motionless in relation to each other, and "wandering stars", which moved relative to the fixed stars over the course of the year.
The Solar System Family Portrait is an image of many of the Solar System's planets and moons acquired by MESSENGER during November 2010 from approximately the orbit of Mercury. The mosaic is intended to be complementary to the Voyager 1's Family Portrait acquired from the outer edge of the Solar System on February 14, 1990.
The following outline is provided as an overview of and topical guide to the Solar System:
The word "transit" refers to cases where the nearer object appears smaller than the more distant object. Cases where the nearer object appears larger and completely hides the more distant object are known as occultations.
In Greco-Roman Classical Mythology, the Astra Planeta are brothers, and are five of Eos' and Astraeus' children--along with the Anemoi and Astraea--personifying the Classical planets ; Uranus, Neptune and Pluto are not included, as they are invisible to the naked eye and were thus unknown to the ancient Hellenic peoples. The Astra Planetas names are:
- Phainon ; the personification of the planet Saturn. Associated with the god, Kronus/Saturn (Kronion).
- Phaethon ; the personification of the planet Jupiter. Associated with the god, Zeus/Jupiter/Jove (Dios). Phaethon also shares a similar name with Phaethon, the son of Helios and a queen of Aethiopia; after his disastrous attempt at driving the solar chariot, Phaethon was struck down by Zeus and placed into the sky as (also) the god of Zeus' planet - Jupiter.
- Pyroeis ; the personification of the planet Mars. He was also called Mesonyx, the Midnight Star. Associated with the gods, Mars/Ares &/or Herakles/Heracles.
- Eosphoros/Phosphorus and Hesperus / ; the personification of the planet Venus. Associated with the goddess, Aphrodite/Venus. The name, Eosphoros/Phosphorus, is specific to Venus in the morning, as the "Morning Star", and the name, Hesperus, is specific to Venus in the evening, as the "Evening Star"; while it was once believed that Venus in the morning and Venus in evening were two different celestial bodies, these two star-gods were later combined.
- Stilbon ; the personification of the planet Mercury. Associated with the god, Hermes/Mercury.
