Astronomy and related astronomical terms

1. One of a set of six parameters that fix uniquely the shape, size, and orientation of a celestial body: The orbital elements describe the size, shape, and orientation of an orbit.

The classical orbital elements include the semi-major axis, eccentricity, inclination, argument of perigee, right ascension of ascending node, mean anomaly, and epoch time.

A very prominent constellation in the equatorial region of the sky: Orion identified with the giant hunter of Greek mythology.

In Greek mythology, a giant and hunter, the son of the sea god Poseidon, who was killed by the goddess Artemis and then transformed into a constellation near the celestial equator containing the Great Nebula and more than 200 stars visible to the naked eye.

A conic section when some comets enter the solar system: Parabola can also refer to a parabolic orbits and are therefore never seen again.

The parabolic shape is also used in telescope mirrors and radio telescope aerials to bring all the radiation gathered from an object into a sharp focus.

The change in the obvious position of an object against its background when observed from two different viewpoints: The parallax is an angular displacement undergone by the position of a star when observed from two different points.

In astronomy, nearby stars show a shift owing to a parallax when seen from different locations on the Earth's orbit around the Sun. A star's parallax is used to determine its distance from the Earth.

Nearer bodies, such as, the Moon, Sun, and planets also show a parallax caused by the motion of the Earth.

A unit used with reference to distances to stars and galaxies: A parsec, short for "parallax section", is equal to the distance at which the mean radius of the Earth's orbit subtends an angle of one second of arc.

One parsec is equal to 3.2616 light-years, 2.063 x 10⁵ astronomical units, and 3.086 x 10¹³ kilometers.

Any very small piece of matter, such as a molecule or atom: Particles can refer to pieces even smaller, such as electrons, protons, and neutrons. Another application is to larger ones, as in interstellar dust.

A grayish area surrounding the dark center of a sunspot: A penumbra is the partial outer shadow that is lighter than the darker inner shadow or umbra, such as the area between complete darkness and complete light in an eclipse.

An observer within the penumbral region will see a partial eclipse. It is also the name given to the outer dark region of a sunspot.

The point in a planet's orbit that is is closest to the Sun: A perihelion is the exact position in the orbit of a planet, comet, or other celestial body, at which it passes closest to the Sun.

A relation obeyed by cepheid variable stars (highly luminous yellow or orange super giant stars that varies regularly in brightness): The period-luminosity relation relates to the period of the changes in luminosity that varies directly with the luminosity of the star.

Irregularity in an object's orbit: Perturbation is caused by the gravitational influence of another object.

1. The appearance of the illuminated surface of a celestial body as seen from the Earth: Jill was very interested in astronomy and didn't want to miss out on any of the phases of the Moon.
2. One of the varying shapes of the lighted portion of the Moon: In her book, Susan read that there were many phases of the Moon, like the Full Moon, Half Moon, and the Waning Crescent, among others.

The measurement of the luminous intensities of visible light sources which is sometimes expanded to include near-infrared and near-ultraviolet light: Such devices for photometry accurately assess the intensity of light from the stars and galaxies.

They are nearly all based on highly sensitive, light-detecting electron tubes, called "photomultipliers".

In these devices, weak starlight causes electrons to be emitted from the surface of a photocathode, and each electron released can be made to produce many millions of secondary electrons.

An electronic device used to amplify low-intensity light signals by converting them into electrons: In the photomultiplier, the acceleration of these electrons leads to a cascade, and a relatively large output signal for a small input signal.

One of many particles that make up electromagnetic radiation: Photons carry varying amounts of energy and travel at the speed of light.

The intensely bright gaseous outer layer of a star: The photosphere is the outermost visible surface, of the Sun which emits light and heat.

The photosphere is the layer of the Sun that corresponds to the solar surface viewed in white light or the region from which light escapes from the Sun into space.

The photosphere is often marked by large, dark patches called "sunspots" and "faculae". The faculae are groups of small shining spots on the surface of the Sun which are brighter than the other parts of the photosphere.

About 300 kilometers, or 200 miles deep, the photosphere consist of incandescent gas at a temperature of 5,800 Kelvin (5,530° Celsius or 9,980° Fahrenheit).

Kelvin is the basic unit of thermodynamic temperature adopted under the Systeme International d'Unites.

Rising cells of hot gas produce a mottling of the photosphere known as granulation and each granule is about 1,000 kilometers or 620 miles in diameter.