Astronomy and related astronomical terms

1. The moment at which a body in the solar system lies opposite the sun in the sky as seen from the earth and crosses the meridian at about midnight.
2. An object lying farther from the sun than the earth is at opposition when it lies on a line from the sun passing through the earth to the body in question.

The object is then directly behind the earth, and is fully illuminated as seen from the earth.

1. The path of one body in space around another body; such as, the orbit of the earth around the sun, or the moon around the earth.

When the two bodies are similar in mass, as in a binary star, both bodies move around their common center of mass.

The movement of objects in orbit follow Johann Kepler's laws, which apply to artificial satellites as well as to natural bodies.

2. The path followed by one object around another as a result of their mutual gravitational interaction.

As a result of the inverse square law of gravitation, planetary orbits are approximately conic sections.

1. A set of six parameters that fix uniquely the shape, size, and orientation of a celestial body.
2. A collection of quantities that, together, 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, identified with the 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 or 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.

1. The angular displacement undergone by the position of a star when observed from two different points.
2. The change in the apparent position of an object against its background when viewed from two different positions.

In astronomy, nearby stars show a shift owing to parallax when viewed from different positions 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.

1. A unit of length equal to the distance at which the mean radius of the earth's orbit subtends an angle of one second of arc.
2. A unit used with reference to distances to stars and galaxies.

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.

It also refers to pieces even smaller; such as, electrons, protons, and neutrons. Another application is to larger ones, as in interstellar dust.

1. A grayish area surrounding the dark center of a sunspot.
2. A 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.

1. The point in its orbit where a planet is closest to the sun.
2. The point 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), and which states that the period of the changes in luminosity varies directly with the luminosity of the star.

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

The appearance of the illuminated surface of a celestial body as seen from the earth.

Referring to a planet or moon with the varying shape of the lighted portion; such as, full, half, crescent, etc.

The measurement of the luminous intensities of visible light sources which is sometimes expanded to include near-infrared and near-ultraviolet light.

Photometric devices accurately measure 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.