Glossary of Astronomical Terms

As with many science-related hobbies, there are a few new words you'll have to add to your vocabulary. This glossary is not meant to be an exhaustive list of techno-definitions. Rather, it is designed as an easy-to-access aid for novices to help them grapple with the terminology of astronomy.

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A - C
D - F
G - I
J - M
S - Z

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N O PQR

N

NADIR: The point directly below in the direction of the center of the earth. For spacecraft the arbitrarily defined 'down' direction. This is usually defined as the direction pointing toward the center of the primary body. Opposite of nadir is zenith.

NEBULA (plural NEBULAE): The term originally applied to any extended (i.e., fuzzy, non-stellar) object in the sky. More recently, it is used to describe clouds of gas in space. Some nebulae are illuminated by nearby stars (bright nebulae), while others remain dark and are only seen if they obscure a brighter object (dark nebulae).

NEON (Ne): Neon is the tenth element on the periodic table, and it is the first of the noble gases to have a full 8-electron orbital. Neon produces a reddish color on "neon" lights, and is completely inert, so it has no electronegativity. At STP, neon is gaseous and has a density of 0.900 g/L. Ne melts at 24.55 K and boils at 27.10 K. The atomic mass of Ne is 20.1797,

NEUTRON STAR:  A celestial body hypothesized to occur in a terminal stage of stellar evolution, essentially consisting of a super dense mass of neutrons and having a powerful gravitational attraction from which only neutrinos and high-energy photons can escape, thus rendering the body invisible except to x-ray detection.

NEWTONIAN: See reflector.

NITROGEN: Nitrogen is a very common gas -- so common, in fact, that it composes almost 4/5 of our atmosphere. Nitrogen forms covalent bonds as well as hydrogen bonds, and has valences of -3, -5, +4, and +2. Nitrogen is element #7, and N's atomic mass is 14.0067. At STP, N has a density of 1.251 g/L. N melts at 63.15 K and boils at 77.344 K. It is a highly electronegative element, with an electronegativity of 3.04.

NODE: The place where the plane of one orbit crosses the plane of another.

NUCLEAR REACTIONS: Combinations between atomic nuclei that become possible when atoms have broken up at high temperatures and pressures, such as those in the center of stars.

O

OBJECTIVE: This is the thing that gathers light from the sky and folds the light into a cone. In a refractor it is the big lens that points at the sky, in a reflector it is the big mirror at the bottom of the tube. The job of the objective is to create a light cone which comes into tight focus at a single focal point.

OBLATE SPHEROID: An object with a shape that looks like a flattened sphere.

OBLATENESS: The flattening of a planet from spherical form because of the centrifugal effect of rotation.

OCCULTATION: When one celestial body passes in front of, or covers, another in the sky, it is called an occultation. A solar eclipse is really just a specific type of occultation (the moon occulting the sun).

OPPOSITION: When a planet is exactly opposite of the sun in the sky. This means that the object will cross the meridian exactly at midnight. Planets are usually best viewed when at opposition. Venus and Mercury never reach opposition because their orbits lie closer to the sun than Earth's.

OPTICAL TUBE: This is the telescope proper. It is the tube which holds the objective. The rest of the stuff are accessories, such as the mount, tripod, and eyepieces. When reading ads, note that some times optical tubes are sold by themselves. You will need to go out and buy (or build) a mount for them before you can use them.

ORBIT: The path taken by an object revolving about a larger object.

ORBITAL INCLINATION: The angle made by the orbital plane of a planet with the orbital plane of Earth

ORBITAL PLANE: The flat surface that would result if every point on the orbit of a body was connected to the center of its primary.

ORBITAL SPEED: The speed with which an object moves in its orbit.

OXYGEN (O): Oxygen is necessary for respiration, and is present in everything from sugar to rust to fat, but is toxic when pure. O is element #8, and has a charge of -2. (And rarely, a charge of -1 when it forms peroxide ions). O has an electronegativity of 3.44, making it the second most electronegative element. At STP, O has a density of 1.429 g/L. O melts at 54.8 K and boils at 90.188 K. An active gas, Oxygen makes up 1/5 of the Earth's atmosphere. This gas is fairly common in the universe.

P Q R

PARALLAX: The apparent change of position of a close object compared to a more distant object when the viewer shifts the position from which he views the object (try this: hold your finger close to your face and look at it first with one eye, and then the other. The finger will appear to move).

PARSEC (pc): 3.26 light years or about 3.084198 x 10^13 Kilometers

PENUMBRA: The edge of the primary body's shadow.

PERIAPSIS: The closest point in an orbit to the primary body. For earth orbits periapsis is known as perigee (see below).

PERIGEE: The closest point on an orbit to the earth.

PERIHELION: The point in a planet's orbit when it is closest to the sun.

PERIOD: The time required for one body to orbit the primary body. The period is related to the semi-major axis as defined by Kepler's Third Law.

PERTURBATION: A small gravitational effect, such as that of one planet on another.

PHASES: The different shapes of the lighted side of a planet or satellite that is shining by reflected light from the sun.

PLANE: A two-dimensional geometric figure that is completely flat.

PLANET: Originally, an object in the sky which moved against that background of stars. Now it is used for any object that circles a star, and shines by reflected light.

PLANETOID: see Asteroid

PRIMARY BODY: The object which is the gravitational center of the system. The sun is the primary body for the Solar System and the earth is the primary body for almost all satellite operations.

PRIME MERIDIAN: The line of longitude that runs through Greenwich, England and from which longitude is measured east or west. 180 degrees from the Prime Meridian lies the International Date Line.

PROBE: A vessel designed to pass and observe an astronomical object. Usually unmanned.

PULSAR: Any of several celestial radio sources emitting short, intense bursts of radio waves, x-rays, or visible electromagnetic radiation at regular intervals, generally believed to be rotating neutron stars.

QUASAR: A star like object that has a large red shift and emits powerful blue light and often radio waves.

RED GIANT: A star of great size and brightness that has a relatively low surface temperature.

REFLECTING TELESCOPE: A telescope that uses a mirrored, concave, surface to bring the image to a focal plane. The most common type is known as a "Newtonian" reflector - named after Sir Isaac himself. These are popular with amateurs due to their low cost, ease of construction, and good images.

REFRACTING TELESCOPE: A telescope that uses a clear (glass) objective to refract, or bend, the light to form an image at the focal plane. Binoculars are really just two small refracting telescopes. Small astronomical refractors are common, but larger ones (4"+) can get very expensive.

RETROGRADE MOTION: Motion in a direction opposite to what is usual (in the solar system clockwise motion as viewed from above the sun's north pole is considered retrogate).

REVOLUTION: The circling of an object about another object.

ROTATION: The spinning of an object about its axis.