What Does THAT Mean?

accretion disk

a disk surrounding a black hole or star in which matter gravitationally falls onto the central object.

achromat

A high-quality objective lens that display very little prismatic color.

alt-az (altazimuth) mounting

A telescope mounting that swings side-to-side and up-and-down, but cannot follow the celestial object with a single motion.

altitude

the angle between an object's position on the celestial sphere and the horizon.

aperture

The diameter of the objective lens or mirror of a telescope; measured in inches or millimeters.

apochromat

A premium objective lens that displays no prismatic color.

arcminute

a unit of angular size equal to 1/60 of a degree.

arcsecond

a unit of angular size equal to 1/3,600 of a degree (or 1/60 of an arcminute).

astronomical Unit (AU)

the average distance from Earth to the sun, equal to about 93,000,000 miles (150,000,000 km).

azimuth

the angle measured eastward from due north to the point on the horizon directly below an object.

Big Bang

the giant explosion that created the universe 10 billion to 20 billion years ago.

binary star

a system of two stars that orbit a common center of gravity; also known as a double star.

black hole

a region of space where gravity is so powerful that not even light can escape.

catadioptric telescope

A telescope with an objective that combines lenses and mirrors; examples are Schmidt-Cassegrain and Maksutov telescopes.

CCD (charge-coupled device)

a silicon chip used to detect light; CCDs are far more efficient at collecting light than conventional film.

clock drive

a motor attached to an equatorial mount that compensates for EarthŒs rotation and thus keeps the telescope pointing at the same area of sky.

conjunction

a time when two or more bodies appear close together in the sky.

dark matter

matter that exerts gravitational force but does not emit any detectable light or radiation; dark matter comprises most of the mass of the universe but its exact nature remains unknown.

declination

the angular distance of an object above or below the celestial equator; the celestial sphere equivalent of latitude.

deep-sky objects

objects located beyond the solar system; consist of stars, nebulae, star clusters, and galaxies.

degree

a unit of angular size equal to 1/360 the circumference of the celestial sphere; the sun and moon both appear about half a degree wide.

Dobsonian mounting

A simple, sturdy alt-az mounting often used on large reflector telescopes; named for inventor John Dobson.

Dobsonian telescope

A Newtonian reflector telescope on a Dobsonian mounting.

eclipse

an event in which one body passes in front of another, blocking it partially or completely from view.

solar: an eclipse of the sun caused by the moon passing between Earth and the sun.

lunar: an eclipse of the moon caused by the Earth passing between the sun and moon.

annular: a solar eclipse in which the moon does not fully cover the sun's disk, allowing observers to see a thin ring of sunlight.

ecliptic

the plane of Earth's orbit around the sun; all the planets except Mercury and Pluto have orbits in nearly the same plane.

elongation

the apparent angular separation of an object from the sun.

equatorial mounting

A mounting that has one axis parallel to Earth's rotation axis; this lets the telescope track stars with a single motion.

eyepiece

A small, high-quality lens that magnifies the image formed by a telescope's optics.

finder scope

A low-magnification, wide-field telescope attached to the main telescope and used to aim it.

fluorite lens

An apochromat lens with one element made of the mineral fluorite.

focal length

The distance from the objective lens or mirror to the place where the image is focused.

focal ratio (f/ratio)

The focal length of a telescope divided by the aperture.

focuser

A telescope fitting that holds the eyepiece and lets you rack it in and out for sharp focus.

galaxy

An aggregation of billions of stars moving under mutual gravity; the Milky Way is a spiral galaxy and there are other types.

gas giant

a large planet made primarily of gas; such as Jupiter, Saturn, Uranus, and Neptune.

gibbous

the phase of the moon between first quarter and last quarter, when the moon appears more than half illuminated.

globular cluster

a roughly spherical congregation of hundreds of thousands of stars; most globular clusters consist of old stars and exist in a galaxy's halo.

halo

the outer region of a galaxy containing globular clusters, a few stray stars, and dark matter.

Hubble constant

the value that expresses the expansion rate of the universe and is equal to the recessional velocity of a galaxy divided by the distance to the galaxy; usually expressed in kilometers per second per megaparsec (3.26 million light-years).

infrared light

a form of light with slightly lower energy than visible light but with greater energy than radio waves.

Kuiper belt

a region in the outer solar system beyond Neptune's orbit that contains billions of small, icy bodies; Pluto is the largest known Kuiper belt member.

light-year

the distance light travels in one year; equivalent to approximately 5.9 trillion miles (9.5 trillion km).

Local Group

the galaxy cluster containing roughly 35 galaxies to which the Milky Way Galaxy belongs.

luminosity

the total amount of light that an object radiates.

magnitude

the measurement of the brightness of an object; the lower the number, the brighter the object.

absolute: the apparent brightness an object would have if it were 10 parsecs (32.6 light-years) from Earth.

apparent: the measure of the brightness of an object as seen from Earth.

Maksutov telescope

A special type of catadioptric telescope; small and portable.

meridian

an imaginary circle on the celestial sphere that connects the zenith to the north (or south) celestial pole.

meteor

a flash of light that occurs when a meteoroid burns up in EarthŒs atmosphere; also popularly known as a shooting star.

meteorite

a rock from space that survives passage through Earth's atmosphere and falls to the ground.

Milky Way

the band of light that encircles the entire sky and results from the combined light of billions of stars in our galaxy's disk.

Milky Way Galaxy

the spiral galaxy to which Earth belongs.

nebula (plural: nebulae)

A cloud of gas, dust, or both, often charged to glow by the energetic radiation of hot stars near it.

neutron star

the collapsed, extraordinarily dense, city-sized remnant of a high-mass star.

Newtonian telescope

A common type of reflecting telescope with a mirror objective and an open-end tube; named for Isaac Newton, who invented it.

nova

a violent explosion on the surface of a white dwarf, which causes the star to temporarily brighten by a factor of several hundred to several thousand.

objective lens (mirror)

The main light-collecting optical element in a telescope.

occultation

the passage of one object in front of a smaller one, temporarily obscuring all or part of the background object from view.

Oort cloud

a cloud of cometary nuclei that surrounds the sun at a distance of many thousands of astronomical units.

open cluster

a system containing a few dozen to a few thousand stars that formed from the same stellar nursery.

opposition

the moment when a planet farther from the sun than Earth appears opposite the sun in the sky; it is the best time to observe a planet.

orthoscopic eyepiece

An eyepiece with four lenses in it to provide long eye relief and corrections to optical aberrations commonly found in less expensive eyepieces.

perihelion

the point closest to the sun in an object's orbit.

phase

the regular cycle of changes in the appearance of a moon or planet.

photons

individual "particles" of light.

planetary nebula

a glowing shell of gas ejected by a dying low-mass star.

Plössl eyepiece

An eyepiece similar to an orthoscopic but has a slightly wider field of view.

prismatic color

A false color seen in the images of bright stars as seen through a refractor; it comes about because the objective lens acts like a prism to break starlight up into its component colors. The better the instrument, the less color you will see (and the more it will cost).

pulsar

a rapidly rotating neutron star that bathes Earth in regular pulses of electromagnetic radiation.

quasar

the highly energetic core of a young galaxy thought to be powered by a supermassive black hole; short for quasi-stellar object.

radio waves

the form of light with the longest wavelength and the least energy.

red giant

a cool star near the end of its life cycle that has expanded to a diameter a few dozen to a hundred times that of the sun.

redshift

an increase in the wavelength of light coming from an object due to its motion away from Earth, the expansion of the universe, or a strong gravitational field.

reflector

A telescope whose objective is a mirror.

refractor

A telescope whose objective is a lens.

resolution (or resolving power)

the ability of a telescope or camera to pick out fine detail.

right ascension

the angular distance of a celestial object east of the vernal equinox; the celestial sphere equivalent of longitude.

Schmidt-Cassegrain telescope

A special design of catadioptric telescope; features a compact, barrel-like tube.

seeing

The amount of turbulence in the atmosphere; often highly variable.

setting circles

Graduated scales on a equatorial mounting that indicate where the instrument is pointing.

star atlas

A collection of maps showing the positions of stars, nebulae, and galaxies in the sky.

star cluster

A closely-bunched group of stars born together at the same time. Open clusters typically have a thousand or less stars while globular clusters contain up to a million stars.

supernova

the cataclysmic explosion of a high-mass star.

terminator

the boundary on a planet or moon separating the illuminated side from the unilluminated.

ultraviolet radiation

a form of light with higher energy than visible light, but without as much energy as x rays.

universal time

the local time of day on a line of longitude centered on Greenwich, England; it forms the basis for all civil timekeeping.

visible light

the form of electromagnetic radiation that humans can see with their eyes.

white dwarf

the dense, collapsed, Earth-sized remnant of an intermediate-mass star like the sun.

x rays

electromagnetic radiation more energetic than ultraviolet light but less energetic than gamma rays.

zenith

the point on the celestial sphere directly over the head of an observer.

zodiacal light

a faint, cone-shaped glow of light seen in the west after nightfall or in the east before dawn caused by sunlight reflecting and scattering off interplanetary dust particles lying along the ecliptic plane.

To Beginning Astronomer