The process in which light or other electromagnetic radiation gives up its energy to an atom or molecule. For example, ozone in our atmosphere absorbs ultraviolet radiation.
A spectrum showing dark lines at some narrow color regions (wavelengths). The lines are formed by atoms absorbing light, which lifts their electrons to higher orbits.
A change in an object's velocity (either its speed or its direction)
A measure of how large an object looks to you. It is defined as the angle between lines drawn from the observer to opposite sides of an object. For example, the angular diameter of the Moon is about 1/2.
An easily identified grouping of stars, often part of a larger constellation. For example, the Big Dipper.
A distance unit based on the average distance of the Earth from the Sun.
astronomical unit (AU)
A wavelength band in which our atmosphere absorbs little radiation. For example, on Earth the visible window ranges from about 300 to 700 nanometers, allowing the light we can see with our eyes to pass through the atmosphere.
A submicroscopic particle consisting of a nucleus and orbiting electrons. The smallest unit of a chemical element.
An object that is an ideal radiator when hot and a perfect absorber when cool. It absorbs all radiation that falls upon it, reflecting no light; hence, it appears black. Stars are approximately blackbodies. The radiation emitted by blackbodies obeys Wien's law and the Stefan-Boltzmann law.
A shift in the wavelength of electromagnetic radiation to a shorter wavelength. For visible light, this implies a shift toward the blue end of the spectrum. The shift can be caused by the motion of a source of radiation toward the observer or by the motion of an observer toward the source. For example, the spectrum lines the star toward the earth exhibit a blueshift. (see also Doppler shift)
An imaginary line on the celestial sphere lying exactly above the Earth's equator. It divides the celestial sphere into northern and southern hemispheres.
An imaginary point on the celestial sphere directly above the Earth's North or South Pole.
An imaginary sphere surrounding the Earth representing the sky. Ancient astronomers pictured celestial objects as attached to it.
A principle of physics stating that the angular momentum of a rotating body remains constant unless forces act to speed it up or slow it down. Mathmatically, conservation of angular momentum states that MVR is a constant, where M is the mass of a body moving with a velocity V in a circle of radius R. One extremely important consequence of this principle is that if a rotating body shrinks, its rotational velocity must increase.
conservation of energy
A grouping of stars on the night sky. Astronomers divide the sky into 88 constellations.
A spectrum with neither dark absorption nor bright emission lines. The intensity of the radiation in such a spectrum changes smoothly from one wavelength to the next.
The appearance of two astronomical objects in approximately the same direction on the sky. For example, if Mars and Jupiter happen to appear near each other on the sky, they are said to be in conjunction. Superior conjunction refers to a planet that is approximately in line with the Sun but on the far side of the Sun from the Earth. Inferior conjunction refers to a planet that lies approximately between the Sun and the Earth.
One part of a coordinate system for locating objects in the sky north or south of the celestial equator. Declination is analogous to latitude on the earth's surface.
The change in the observed wavelength of radiation caused by the motion of the emitting body or the observer. The shift is an increase in the wavelength if the source and observer move apart and a decrease in the wavelength if the source and observer approach. (see also redshift and blueshift)
The times of year, separated by about 6 months, when eclipses are possible. At any given eclipse season, both a solar eclipse and lunar eclipse generally occur.
The path that the Sun appears to make around the celestial sphere as the Earth moves along it's orbit. The path gets it's name because eclipses can occur only when the Moon crosses the ecliptic.
A general term for any kind of electromagnetic wave.
The assemblage of all wavelengths of electromagnetic radiation. The spectrum includes the following wavelengths, from long to short: radio, microwave, infrared, visible light, ultraviolet, X rays and gamma rays.
Wave consisting of alternating electric and magnetic energy. Ordinary visible light is an electromagnetic wave, and the wavelength determines the lights color.
A low-mass, negatively charged subatomic particle. Electrons orbit the atomic nucleus, but may at times be torn free.
A geometric figure related to a circle but flattened into an oval shape.
The production of light, or more generally, electromagnetic radiation by an atom or other object.
A spectrum consisting of bright lines at certain wavelengths separated by dark regions in which there is no light.
Any of the numerous orbitals that an electron can occupy in an atom or molecule, roughly corresponding to an electron orbit.
A fictitious, small, and circular orbit superimposed on another circular orbit and proposed by early astronomers to explain the retrograde motion of the planets.
The time of year when the sun appears to cross the celestial equator. At this time, the number of hours of daylight and night are approximately equal. The vernal and autumnal equinoxes mark the beginning of the spring and fall seasons.
The speed an object needs to move away from another body in order not to be pulled back by it's gravitational attraction.
The planet Venus seen low in the western sky after sunset. (sometimes used for other bright planets.)
The condition in which the electrons of an atom are not in their lowest energy level (orbit).
(1)one of 2 points within an ellipse used to generate the elliptical shape. Planets orbit along ellipses with the sun at one focus of the ellipse. (2)a point in an optical system in which light rays are brought together. The location where an image forms in such systems.
The number of times per second that a wave vibrates.
A massive system of stars held together by their mutual gravity. Our Galaxy is the Milky Way.
A group of galaxies held together by their mutual gravitational attraction. The Milky Way belongs to the Local Group galaxy clusters.
Models of the Solar System centered on the Earth. Many of the earliest attempts to describe the Solar System were geocentric in that they supposed that the planets moved around the Earth rather than the Sun.
The force of attraction that is between 2 bodies and is generated by their masses.
The position of an inner Planet (Mercury or Venus) when it lies farthest from the sun on the sky. Mercury and Venus are particularly easy to see when they are at greatest elongation. Objects may be at greatest eastern or western elongation according to whether they lie east or west of the sun.
Models of the Solar System centered on the Sun.
The tendency of an object at rest to remain at rest and of a body in motion to continue in motion in a straight line at a constant speed.
Laws that describe the motion of Planets around the Sun. The first law states that Planets move in elliptical orbits with the Sun off-center at a focus of the ellipse. The second law states that a line joining the Planet and the Sun sweeps out equal areas in equal times. The third law relates a Planet's orbit Period, P, to the semimajor access of it's elliptical orbit, a.
Kepler's three laws
A description of the gravitational force exerted by one body on another. The gravitational force is proportional to the product of the bodies' masses and the inverse square of their separation.
law of gravity
A unit of distance equal to the distance light travels in one year. A light-year is roughly 6 trillion miles.
The small cluster or group of several dozen galaxies to which the Milky Way belongs.
The passage of the Earth between the Sun and the Moon so that the Earth's shadow falls on the Moon.
A measure of the amount of material an object contains. A quantity measuring a body's inertia.
The Galaxy to which the Sun belongs. Seen from the Earth, the Galaxy is a pale, milky white band in the night sky.
Milky Way Galaxy
A theoretical representation of some object or system.
The illusion that the Moon appears larger when near the horizon than when seen high in the sky.
The planet Venus seen in the eastern sky before dawn. (Sometimes applied to other bright planets.)
A unit of length equal to 1 billionth of a meter and abbreviated nm. Wavelengths of visible light are several hundred nanometers. The diameter of a hydrogen atom is roughly 0.1nm.
A subatomic particle of nearly the same mass as the proton but with no electric charge. Neutrons and protons compose the nuclei of atoms.
The law that a body continues in a state of rest or uniform motion in a straight line unless made to change that state by forces acting on it. (see also inertia)
Newton's first law of motion
In words, the amount of acceleration, a, that a force, F, produces depends on the mass, m, of the object being accelerated.
Newton's second law of motion
The law that when two bodies interact, they exert equal and opposite forces on each other.
Newton's third law of motion
The point on the celestial sphere directly above the Earth's North Pole. Objects on the sky appear to circle around this point.
north celestial pole
The core of an atom around which the electrons orbit. The nucleus has a positive electric charge and constitutes most of an atom's mass.
The configuration of a planet when it is opposite the Sun in the sky. If a planet is in opposition, it rises when the Sun sets and sets when the Sun rises.
A description of an electron's possible location in an atom as it "orbits" the nucleus. At these tiny scales, the wave nature of matter only allows a description of position in terms of probabilities.
The shift in an object's position caused by the observer's motion. A method for finding distances based on that shift.
The changing illumination of the Moon or other body that causes its apparent shape to change. The following is the cycle of lunar phases: new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, third quarter, waning crescent, new.
A particle of visible light or other electromagnetic radiation.
A body in orbit around a star that is large enough to have taken on a round shape, and which has cleared the path of its orbit of all bodies of comparable size.
The slow change in direction of the pole (rotation axis) of a spinning body or of the orientation of an orbit.
A positively charged subatomic particle. One of the constituents of the nucleus of an atom along with neutrons.
A shift in the wavelength of electromagnetic radiation to a longer wavelength. For visible light, this implies a shift toward the red end of the spectrum. The shift can be caused by a source of radiation moving away from the observer or by the observer moving away from the source. For example, if a star is moving away from Earth, its spectrum lines exhibit a redshift. (see also Doppler shift)
The ability of a telescope or instrument to discern fine details. Larger-diameter telescopes have greater (that is, better) resolving power.
The drift of a planet westward against the background stars. Normally planets shift eastward because of their orbital motion. The planet does not actually reverse its motion. The change in its direction is caused by the change in the position from which we view the planet as the Earth overtakes and passes it.
A coordinate for locating objects on the sky, analogous to longitude on the Earth's surface. Measured in hours and minutes of time.
An imaginary line through the center of a body about which the body spins.
A body orbiting a planet.
The process of observing a phenomenon, proposing a hypothesis on the basis of the observations, and then testing the hypothesis.
A shorthand way to write numbers using ten to a power. Also called "powers-of-ten notation."
Half the long dimension of an ellipse.
The passage of the Moon between the Earth and the Sun so that our view of the Sun is partially or totally blocked.
The Sun, planets, their moons, and other bodies that orbit the Sun.
(winter and summer) the beginning of winter and summer. Astronmically the solstice occurs when the Sun is at its greatest distance north (June) or south (December)of the celestial equator.
The imaginary point on the celestial sphere directly over the Earth's South Pole.
south celestial pole
A massive, gaseous body held together by gravity and generally emitting light. Normal stars generate energy by nuclear reactions in their interiors.
The acceleration caused by gravity at the surface of a planet or other body.
The time between repeated configurations of a planet or moon. For example, the time between oppositions of a planet or between full moons.
The passage of a planet directly between the observer and the Sun. At a transit, we see the planet as a dark spot against the Sun's bright disk. From Earth, only Mercury and Venus can transit the Sun.
The largest astronomical structure we know of. The Universe contains all matter and radiation and encompasses all space.
The cluster of galaxy clusters in which the Milky Way is located. The Local Group is one of its members.
The part of the electromagnetic spectrum that we can see with our eyes. It consists of the familiar colors violet, blue, green, yellow, orange and red, extending from about 400 nm to 700 nm.
The distance between wave crests. It determines the color of visible light.
The theory that electromagnetic radiation may be treated as either a particle (photon) or an electromagnetic wave.
Visible light exhibiting no color of its own but composed of a mix of all colors. Sunlight and many artificial light sources are "white".
A relation between a body's temperature and the wavelength at which it emits radiation most intensely. Hotter bodies radiate more intensely at shorter wavelengths.
A band running around the celestial sphere in which the planets move.