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asteroid belt : the region of the solar system between the orbits of Mars and Jupiter in which most asteroids are located; the main belt, where the orbits are generally the most stable, extends from 2.2 to 3.3 AU from the Sun | https://openstax.org/books/astronomy-2e/pages/3-key-terms |
astronomical unit (AU) : the unit of length defined as the average distance between Earth and the Sun; this distance is about 1.5 Ã 108kilometers | https://openstax.org/books/astronomy-2e/pages/3-key-terms |
density : the ratio of the mass of an object to its volume | https://openstax.org/books/astronomy-2e/pages/3-key-terms |
eccentricity : in an ellipse, the ratio of the distance between the foci to the major axis | https://openstax.org/books/astronomy-2e/pages/3-key-terms |
ellipse : a closed curve for which the sum of the distances from any point on the ellipse to two points inside (called the foci) is always the same | https://openstax.org/books/astronomy-2e/pages/3-key-terms |
escape speed : the speed a body must achieve to break away from the gravity of another body | https://openstax.org/books/astronomy-2e/pages/3-key-terms |
focus : (plural: foci) one of two fixed points inside an ellipse from which the sum of the distances to any point on the ellipse is constant | https://openstax.org/books/astronomy-2e/pages/3-key-terms |
gravity : the mutual attraction of material bodies or particles | https://openstax.org/books/astronomy-2e/pages/3-key-terms |
Keplerâs first law : each planet moves around the Sun in an orbit that is an ellipse, with the Sun at one focus of the ellipse | https://openstax.org/books/astronomy-2e/pages/3-key-terms |
Keplerâs second law : the straight line joining a planet and the Sun sweeps out equal areas in space in equal intervals of time | https://openstax.org/books/astronomy-2e/pages/3-key-terms |
Keplerâs third law : the square of a planetâs orbital period is directly proportional to the cube of the semimajor axis of its orbit | https://openstax.org/books/astronomy-2e/pages/3-key-terms |
major axis : the maximum diameter of an ellipse | https://openstax.org/books/astronomy-2e/pages/3-key-terms |
mass : a measure of the amount of material within an object | https://openstax.org/books/astronomy-2e/pages/3-key-terms |
momentum : the measure of the amount of motion of a body; the momentum of a body is the product of its mass and velocity; in the absence of an unbalanced force, momentum is conserved | https://openstax.org/books/astronomy-2e/pages/3-key-terms |
Newtonâs first law : every object will continue to be in a state of rest or move at a constant speed in a straight line unless it is compelled to change by an outside force | https://openstax.org/books/astronomy-2e/pages/3-key-terms |
Newtonâs second law : the change of motion of a body is proportional to and in the direction of the force acting on it | https://openstax.org/books/astronomy-2e/pages/3-key-terms |
Newtonâs third law : for every action there is an equal and opposite reaction (or:the mutual actions of two bodies upon each other are always equal and act in opposite directions) | https://openstax.org/books/astronomy-2e/pages/3-key-terms |
orbit : the path of an object that is in revolution about another object or point | https://openstax.org/books/astronomy-2e/pages/3-key-terms |
orbital period (P) : the time it takes an object to travel once around the Sun | https://openstax.org/books/astronomy-2e/pages/3-key-terms |
orbital speed : the speed at which an object (usually a planet) orbits around the mass of another object; in the case of a planet, the speed at which each planet moves along its ellipse | https://openstax.org/books/astronomy-2e/pages/3-key-terms |
perigee : the point in its orbit where an Earth satellite is closest to Earth | https://openstax.org/books/astronomy-2e/pages/3-key-terms |
perihelion : the point in its orbit where a planet (or other orbiting object) is nearest to the Sun | https://openstax.org/books/astronomy-2e/pages/3-key-terms |
perturbation : a small disturbing effect on the motion or orbit of a body produced by a third body | https://openstax.org/books/astronomy-2e/pages/3-key-terms |
satellite : an object that revolves around a planet | https://openstax.org/books/astronomy-2e/pages/3-key-terms |
semimajor axis : half of the major axis of a conic section, such as an ellipse | https://openstax.org/books/astronomy-2e/pages/3-key-terms |
velocity : the speed and direction a body is movingâfor example, 44 kilometers per second toward the north galactic pole | https://openstax.org/books/astronomy-2e/pages/3-key-terms |
Tycho Braheâs accurate observations of planetary positions provided the data used by Johannes Kepler to derive his three fundamental laws of planetary motion. Keplerâs laws describe the behavior of planets in their orbits as follows: (1) planetary orbits are ellipses with the Sun at one focus; (2) in equal interval... | https://openstax.org/books/astronomy-2e/pages/3-summary |
In hisPrincipia, Isaac Newton established the three laws that govern the motion of objects: (1) objects continue to be at rest or move with a constant velocity unless acted upon by an outside force; (2) an outside force causes an acceleration (and changes the momentum) for an object; and (3) for every action there is a... | https://openstax.org/books/astronomy-2e/pages/3-summary |
Gravity, the attractive force between all masses, is what keeps the planets in orbit. Newtonâs universal law of gravitation relates the gravitational force to mass and distance: | https://openstax.org/books/astronomy-2e/pages/3-summary |
The force of gravity is what gives us our sense of weight. Unlike mass, which is constant, weight can vary depending on the force of gravity (or acceleration) you feel. When Keplerâs laws are reexamined in the light of Newtonâs gravitational law, it becomes clear that the masses of both objects are important for th... | https://openstax.org/books/astronomy-2e/pages/3-summary |
The closest point in a satellite orbit around Earth is its perigee, and the farthest point is its apogee (corresponding to perihelion and aphelion for an orbit around the Sun). The planets follow orbits around the Sun that are nearly circular and in the same plane. Most asteroids are found between Mars and Jupiter in t... | https://openstax.org/books/astronomy-2e/pages/3-summary |
The orbit of an artificial satellite depends on the circumstances of its launch. The circular satellite velocity needed to orbit Earthâs surface is 8 kilometers per second, and the escape speed from our planet is 11 kilometers per second. There are many possible interplanetary trajectories, including those that use g... | https://openstax.org/books/astronomy-2e/pages/3-summary |
Calculating the gravitational interaction of more than two objects is complicated and requires large computers. If one object (like the Sun in our solar system) dominates gravitationally, it is possible to calculate the effects of a second object in terms of small perturbations. This approach was used by John Couch Ada... | https://openstax.org/books/astronomy-2e/pages/3-summary |
apparent solar time : time as measured by the position of the Sun in the sky (the time that would be indicated by a sundial) | https://openstax.org/books/astronomy-2e/pages/4-key-terms |
declination : the angular distance north or south of the celestial equator | https://openstax.org/books/astronomy-2e/pages/4-key-terms |
great circle : a circle on the surface of a sphere that is the curve of intersection of the sphere with a plane passing through its center | https://openstax.org/books/astronomy-2e/pages/4-key-terms |
International Date Line : an arbitrary line on the surface of Earth near longitude 180° across which the date changes by one day | https://openstax.org/books/astronomy-2e/pages/4-key-terms |
lunar eclipse : an eclipse of the Moon, in which the Moon moves into the shadow of Earth; lunar eclipses can occur only at the time of full moon | https://openstax.org/books/astronomy-2e/pages/4-key-terms |
mean solar time : time based on the rotation of Earth; mean solar time passes at a constant rate, unlike apparent solar time | https://openstax.org/books/astronomy-2e/pages/4-key-terms |
meridian : a great circle on the terrestrial or celestial sphere that passes through the poles | https://openstax.org/books/astronomy-2e/pages/4-key-terms |
phases of the Moon : the different appearance of light and dark on the Moon as seen from Earth during its monthly cycle, from new moon to full moon and back to new moon | https://openstax.org/books/astronomy-2e/pages/4-key-terms |
right ascension : the coordinate for measuring the east-west positions of celestial bodies; the angle measured eastward along the celestial equator from the vernal equinox to the hour circle passing through a body | https://openstax.org/books/astronomy-2e/pages/4-key-terms |
sidereal day : Earthâs rotation period as defined by the positions of the stars in the sky; the time between successive passages of the same star through the meridian | https://openstax.org/books/astronomy-2e/pages/4-key-terms |
sidereal month : the period of the Moonâs revolution about Earth measured with respect to the stars | https://openstax.org/books/astronomy-2e/pages/4-key-terms |
solar day : Earthâs rotation period as defined by the position of the Sun in the sky; the time between successive passages of the Sun through the meridian | https://openstax.org/books/astronomy-2e/pages/4-key-terms |
solar eclipse : an eclipse of the Sun by the Moon, caused by the passage of the Moon in front of the Sun; solar eclipses can occur only at the time of the new moon | https://openstax.org/books/astronomy-2e/pages/4-key-terms |
solar month : the time interval in which the phases repeatâsay, from full to full phase | https://openstax.org/books/astronomy-2e/pages/4-key-terms |
synchronous rotation : when a body (for example, the Moon) rotates at the same rate that it revolves around another body | https://openstax.org/books/astronomy-2e/pages/4-key-terms |
tides : alternate rising and falling of sea level caused by the difference in the strength of the Moonâs gravitational pull on different parts of Earth | https://openstax.org/books/astronomy-2e/pages/4-key-terms |
The terrestrial system of latitude and longitude makes use of the great circles called meridians. Longitude is arbitrarily set to 0° at the Royal Observatory at Greenwich, England. An analogous celestial coordinate system is called right ascension (RA) and declination, with 0° of declination starting at the vernal eq... | https://openstax.org/books/astronomy-2e/pages/4-summary |
The familiar cycle of the seasons results from the 23.5° tilt of Earthâs axis of rotation. At the summer solstice, the Sun is higher in the sky and its rays strike Earth more directly. The Sun is in the sky for more than half of the day and can heat Earth longer. At the winter solstice, the Sun is low in the sky and... | https://openstax.org/books/astronomy-2e/pages/4-summary |
The basic unit of astronomical time is the dayâeither the solar day (reckoned by the Sun) or the sidereal day (reckoned by the stars). Apparent solar time is based on the position of the Sun in the sky, and mean solar time is based on the average value of a solar day during the year. By international agreement, we de... | https://openstax.org/books/astronomy-2e/pages/4-summary |
The fundamental problem of the calendar is to reconcile the incommensurable lengths of the day, month, and year. Most modern calendars, beginning with the Roman (Julian) calendar of the first century BCE, neglect the problem of the month and concentrate on achieving the correct number of days in a year by using such co... | https://openstax.org/books/astronomy-2e/pages/4-summary |
The Moonâs monthly cycle of phases results from the changing angle of its illumination by the Sun. The full moon is visible in the sky only during the night; other phases are visible during the day as well. Because its period of revolution is the same as its period of rotation, the Moon always keeps the same face tow... | https://openstax.org/books/astronomy-2e/pages/4-summary |
The twice-daily ocean tides are primarily the result of the Moonâs differential force on the material of Earthâs crust and ocean. These tidal forces cause ocean water to flow into two tidal bulges on opposite sides of Earth; each day, Earth rotates through these bulges. Actual ocean tides are complicated by the add... | https://openstax.org/books/astronomy-2e/pages/4-summary |
The Sun and Moon have nearly the same angular size (about 1/2°). A solar eclipse occurs when the Moon moves between the Sun and Earth, casting its shadow on a part of Earthâs surface. If the eclipse is total, the light from the bright disk of the Sun is completely blocked, and the solar atmosphere (the corona) comes... | https://openstax.org/books/astronomy-2e/pages/4-summary |
absorption spectrum : a series or pattern of dark lines superimposed on a continuous spectrum | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
blackbody : an idealized object that absorbs all electromagnetic energy that falls onto it | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
continuous spectrum : a spectrum of light composed of radiation of a continuous range of wavelengths or colors, rather than only certain discrete wavelengths | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
dispersion : separation of different wavelengths of white light through refraction of different amounts | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
Doppler effect : the apparent change in wavelength or frequency of the radiation from a source due to its relative motion away from or toward the observer | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
electromagnetic radiation : radiation consisting of waves propagated through regularly varying electric and magnetic fields and traveling at the speed of light | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
electromagnetic spectrum : the whole array or family of electromagnetic waves, from radio to gamma rays | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
emission spectrum : a series or pattern of bright lines superimposed on a continuous spectrum | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
energy flux : the amount of energy passing through a unit area (for example, 1 square meter) per second; the units of flux are watts per square meter | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
energy level : a particular level, or amount, of energy possessed by an atom or ion above the energy it possesses in its least energetic state; also used to refer to the states of energy an electron can have in an atom | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
excitation : the process of giving an atom or an ion an amount of energy greater than it has in its lowest energy (ground) state | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
frequency : the number of waves that cross a given point per unit time (in radiation) | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
gamma rays : photons (of electromagnetic radiation) of energy with wavelengths no longer than 0.01 nanometer; the most energetic form of electromagnetic radiation | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
ground state : the lowest energy state of an atom | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
infrared : electromagnetic radiation of wavelength 103â106nanometers; longer than the longest (red) wavelengths that can be perceived by the eye, but shorter than radio wavelengths | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
inverse square law : (for light) the amount of energy (light) flowing through a given area in a given time decreases in proportion to the square of the distance from the source of energy or light | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
ion : an atom that has become electrically charged by the addition or loss of one or more electrons | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
ionization : the process by which an atom gains or loses electrons | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
isotope : any of two or more forms of the same element whose atoms have the same number of protons but different numbers of neutrons | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
microwave : electromagnetic radiation of wavelengths from 1 millimeter to 1 meter; longer than infrared but shorter than radio waves | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
nucleus (of an atom) : the massive part of an atom, composed mostly of protons and neutrons, and about which the electrons revolve | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
photon : a discrete unit (or âpacketâ) of electromagnetic energy | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
radial velocity : motion toward or away from the observer; the component of relative velocity that lies in the line of sight | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
radio waves : all electromagnetic waves longer than microwaves, including radar waves and AM radio waves | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
spectrometer : an instrument for obtaining a spectrum; in astronomy, usually attached to a telescope to record the spectrum of a star, galaxy, or other astronomical object | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
Stefan-Boltzmann law : a formula from which the rate at which a blackbody radiates energy can be computed; the total rate of energy emission from a unit area of a blackbody is proportional to the fourth power of its absolute temperature:F= ÏT4 | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
ultraviolet : electromagnetic radiation of wavelengths 10 to 400 nanometers; shorter than the shortest visible wavelengths | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
visible light : electromagnetic radiation with wavelengths of roughly 400â700 nanometers; visible to the human eye | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
wavelength : the distance from crest to crest or trough to trough in a wave | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
Wienâs law : formula that relates the temperature of a blackbody to the wavelength at which it emits the greatest intensity of radiation | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
X-rays : electromagnetic radiation with wavelengths between 0.01 nanometer and 20 nanometers; intermediate between those of ultraviolet radiation and gamma rays | https://openstax.org/books/astronomy-2e/pages/5-key-terms |
James Clerk Maxwell showed that whenever charged particles change their motion, as they do in every atom and molecule, they give off waves of energy. Light is one form of this electromagnetic radiation. The wavelength of light determines the color of visible radiation. Wavelength (λ) is related to frequency (f) and th... | https://openstax.org/books/astronomy-2e/pages/5-summary |
The electromagnetic spectrum consists of gamma rays, X-rays, ultraviolet radiation, visible light, infrared, and radio radiation. Many of these wavelengths cannot penetrate the layers of Earthâs atmosphere and must be observed from space, whereas othersâsuch as visible light, FM radio and TVâcan penetrate to Eart... | https://openstax.org/books/astronomy-2e/pages/5-summary |
A spectrometer is a device that forms a spectrum, often utilizing the phenomenon of dispersion. The light from an astronomical source can consist of a continuous spectrum, an emission (bright line) spectrum, or an absorption (dark line) spectrum. Because each element leaves its spectral signature in the pattern of line... | https://openstax.org/books/astronomy-2e/pages/5-summary |
Atoms consist of a nucleus containing one or more positively charged protons. All atoms except hydrogen can also contain one or more neutrons in the nucleus. Negatively charged electrons orbit the nucleus. The number of protons defines an element (hydrogen has one proton, helium has two, and so on) of the atom. Nuclei ... | https://openstax.org/books/astronomy-2e/pages/5-summary |
When electrons move from a higher energy level to a lower one, photons are emitted, and an emission line can be seen in the spectrum. Absorption lines are seen when electrons absorb photons and move to higher energy levels. Since each atom has its own characteristic set of energy levels, each is associated with a uniqu... | https://openstax.org/books/astronomy-2e/pages/5-summary |
If an atom is moving toward us when an electron changes orbits and produces a spectral line, we see that line shifted slightly toward the blue of its normal wavelength in a spectrum. If the atom is moving away, we see the line shifted toward the red. This shift is known as the Doppler effect and can be used to measure ... | https://openstax.org/books/astronomy-2e/pages/5-summary |
adaptive optics : systems used with telescopes that can compensate for distortions in an image introduced by the atmosphere, thus resulting in sharper images | https://openstax.org/books/astronomy-2e/pages/6-key-terms |
aperture : diameter of the primary lens or mirror of a telescope | https://openstax.org/books/astronomy-2e/pages/6-key-terms |
charge-coupled device (CCD) : array of high-sensitivity electronic detectors of electromagnetic radiation, used at the focus of a telescope (or camera lens) to record an image or spectrum | https://openstax.org/books/astronomy-2e/pages/6-key-terms |
chromatic aberration : distortion that causes an image to appear fuzzy when each wavelength coming into a transparent material focuses at a different spot | https://openstax.org/books/astronomy-2e/pages/6-key-terms |
detector : device sensitive to electromagnetic radiation that makes a record of astronomical observations | https://openstax.org/books/astronomy-2e/pages/6-key-terms |
eyepiece : magnifying lens used to view the image produced by the objective lens or primary mirror of a telescope | https://openstax.org/books/astronomy-2e/pages/6-key-terms |
focus : (of telescope) point where the rays of light converged by a mirror or lens meet | https://openstax.org/books/astronomy-2e/pages/6-key-terms |
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