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Astronomy DTU
Chapter 1 - 4
| Question | Answer |
|---|---|
| Angle | the opening between two straight lines that meet at a point |
| Angular diameter (angular size) | the arc angle across an object |
| Annular eclipse | an eclipse of the Sun in which the Moon is too distant to cover the Sun completely so that a ring of sunlight is seen around the Moon at mideclipse |
| Arc angle | the measurement of the angle between two objects or two parts of the same object |
| Asteroid (minor planet) | any of the rocky objects larger than a few hundred meters in diameter (and not classified as a planet or moon) that orbits the Sun |
| Autumal equinox | the intersection of the ecliptic and the celestial equator where the Sun crosses equator moving from north to south and the beginning of autumn (around September 23) |
| Black hole | an object whose gravity is so strong that the escape velocity from it exceeds the speed of light |
| Celestial equator | a great circle on the celestial sphere 90 degrees from the celestial poles |
| Celestial pole | point about which the celestial sphere appears to rotate |
| Celestial Sphere | a hypothetical sphere of very large radius centered on the observe; the apparent sphere of the night sky |
| Circumpolar stars | all the stars that never set at a given latitude; all the stars between Polaris and the northern horizon |
| Comet | A small body of ice and dust in orbit about the Sun. While passing near the Sun, a comet's vaporized ices give rise to a coma, tails, and a hydrogen envelope |
| Constellation | any of the 88 contiguous regions that cover he entire celestial sphere, including all the objects in each region; also, a configuration of stars ofen named after an object, a person, or an animal |
| Declination | the coordinate on the celestial sphere exactly analogous to latitude on Earth: measured north and south of the celestial equator |
| Degree | a unit of angular measure or a temperature measure |
| Diurnal motion | cyclic motion with a 1-day period |
| Eclipse path | track of the tip of the Moon's shadow along the Earth's surface during a total or annular solar eclipse |
| Ecliptic | annual path of the Sun on the celestial sphere; the plane of the Earth's orbit around the Sun |
| Equinox | either of the two days of the two days of the year when the Sun crosses the celestial equator and is therefore directly over the Earth's equator |
| Gravitation | tendency of all matter to attract all other matter |
| Line of nodes | line along which the plane of the Moon's orbit intersects the plane of the ecliptic |
| Lunar eclipse | eclipse during which the Earth blocks light that would have struck the Moon |
| Lunar phase | names given to the apparent shapes of the Moon as see3n from Earth |
| Meteoroid | small rock in interplanetary space |
| North celestial pole | location on the celestial sphere directly above the Earth's northern rotation pole |
| Partial eclipse | lunar or solar eclipse in which the eclipsed object does not appear completely covered |
| Penumbra | portion of a shadow in which only part of the light source is covered by the shadow making body |
| Penumbral eclipse | lunar eclipse in which the Moon passes only through the Earth's penumbra |
| Precession | slow, conical motion of the Earth's axis of rotation caused by the gravitational pull of the Moon and Sun on the Earth's equatorial bulge |
| Precession of the equinoxes | slow westward motion of the equinoxes along the ecliptic because of the Earth's precession |
| Revolution | orbit of one body about another |
| Right ascension | celestial coordinate analogous to longitude on Earth and measured around the celestial equator from the vernal equinox |
| Rotation | spinning of a body about an axis passing through it |
| Scientific notation | style of writing large and small numbers using powers of ten |
| Sidereal month | period of the Moon's revolution about the Earth measured with respect to the Moon's location among the stars; 27 & 1/3 Earth days |
| Sidereal period | orbital period of one object about another measured with respect to the stars |
| Solar corona | Sun's outer atmosphere |
| Solar day | From noontime to the next noontime; for Earth it is 24 hours |
| Solar eclipse | eclipse during which the Moon blocks the Sun |
| South celestial pole | location on the celestial sphere directly above the Earth's south rotation pole |
| Summer solstice | point on the ecliptic where the Sun is farthest north of the celestial equator; the day with the largest number of daylight hours in the norther hemisphere |
| Synodic month (lunar month) | period of revolution of the Moon with respect to the Sun; the length of one cycle of lunar phases; 29 and 1/2 Earth days |
| Terminator | line dividing day and night on the surface of any body orbiting the Sun; the line of the sunset or sunrise |
| Time zone | one of the 24 divisions of the Earth's surface separated by 15 degrees along lines of constant longitude ( with allowances for some political boundaries) |
| Total eclipse | solar eclipse during which the Sun is completely hidden by the Moon, or a lunar eclipse during which the Moon is completely immersed in the Earth's umbra |
| Umbra | central, completely dark portion of a shadow |
| Vernal equinox | point on the ecliptic where the Sun crosses the celestial equator from south to north; the beginning of spring, around March 21 |
| Winter solstice | point on the ecliptic where the Sun is farthest south of the celestial equator; fewest hours of daylight in the northern hemisphere, around December 22 |
| Zenith | point on the celestial sphere directly overhead |
| Zodiac | a band of 13 constellations around the sky through which the Sun appears to move throughout the year |
| Acceleration | change in direction or magnitude of a velocity |
| Angular momentum | measure of how much energy an object has stored in its rotation and/or revolution |
| Aphelion | point in its orbit where a planet or other solar system body is farthest from the Sun |
| Astronomical unit | average distance between the Earth and the Sun: 1.5 X 10 to the 8th power km = 93 million mi |
| Configuration (of a planet) | particular geometric arrangement of the Earth, a planet, and the Sun |
| Conjunction | alignment of two bodies in the solar system so that they appear in the same part of the sky as seen from Earth |
| Conservation of angular momentum | law of physics stating that the total amount of angular momentum in an isolated system remains constant |
| Cosmology | study of the formation, organization, and evolution of the universe |
| Direction motion | gradual, eastward apparent motion of a planet against the background stars as seen from Earth |
| Ellipse | closed curve obtained by cutting completely through a circular cone with a plane; the shape of planetary orbits |
| Elongation | angle between a planet and the Sun as seen from Earth |
| Focus (of an ellipse) | two points inside an ellipse, the sum of whose distances from any point on the ellipse is constant |
| Force | which can change the momentum of an object |
| Galilean moons (satellites) | any one of the four large moons of Jupiter (Callisto, Europa, Ganymede, Io) that is visible from Earth through a small telescope |
| Gravity (gravitation) | tendency of all matter to attract all other matter |
| Heliocentric cosmology | theory of the formation and evolution of the solar system with the Sun at the center |
| Hyperbola | open curve obtained by cutting a cone with a plane |
| Inferior conjunction | configuration when Mercury or Venus is directly between the Sun and the Earth |
| Kepler's laws | 3 statements, formulated by Johannes Kepler, that describe the motions of the planets |
| Kinetic energy | energy an object has as a result of its motion |
| Law of equal areas | Kepler's 2nd law |
| Law of inertia | (Newton's 1st law of motion) physical law that an object will stay at rest or move at a constant speed in a fixed direction unless acted upon by an outside force |
| Law of universal gravitation | Newton's law of gravitation, which describes how the gravitational force between 2 bodies depends on their masses and separation |
| Light-year | distance that light travels in a vacuum in 1 year |
| Mass | measure of the total amount of material in an object |
| Model | hypothesis that has withstood observational or experimental tests |
| Moment of inertia | measure of the inertial resistance of an object to changes in the object's rotational motion about the axis |
| Momentum | measure of the inertial resistance of an object to changes in the object's rotational motion about the axis |
| Newton's laws of motion | Newton's equations that describe the motion of matter as a result of forces action on it |
| Occam's razor | principle of choosing the simplest scientific theory that correctly explains any phenomenon |
| Opposition | configuration of a planet when it is at an elongation of 180 degrees and thus appears opposite the Sun in the sky |
| Parabola | open curve formed by cutting a circular cone at an angle parallel to the sides of the cone |
| Parallax | apparent displacement of an object relative to more distant objects caused by fiewing it from different locations |
| Parsec | unit of distance equal to 3.26 light-years |
| Perihelion | point in its orbit where a planet is nearest the Sun |
| Potential energy | energy stored in an object as a result of its location in space |
| Retrograde motion | occasional backward (that is, westward) apparent motion of a planet against the background stars as seen from Earth, Retrograde motion is an optical illusion |
| Scientific method | method of doing science based on observation, experimentation, and he formation of hypotheses (theories) that can be tested |
| Scientific theory | idea about the natural world that is subject to verification and refinement |
| Semimajor axis (of an ellipse) | half of the longest dimension of an ellipse |
| Sideral period | orbital period of the one object about another measured with respect to the stars |
| Superior conjunction | configuration when Mercury or Venus is on the opposite side of the Sun from Earth |
| Synodic period | interval between successive occurrences of the same configuration of a planet as seen from Earth |
| Theory | see scientific theory |
| Universal constant of gravitation | constant of proportionality in Newton's law of gravitation, usually denoted G |
| Velocity | quantity that specifies both direction and speed of an object |
| Weight | force with which a body presses down on the surface of a world such as Earth |
| Work | change in a object's energy as a result of a force being applied to it |
| Active optics | system that adjusts a reflecting telescope in response to changes in temperature and shape of the mount; it helps optimize an image |
| Adaptive optics | primary telescope mirrors that are continuously and automatically adjusted to compensate for the distortion of starlight due to the motion of the Earth's atmosphere |
| Angular resolution (resolution) | angular size of the smallest detail of an astronomical object has stored in its rotation and/or revolution |
| Cassegrain focus | optical arrangement in a reflecting telescope in which light rays are reflected by a secondary mirror through a hole in the primary mirror |
| Charge-coupled device (CCD) | type of solid-state silicon wafer designed to detect photons |
| Coude focus | reflecting telescope in which a series of mirrors direct light to a remote focus away from the moving parts of the telescope |
| Electromagnetic spectrum | entire array of electromagnetic radiation |
| Electromagnetic radiation | radiation consisting of oscillating electric & magnetic fields, namely gamma rays, X rays, visible light, ultraviolet & infrared radiation, & radio waves |
| Eyepiece lens | magnifying lens used to view the image produced at the focus of a telescope |
| Focal length | distance from a lens or concave mirror to where converging light rays meet |
| Focal plane | plane at the focal length of a lens or concave mirror on which an extended object is focused |
| Focal point | aka focus - place at the focal length where light rays from a point object (that is, one that is too distant or tiny to resolve) are converged by a lends or concave mirror |
| Frequency | number of peaks or troughs of a wave that pass a fixed point each second and number also, number of complete vibrations or oscillations per second |
| Gamma ray | most energetic form of electromagnetic radiation |
| Infrared radiation | electromagnetic radiation of a wavelength longer than visible light but shorter than radio waves |
| Interferometry | method of increasing resolving power by combining electromagnetic radiation obtained by 2 or more telescopes |
| Light-gathering power | measure of how much light a telescope intercepts & brings to a focus |
| Magnification (magnifying power) | number of times larger in angular diameter an object appears through a telescope than when it is seen by the naked eye |
| Newtonian reflector | optical arrangement in a reflecting telescope in which a small, flat mirror reflects converging light rays to a focus on 1 side of the telescope tube |
| Objective lens | principal lens of a refracting telescope |
| Photon | discrete unit of electromagnetic energy |
| Pixel | contraction of the term "picture element"; usually refers to 1 square of a grid into which the light-sensitive component of a charge-coupled device is divided |
| Primary mirror | large, concave, light-gathering mirror in a reflecting telescope, analogous to the objective lens on a refracting telescope |
| Prime focus | point in a reflecting telescope where the primary mirror focuses light |
| Radio telescope | telescope designed to detect radio waves |
| Radio wave | long-wavelength electromagnetic radiation |
| Reflecting telescope (reflector) | telescope in which the principal light-gathering component is a concave mirror |
| Reflection | rebounding of light rays off a smooth surface |
| Refracting telescope (refractor) | telescope in which the principal light-gathering component is a lens |
| Refraction | bending of light rays when they pass from 1 transparent medium to another |
| Schmidt corrector plate | specially shaped lens used with spherical mirrors that corrects for spherical aberration and provides an especially wide field of view |
| Secondary mirror | relatively small mirror used in reflecting telescopes to guide the light out the side or bottom of the telescope |
| Seeing disk | size that a star appears to have on a photographic or charge-coupled-device image as a result of the changing refraction of the starlight passing through the Earth's atmosphere |
| Spectrum (plural spectra) | result of electromagnetic radiation passing through a prism or grating so that different wvelengths are separated |
| Spherical aberration | optical property where by different portions of a spherical lens or spherical, concave mirror have slightly different focal lengths, thereby producing a fuzzy image |
| Twinkling | apparent change in a star's brightness, position, or color due to the motion of gases in the Earth's atmosphere |
| Ultraviolet (UV) radiation | electromagnetic radiation of wavelengths shorter than those of visible light but longer than those of X rays |
| Very-long-baseline interferometry (VLBI) | method of connecting widely separated radio telescopes to make observations of very high resolution |
| Wavelength | distance between 1 successive peaks in a wave |
| X ray | electromagnetic radiation whose wavelength is between that of ultraviolet light and gamma rays |
| Absorption line | dark line in a continuous spectrum created when photons of a certain energy are absorbed by atoms or molecules |
| Absorption line spectrum | dark lines superimposed on a continuous spectrum |
| Atom | smallest particle of an element that has the properties characterizing that element |
| Atomic number | number of protons in the nucleus of an atom |
| Blackbody | hypothetical perfect radiator that absorbs & reemits all radiation falling upon it |
| Blackbody curve | curve obtained when the intensity of radiation from a blackbody at a particular temperature is plotted against wavelength |
| Blueshift | shift of all spectral features toward shorter wavelengths; the Doppler shift of light from an approaching source |
| Continuous spectrum (continuum) | spectrum of light over a range of wavelengths without any spectral lines |
| Diffraction grating | optical device consisting of closely spaced lines ruled on a piece of glass that is used like a prism to disperse light into a spectrum |
| Doppler shift or Doppler effect | change in wavelength or radiation due to relative motion between the source & the observer along the line of sight |
| Electromagnetic force | interaction between charged particles, the 2nd of 4 fundamental forces in nature |
| Electron | negatively charged subatomic particle usually found in orbit about the nucleus of an atom |
| Element | substance that cannot be decomposed by chemical means into simpler substances. every atom of the same element contains the same number of protons |
| Emission line | bright line of electromagnetic radiation |
| Emission line spectrum | spectrum that contains only bright emission lines |
| Energy flux | amount of energy emitted from each square meter of an object's surface per second |
| Excited state | orbit of an electron with energy greater than the lowest energy orbit (or state) available to that election |
| Ground state | lowest energy level of an atom |
| Ion | atom that has become electrically charged due to the loss or addition of 1 or more electrons |
| Ionization | process by which an atom loses or gains electrons |
| Isotope | atoms that all have the same number of protons (atomic number) but different numbers of neutrons. Their nuclear properties often differ greatly |
| Kirchhoff's laws | 3 statements formulated by Gustav Kirchhoff describing what physical conditions produce each type of spectra |
| Luminosity | rate at which electromagnetic radiation is emitted from a star or other object |
| Molecule | 1 or more atoms bonded together |
| Neutron | nuclear particle with no electric charge & with a mass nearly equal to that of the proton |
| Nucleus (of an atom) | massive part of an atom, composed of protons & neutrons; electrons surround a nueleus |
| Periodic table | listing of the chemical elements according to their properties; created by D. Mendeleev |
| Planck's law | relationship between the energy carried by a photon & its wavelength |
| Proper motion | change in the location of a star on the celestial sphere |
| Proton | heavy, positively charged nuclear particle |
| Quantum mechanics | branch of physics dealing with the structure & behavior of atoms & their interactions with each other & with light |
| Radial velocity | portion of an object's velocity parallel to the line of sight |
| Radioactive zone | region inside a star where energy is transported outward by the movement of photons through a gas from a hot location to a cooler 1 |
| Redshift | shifting to longer wavelengths of the light from remote galaxies & quasars; the Doppler shift of light from any receding source |
| Spectral analysis | identification of chemicals by the appearance of their spectra |
| Spectrograph | device for photographing a spectrum |
| Spectroscope | device for directly viewing a spectrum |
| Stefan-Boltzmann law | relationship stating that an object emits energy at a rate proportional to the 4th power of its temperature, in Kelvins |
| Strong nuclear force | force that finds protons & neutrons together in nuclei |
| Transition (of an electron) | change in energy & orbit of an electron around an atom or molecule |
| Transverse velocity | portion of an object's velocity perpendicular to our line of sight to it |
| Weak nuclear force | nuclear interaction involved in certain kinds of radioactivity decay |
| Wien's law | relationship that the dominant wavelength of radiation emitted by a blackbody varies inversely with its temperature |
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