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Astronomy 1-3
| Term | Definition |
|---|---|
| field of view | the area visible in an image. usually given as the diameter of a region |
| scientific notation | the system of recording very large or very small numbers by using powers of 10 |
| solar system | the sun and its planets, asteroids, comets, and so on |
| planet | a nonluminous body in orbit around a star, large enough to be spherical and to have cleared its orbital zone of other objects |
| star | a globe of gas held together by its own gravity and supported by the internal pressure of its hot gases, which generate energy by nuclear fusion |
| astronomical unit | average distance from Earth to the sun; 1.5x10^8 kilometers (93 million miles) |
| light year (ly) | unit of distance equal to the distance light travels in one year |
| galaxy | a large system of stars, star clusters, gas, dust, and nebulae orbiting a common center of mass |
| milky way | the hazy band of light that circles our sky, produced by the glow of our galaxy |
| milky way galaxy | the spiral galaxy containing our sun, visible in the night sky as the milky way |
| spiral arms | long spiral pattern of bright stars, star clusters, gas and dust. they extend across the disks of spiral galaxies |
| supercluster | a cluster of galaxy clusters |
| constellations | one of the stellar patterns identified by name, usually of mythological gods, people, animals, or objects. also the region of the sky containing that star pattern |
| asterism | a named grouping of stars that is not one of the recognized constellations |
| magnitude scale | the astronomical brightness scale. the larger the number, the fainter the star |
| apparent visual magnitude | a measure of the brightness of a star as seen by human eyes on Earth |
| flux | a measure of the flow of energy through a surface. usually applied to light |
| celestial sphere | an imaginary sphere of very large radius surrounding Earth to which the planets, stars, sun, and moon seem to be attached |
| scientific model | a concept that organizes thought about an aspect of nature without necessarily being literally true |
| precession | the slow change in orientation of the Earth's axis of rotation. one cycle takes nearly 26,000 years |
| zenith | the top of the sky above an observers head |
| north celestial pole | located directly above Earth's north pole |
| south celestial pole | located directly below Earth's south pole |
| celestial equator | lies halfway between the celestial poles |
| north point and south point | points on the horizon closest to the celestial poles |
| east point and west point | lie halfway between the north and south points, where the celestial equator always touches the horizon |
| angular distance | angle between two lines extending from your eye to the two objects |
| arc minutes | 1/60th of a degree measurement |
| arc seconds | 1/60th of an arc minute |
| angular diameter | angular distance from one edge to the other |
| circumpolar constellations | groups of stars that never rise or set |
| rotation | motion around an axis passing through the rotating body |
| revolution | orbital motion about a point located outside the orbiting body |
| ecliptic | the apparent path of the sun around the sky |
| zodiac | a band centered on the ecliptic and enciricling the sky |
| evening star | any planet visible in the sky just after sunset |
| morning star | any planet visible in the sky just before sunrise |
| vernal equinox | point where the sun crosses the celestial equator going northward |
| summer solstice | point where the sun is farthest north |
| autumnal equinox | point where the sun crosses the celestial equator going southward |
| winter solstice | point where the sun is farthest south |
| perihelion | Earth's closest point to the sun |
| aphelion | Earth's most distant point from the sun |
| sidereal period | how long the moon takes to circle the sky once and return to the same position among the stars |
| 27.32 days | how long does it take for the moon to orbit the Earth |
| sydonic period | a complete cycle of lunar phases |
| 29.53 days | how long is the moon's sydonic period |
| solar eclipse | the event that occurs when the moon passes directly between Earth and the sun, blocking your view of the sun |
| umbra | the region of a shadow that is totally shaded |
| penumbra | the portion of a shadow that is only partially shaded |
| annular eclipse | a solar eclipse in which the solar photosphere appears around the edge of the moon in a bright ring, or annulus. features of the solar atmosphere cannot be seen during this type of eclipse |
| lunar eclipse | the darkening of the moon when it moves through Earth's shadow |
| Saros cycle | an 18-year, 11 1/3-day period after which the pattern of lunar and solar eclipses repeats |
| first principles | something that seems obviously true and needs no further examination |
| geocentric universe | a model universe with Earth at the center, such as the Ptolemaic universe |
| Heliocentric universe | a model of the universe with the sun at the center, such as the Copernican universe |
| uniform circular motion | all motion in the heavens must be made up of combinations of circles turning at uniform rates |
| parallax | the apparent motion of an object because of the motion of the observer |
| retrograde motion | backward motion |
| epicycle | small circle making loops around a larger circle |
| deferent | the larger circle with loops rotating around |
| equant | point from which the offset Earth seems to be moving at a constant rate |
| paradigm | a commonly accepted set of scientific ideas and assumptions |
| ellipse | a closed curve around two points called the foci, such that the total distance from one focus to the curve and back to the other focus remains constant |
| semi-major axis | half of the longest diameter of an ellipse |
| eccentricity | a number between 1 and 0 that describes the shape of an ellipse; the distance from one focus to the center of the ellipse divided by the semi-major axis |
| empirical | description of a phenomenon without explaining why it occurs |
| hypothesis | a conjecture, subject to further tests, that accounts for a set of facts |
| theory | a system of assumptions and principles applicable to a wide range of phenomena that has been repeatedly verified |
| natural law | a theory that has been so well confirmed that it is almost universally accepted as correct |
| mass | a measure of the amount of matter making up an object |
| weight | the force that gravity exerts on an object |
| inverse square relation | a rule that the strength of an effect (such as gravity) decreases in proportion as the distance squared increases |
| spring tide | ocean tide of large range that occurs at full and new moon |
| neap tide | ocean tide of small range occurring at first- and third- quarter moons |
| circular velocity | the velocity needed to stay in a circular orbit |
| geosynchronous satellite | orbits eastward with the rotation of Earth and remains above a fixed spot - ideal for communications and weather satellites |
| center of mass | the balance point of the gravitational system |
| closed orbits | return the orbiting object to its starting point |
| escape velocity | the velocity needed to leave a body |
| open orbit | does not return the object to Earth |
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