Astronomy Chapter 2 — Study Materials Flashcards

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Scientific method

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A systematic process for developing and testing explanations about the natural world. It includes observation, making testable hypotheses, conducting experiments or tests, forming theories, and deriving scientific laws when appropriate.

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Observation

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The first step of the scientific method involving careful recording of phenomena using the senses or instruments. Observations provide the data needed to form hypotheses and test theories.

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Hypothesis

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A tentative, testable statement that explains a set of observations. A good hypothesis must be falsifiable and lead to predictions that can be tested by experiment or observation.

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Theory

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A well-supported explanation of some aspect of the natural world that integrates facts, laws, inferences, and tested hypotheses. Theories can be refined as new evidence becomes available but are broader and better supported than single hypotheses.

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Scientific law

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A concise statement or mathematical expression that reliably describes a pattern in nature under given conditions. Laws summarize observations but do not explain why the pattern exists.

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Triangulation

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A geometric method for measuring distances to faraway objects by observing them from two different viewpoints. It relies on measuring angles and applying trigonometry to determine the distance.

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Parallax

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The apparent shift in position of a nearby object when viewed from different vantage points. Parallax is the angular effect used in triangulation to measure stellar and other distances.

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Eratosthenes

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A 3rd-century BC Greek scholar who estimated Earth's circumference by comparing shadow angles at two cities on the summer solstice. He concluded Earth is spherical and produced an early, surprisingly accurate size estimate.

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Geocentric model

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An Earth-centered model of the universe that places Earth at the center of planetary and stellar motions. The Ptolemaic geocentric model used deferents and epicycles to explain observed planetary retrograde motion.

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Heliocentric model

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A Sun-centered model in which planets orbit the Sun. Copernicus revived this model, which better explained variations in planetary brightness and apparent looping motions, though circular orbits were initially assumed.

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Retrograde motion

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The apparent westward looping motion of a planet against the background stars as seen from Earth. It is an observational effect caused when Earth overtakes and passes an outer planet, or when orbital geometry causes inner-planet viewing effects.

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Epicycle

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A small circular motion whose center moves along a larger circular orbit (deferent) in the Ptolemaic geocentric model. Epicycles were introduced to reproduce the observed complex planetary motions such as retrograde loops.

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Galileo's discoveries

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Observations made with an early telescope that supported heliocentrism: Moon topography, sunspots demonstrating solar rotation, four large moons orbiting Jupiter, and the phases of Venus. These findings challenged the notion that everything orbited Earth.

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Kepler's First Law

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Planetary orbits are ellipses with the Sun at one focus. This replaced the idea of perfect circular orbits and introduced orbital eccentricity as a key descriptor of orbit shape.

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Kepler's Second Law

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A line joining a planet and the Sun sweeps out equal areas in equal times, which implies planets move faster when nearer the Sun and slower when farther away. This law reflects conservation of angular momentum in orbital motion.

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Kepler's Third Law

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The square of a planet's orbital period is proportional to the cube of its semimajor axis, commonly expressed as $P^2=a^3$ when $P$ is in years and $a$ in astronomical units. It relates orbital size to orbital period.

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Orbital eccentricity

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A measure of how stretched an ellipse is, defined as the ratio of the distance between the center and a focus to the semimajor axis. Eccentricity ranges between 0 (circle) and 1 (parabolic limit).

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Newton's law of gravitation

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A universal law stating the gravitational force between two masses is $F=G\frac{M_1 M_2}{R^2}$. The force is proportional to the product of the masses and inversely proportional to the square of their separation.

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Inverse-square law

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A relationship where intensity or force decreases in proportion to the square of the distance from the source. For gravity and light, doubling the distance reduces the effect to one quarter of its original strength.