| A | B |
|---|
| geocentric | describes the solar system when it is regarded as having the Earth as its center |
| heliocentric | with the Sun at the center |
| eccentricity | the deviation of the path of an orbiting body from a true circle |
| ellipse | a two-dimensional shape like a stretched circle with slightly longer flatter sides |
| parallax | the angle between two imaginary lines from two different observation points meeting at an astronomical object; used to measure the object's distance from Earth |
| retrograde motion | orbiting in a direction opposite to that of Earth's orbit around the Sun or of the Moon's orbit around the Earth |
| epicycle | (literally: on the circle in Greek) was a geometric model used to explain the variations in speed and direction of the apparent motion of the Moon or the Sun and the planets. |
| deferent | the difference of an objects path as it orbits the earth in the ptolemaic model. |
| semimajor axis | either of the two halves of the longest axis of an ellipse. |
| period | the time required for one complete cycle of a repetitive system |
| gravity | the attraction due to gravitation that the Earth or another astronomical object exerts on an object on or near its surface |
| force | a physical influence that tends to change the position of an object with mass, equal to the rate of change in momentum of the object. |
| acceleration | the rate at which something increases in velocity |
| mass | the property of an object that is a measure of its inertia; the amount of matter it contains; and its influence in a gravitational field. |
| inertia | the property of a body by which it remains at rest or continues moving in a straight line unless acted upon by a directional force |
| inverse square law | a law stating that the magnitude of a physical quantity varies inversely with the square of its distance from its source |
| field | an area or region within which a force exerts an influence at every point |
| velocity | the rate of change in the position of an object as it moves in a particular direction |
| general relativity | describing the relationship of matter and time and space; extending the principles of the first to gravitation and phenomena related to acceleration |
| Ptolemy | (c. AD 90 – c. AD 168) was a Greek-Roman citizen of Egypt who wrote in Greek. He was a mathematician astronomer geographer astrologer and poet of a single epigram in the Greek Anthology. |
| Copernicus | (19 February 1473 – 24 May 1543) was a Renaissance mathematician and astronomer who formulated a comprehensive heliocentric model which placed the Sun- rather than the Earth at the center of the universe. |
| Tycho | (14 December 1546 – 24 October 1601) As an astronomer- Tycho worked to combine what he saw as the geometrical benefits of the Copernican system with the philosophical benefits of the Ptolemaic system into his own model of the universe- the Tychonic system. He was the last of the major naked eye astronomers working without telescopes for his observations. |
| Kepler | December 27, 1571 – November 15, 1630) was a German mathematician, astronomer and astrologer. A key figure in the 17th century scientific revolution- he is best known for his eponymous laws of planetary motion- codified by later astronomers based on his works Astronomia nova Harmonices Mundi and Epitome of Copernican Astronomy. (These works also provided one of the foundations for Isaac Newton's theory of universal gravitation. |
| Newton | (25 December 1642 – 20 March 1726) Developed the three laws of motion and the universla law of gravity to help explain orbital motion |
| Galileo | (15 February 1564 – 8 January 1642)championing of heliocentrism- was controversial within his lifetime- when most subscribed to either geocentrism or the Tychonic system. He met with opposition from astronomers- who doubted heliocentrism due to the absence of an observed stellar parallax. The matter was investigated by the Roman Inquisition in 1615 and they concluded that it could be supported as only a possibility not an established fact. |
| Einstein | (14 March 1879 – 18 April 1955)the development of his special theory of relativity. He realized that the principle of relativity could also be extended to gravitational fields; and with his subsequent theory of gravitation in 1916- he published a paper on the general theory of relativity. |
| Ptolemaic model of the universe | earth centered everything revolved around the earth - later proved incorrect. |
| Newton’s first law of Motion | If an object experiences no net force- then its velocity is constant: the object is either at rest (if its velocity is zero) or it moves in a straight line with constant speed (if its velocity is nonzero). |
| Newton’s second law of Motion | The acceleration a of a body is parallel and directly proportional to the net force (F) acting on the body is in the direction of the net force; and is inversely proportional to the mass m of the body (F = ma). |
| Newton’s third law of Motion | When a first body exerts a force F1 on a second body - the second body simultaneously exerts a force F2 = ?F1 on the first body. This means that F1 and F2 are equal in magnitude and opposite in direction. |
| Kepler's first law | The orbit of every planet is an ellipse with the Sun at one of the two foci. |
| Kepler's second law | A line joining a planet and the Sun sweeps out equal areas during equal intervals of time. |
| Kepler’s third law | The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit. |
| Newton’s law of Universal Gravitation | states that every point mass in the universe attracts every other point mass with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. |
| weight | the affect of gravity on a mass |
| speed | the rate of change in position of an object |
| paragee | the point of an orbit where it is the closest to the object it is orbiting |
| apogee | the point of an orbit where it is the furthest from the object it is orbiting |
| hypothesis | a tentative explanation for a phenomenon, used as a basis for further investigation |
| theory | a set of facts or propositions or principles analyzed in their relation to one another and used to explain phenomena |
| law | a statement of a scientific fact or phenomenon that is invariable under given conditions |
| model | a simplified version of something complex used in analyzing and solving problems or making predictions |
| Black Hole | an area in space with such a strong gravitational pull that no matter or energy can escape from it. Black holes are believed to form when stars collapse in on themselves. |
