# Astronomy Unit 1

 Length of the Milky Way 100,000 Light Years Distance of the Sun from center of the Milky Way 25,000 Light Years Estimated Star Population in the Milky Way 200 Billion Distance of Milky Way from Andromeda 2.5 Million Zodiac Constellations Constellations that lie on the elliptic plane of the solar system. Rotation Spinning of PlanetsOnly Uranus and Venus rotate clockwise Revolution Orbital motion of the planets around the sun.All planets orbit counter-clockwise.Orbital paths are elliptic. Parahelion Where the planet is closest to the sun in its orbit. Aphelion Where the planet is farthest away from the sun in its orbit. Eccentricity How elliptical, or squashed, the orbital path of a planet is. Obliquity How tilted the planet is on its axis. Obliquity of Earth 23.5 degrees. Solstice The maximum tilt toward or away from the sun.Summer solstice = north pole has 24 hour daylight.Winter solstice = north pole has 24 hour nighttime. Equinox Point where the Earth is neither tilted away or toward the sun.Equal daylight and nighttime hours. Precession The rotational wobble of the earth on its axisTakes 26,000 years to finish one full precession cycle. View of Inferior Planets They rise to greatest elongation and then drop back under the horizon.This is because Earth revolves around the Sun slower than the inferior planets. View of Superior Planets They move forewards, then backwards, then forwards again.This is called retrograde motion.This is because earth revolves around the Sun faster than the superior planets. Eratosthenes Calculated the size of the Earth by using the angle of the Sun's rays on the summer solstice, which still casted a shadow in his region, compared to no shadow at all to an accoplice elsewhere in the world. Aristarchus Calculated the size of the moon by watching the shadow of the Earth during a lunar eclipse.Also calculated the distance of the Earth from the moon and even suggested heliocentric theory. Ptolemy Created the geocentric model, which explained the retrograde motion of inner planets with epicycles, which was worked on a wheeled model of the universe. He also recalculated the circumference of the Earth, which is smaller and wrong. Kepler's Laws of Planetary Motion Planets moved in an elliptical orbit with the Sun at one focus.The closer a planet is to the sun, the faster its motion.There is a mathematical relationship between the planet's distance from the Sun and how long it takes to orbit. (d3 = time it takes to orbit squared) Halley First one to discover the comets are objects orbiting in space, not in theatmosphere. Isaac Newton's Law of Gravity Force of gravity = m1m2 / r2As seen, the force of gravity weakens considerably with distance. Herschel First person to discover a planet unknown to the ancients. (Uranus) Adams/Leverrier Discovered Neptune by investigating Uranus' irregular orbit, leading to the conclusion that another planet must be pulling on it. Clyde Discovered Pluto by taking dual pictures of the night sky and blinking his eyes between each one to see if anything moved. Authorkingofaces11 ID90508 Card SetAstronomy Unit 1 DescriptionDellilo's Class Updated2011-06-13T21:41:22Z Show Answers