22.2 The Earth-Moon-Sun System
Motions of Earth Rotation – turning or spinning of a body on its axis Day & Night 24 hr. time frame Solar Day or Sidereal Day Revolution – motion of a body along a path around some point in space Perihelion – closest to sun (Jan. 3) Aphelion – Farthest from sun (July 4)
Earth’s Axis & Seasons Axis tilted about 23.5 degrees Tilted axis = yearly cycle of seasons Spring Equinox (March 20-21) Autumn Equinox (September 22-23) Summer Solstice (June 21-22) Winter Solstice (December 21-22)
Precession Slow Axis traces a circle on the sky Similar to the wobble of a spinning top Minor effect on seasons b/c the angle of tilt changes only slightly
Earth-Sun Motion Sun revolves around the galaxy Trip takes 230 million years to traverse at speeds approaching 250 km per second Galaxies themselves are also in motion
Motions of the Earth-Moon System Moon orbits Earth in about a month’s time Perigee – moon is closest to Earth Apogee – Moon is farthest from Earth Constantly change the relative positions of the sun, Earth, and Moon
Phases of the Moon Monthly Crescent Phase Waxing First-quarter phase Full Moon phase Waning New-moon phase Lunar phases are a result of the motion of the moon & the sunlight that is reflected from its surface pg. 626 Fig. 15 A & B
Lunar Motions Cycle requires 29 ½ days = moon cycle = synodic month Basis for the Roman calendar Moon’s revolution around Earth & not the true period which only takes 27 1/3 days = sidereal month Moon’s period of rotation on its axis & its revolution around Earth are the same
Eclipses Shadow effects When the moon moves in a line directly btw Earth & Sun it casts a dark shadow on Earth = Solar Eclipse Moon is eclipsed when it moves within Earth’s shadow = Lunar Eclipse During a new-moon or full-moon phase, the moon’s orbit must cross the plane of the ecliptic for an eclipse to take place Usually 4 eclipses (in sets)
REVIEW REVIEW REVIEW Why don’t eclipses occur during every full- moon or new-moon phase? In what ways does Earth move? What causes the phases of the moon? Describe the locations of the sun, moon, & Earth during a solar eclipse & during a lunar eclipse.
22.3 Earth’s Moon
The Lunar Surface Moon has no atmosphere or water Erosion occurs b/c the lack of a protective atmosphere Most obvious structures are craters (round depressions in the surface) Produced mainly by the impact of rapidly moving debris The heat generated by the impact is enough to melt rock
Continued… Most of lunar surface is made up of densely pitted, light-colored areas = Highlands Within the highland regions are mountain ranges Dark relatively smooth area on the moon’s surface = Mare Maria, ancient beds of basaltic lava, originated when asteroids punctured the lunar surface, letting magma bleed out. Long channels that are associated with maria = Rilles
Continued… All lunar terrains are mantled with a layer of gray debris derived from a few billion years of bombardment from meteorites = lunar regolith Composed of igneous rocks, glass beads, & fine lunar dust
Lunar History Moon is our nearest planetary neighbor Most widely accepted model for the origin of the moon is that when the solar system was forming, a body the size of Mars impacted Earth Impact would have liquefied Earth’s surface & ejected huge amounts of crustal & mantle rock from an infant Earth A portion of this ejected debris would have entered an orbit around Earth where it combined to form the moon
REVIEW REVIEW REVIEW How do craters form? How did maria originate? What are the stages that formed the moon?