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27.3 The Sun-Earth-Moon System Daily Motions • The moon rises in the east and sets in the west • These daily motions result from the Earth’s rotation • The sun, moon, planets, and stars do not orbit around the Earth every day • It appears that way because we observe the sky from a planet that rotates Earth’s rotation • We have to look at the Earth’s rotation first before we can understand the moon’s daily motions • The time period from one noon to the next is called a SOLAR DAY • Our timekeeping system is based on a solar day • The Earth requires 4 more minutes each day so it can turn a little farther each day to align with the sun • This is because the Earth and spinning and orbiting Annual Motions • Earth orbits the Sun in a slightly elliptical orbit. • This plan of Earth’s orbit is called the ECLIPTIC PLANE • As the Earth rotates, the sun, planets, and constellations appear to move across the sky in a path known as the ECLIPTIC • As Earth moves in orbit, different constellations are visible The Effects of Earth’s Tilt • Earth’s axis is tilted relative to the ecliptic at approximately 23.5° • As Earth orbits the Sun, the orientation of Earth’s axis remains fixed in space so that, the northern hemisphere of Earth is tilted towards the Sun • After six months, the northern hemisphere is tipped away from the sun • What is the result of this? • Seasons! Solstices • At a given solstice, the Sun is overhead at its farthest distance either north or south of the equator • There are 4 positions (1, 2, 3, 4) • Position 1 corresponds to the summer solstice in the northern hemisphere when the sun is directly overhead at the Tropic of Cancer • At this time, June 21st, the number of daylight reaches its maximum • The sun is up all day within the Artic Circle Solstices • On this day, the number of daylight house in the southern hemisphere is at its minimum, and the sun does not appear in the region within the Antarctic Circle • Solstices explained Solstices • As Earth moves past position 2, the suns altitude decreases in the northern hemisphere until earth reaches Position 3, known at the WINTER SOLSTICE • This happens around December 21st • The earth continues around its orbit past Position 4 and onto Position 1 Equinoxes • An equinox are found in position 2 and 4, where the Earth’s axis is perpendicular with the Sun’s rays • The term equinox means ‘equal nights’ Phases of the Moon • Just as the sun appears to change position in the sky throughout the year, the moon also changes position relative to the ecliptic plane as it orbits Earth • The Moon’s cycle is more complex! • The moon’s cycles are called lunar phases Waxing and Waning • Starting at the new moon, as you increase the sunlit side of the moon, it is known as the WAXING PHASE • The waxing phases are called • Waxing crescent • First Quarter • Waxing Gibbous Waxing and Waning • Once you have a full moon, the entire sunlit side of the moon faces the Earth • After this, the moon begins to decrease the amount of sunlit side, known as the WANING PHASE • Waning Gibbous • Third Quarter • Waning Crescent Explained Video Synchronous Rotation • The surface features of the moon always look the same • As the moon orbits Earth, the same sides faces Earth at all times • The Moon spins exactly once each time it goes around the Earth • Explained Lunar Motions • The length of time it takes for the Moon to go through a complete cycle of phases, is called a LUNAR MONTH • The lunar month is about 29.5 days • The moon rises and sets 50 minutes later each day Tides • One effect the Moon has on Earth is causing ocean tides • The moons gravity pulls on Earth along an imaginary line connecting the Earth and the Moon • Spring Tides = High • Neap tides = Low • Tides Explained Solar Eclipses • Occur when the Moon passes directly between the Sun and the Earth and blocks the sun from view • You can have complete solar eclipses and partial solar eclipses • Explained Annular Eclipses • Occurs when the Moon is too far away for its umbral shadow to reach Earth • A ring or annulus, is left uncovered Lunar Eclipses • Occurs when the Moon passes through Earth’s shadow • A total lunar eclipse occurs when the entire moon is within the Earth’s umbra. • Lasts usually 2 hours • Explained