Earth in Space Astronomy; Science Explorer, Prentice Hall Why does the Earth have day and night? Astronomy: the study of the moon, stars, and other objects in space Axis: the imaginary line that passes through the Earths center and the North and South poles at an angle of 23.5; Polaris is the North Star. Rotation: Earths spinning on its axis; A point on the equator spin at 1600 km/hr Earths rotation causes day and night Whats in a day? In what cardinal direction does Earth rotate? Where does the sun appear to set? How many hours does one rotation take? How are daytime and nighttime defined? Think about it: Why do the sun and moon appear to move each day? 3 rd Rock From the Sun In addition to rotating on its axis, Earth travels around the sun, our closest star. Revolution: the movement of one object around another object Orbit: the elliptical (or oval) path of revolution around the sun Think about it: How long does one trip around the sun take? The Calendar 1500 B.C., British Isles Stonehenge build by ancient people Giant stones mark directions in which the sun rises and sets on the longest day of the year The Calendar 1300 B.C., China During the Shang dynasty, Chinese astronomers calculated the length of a year is days. 300 B.C., Egypt Egyptian astronomers counted the numbers of days between the first appearance of the star Sirius. Sirius appeared before the Niles spring floods. There were about 365 days. The Calendar 900 A.D., Mexico Mayan astronomers studied the sun, moon, and Venus. Their calendar had 365 days. The Mayans were also able to predict astronomical events 3000 years in the future. Chichen Itza: El Caracol (Mayan Observatory) The Calendar A.D. 1450, Wyoming, USA Big Horn Medicine Wheel built by Native Americans. Individual stones are aligned with rising and setting of sun and stars. Used to determine migration times. The Calendar Earths orbit around the sun takes slightly more that 365 days: 365 days Leap year was created to compensate. Every four years the years has 366 days. The time between two full moons is about 29 days. Hence the month (moonth). Romans altered Egyptian calendar of day months ( days) to the one currently used. Seasons on Earth The Earth is warmer at the equator and cooler at the poles. The equator receives the most direct sunlight. The poles receive sunlight at an angle. Seasons on Earth Earth has seasons due to the 23.5 degree tilt of its axis as Earth moves around the sun. When does the Northern Hemisphere have summer? Winter? Why do the Northern and Southern Hemispheres have opposite seasons? Which areas on Earth do not experience a change of season? Seasons Vocabulary Latitude: measurement of distance from the equator, expressed in degrees north or south (equator = 0, North Pole = 90) Summer: Hemisphere has more direct sunlight and more hours of daylight; winter is the opposite. Solstice: The two days each year when the noon sun is overhead at 23.5 south (winter) or 23.5 north (summer). For the Northern Hemisphere, summer solstice is June 21 and winter solstice is Dec. 21. Southern Hemisphere? Seasons Vocabulary Equinox: Halfway between the solstices, the noon sun is directly over the equator for one day. Means equal night. Day and night are the same length. Vernal equinox (spring): Around March 21 for the Northern Hemisphere. Autumnal equinox (fall): Around Sept. 23 for the Northern Hemisphere. Phases, Eclipses, and Tides Motions of the Moon The moon revolves around Earth in 29.5 days. The moon also completes one rotation in 29.5 days. The moon day is the same length as the moon year and is approximately one month on Earth. The moon has an elliptical orbit. Phases of the Moon Why can you observe the moon in the sky at night? The moon takes many shapes: round circle, thin sliver, crescent shape. Phases: Different shapes of the moon The moon goes through all of its phases each month (1 revolution). What Causes the Phases? Because the sun lights the moon, half of the moon is almost always in sunlight. However, the half of the moon facing the Earth is not always the half that is reflecting sunlight. The phase of the moon depends on how much of the sunlit side of the moon faces Earth. Phases of the Moon The Eight Phases 1.New Moon (dark) 2.Waxing Crescent 3.First Quarter (half lighted) 4.Waxing Gibbous 5.Full Moon (fully lighted) 6.Waning Gibbous 7.Third Quarter (half lighted) 8.Waning Crescent About 29.5 days later: New Moon again 1999, Salzburg - Austria Eclipses What causes an Eclipse? Eclipse: an object (moon) in space comes between the sun and a third object (Earth), casting a shadow on the third object. Solar and Lunar Eclipses Why are eclipses rare? The moons orbit is slightly tilted with respect to the Earths orbit around the sun. The moons shadow rarely hits Earth and Earths shadow rarely covers the moon. Solar Eclipses Solar Eclipse: the new moon passes between the Earth and the sun blocking the sunlight (during the day) Umbra: Darkest part of moons shadow; cone-shaped Small part of Earth experiences a total solar eclipse Penumbra: Lighter part of moons shadow; part of sun is visible Larger part of Earth experiences a partial solar eclipse (Do not look directly at a partial solar eclipse.) Lunar Eclipse sequence from 28th August 2007 taken with SkyWatcher ED100 refractor and Canon PowerShot compact camera. by Steve Massey Lunar Eclipses Lunar Eclipse: during a full moon the Earth passes directly between the moon and the sun; Earths shadow blocks sunlight from hitting the moon Umbra: total lunar eclipse occurs Penumbra: partial lunar eclipse occurs A lunar eclipse can be observed from anywhere on Earth (unlike a solar eclipse) The Tides Tides: The rise and fall of the water approximately every 12.5 hours; The water rises for 6 hours and then falls for 6 hours twice a day, in a regular cycle. Tides occur because of differences in the force of gravity between the moon and different parts of Earth Force of Gravity: Depends on the mass and distance between two objects C & D: Water flowing towards the high tides and away from points C and D creates low tides. A: The moons gravity pulls the oceans more strongly than the Earth. The oceans are drawn towards the moon creating high tide. B: The moons gravity pulls the Earth more strongly than the oceans. The oceans are left behind creating a second high tide. AB C D The Tide Cycle There are always two high tides and two low tides occurring on Earth. Why does Ocean City Beach experience two high tides in one day? Spring and Neap Tides Spring Tides: The highest high tide; The tide with the greatest difference between high and low occurs when the sun, moon, and Earth line up. The gravitational pull of the sun and moon combine forces. Neap Tides: The tide with the least difference between high and low occurs when the gravitational pull of the sun and moon are at right angles. How many times a month do Spring and Neap Tides occur? During which phases? 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