The Earth and Its Moon Part 3: The Earth, the Moon, and the Sun
Phases of the Moon • The revolution of the moon around the Earth causes the moon to appear to change shape in the sky. • We see the part of the moon that is reflecting sunlight toward the Earth.
Phases of the Moon • New moon: moon between the sun and the Earth so dark side is facing Earth; usually not visible • Waxing moon: lighted area increasing from new moon to full (right side lit) • Full moon: entire lighted side of the moon is visible • Waning moon: lighted area decreasing from full moon to new (left side lit)
Earthshine • Sometimes the new moon is faintly visible due to light reflected off the Earth onto the moon, or Earthshine. • Earthshine also lets us see the part of the moon facing us that is not sunlit. Earthshine reflecting off the Moon. Photo taken in Helsinki on 21 July 2006, by Ilmari Karonen. The star visible on the right is Beta Tauri
Moonrise and Moonset The moon rises and sets every day The time it does so depends on the phase of the moon. Each day it rises 30 to 70 minutes later than the previous day. The new moon rises at about the same time the sun rises, and it sets at about the same time the sun sets. A waxing moon rises during the day and sets during the night. The full moon rises at about sunset and sets at about sunrise. A waning moon rises during the night and sets during the day.
The lunar phase depends on the Moon's position in orbit around the Earth and the Earth's position in orbit around the sun. This diagram looks down on Earth from north. Earth's rotation and the Moon's orbit are both counter-clockwise here. Sunlight is coming in from the right, as indicated by the yellow arrows.
Moonrise and Moonset Information: http://www.timeanddate.com/worldclock/moonrise.html
Solar Eclipse • Occurs when the new moon comes directly between the sun and the Earth • From Earth, the moon and the sun appear to be about the same size • In a total eclipse, the Moon completely covers the body of the Sun and the solar corona becomes visible. Photo Credit: Luc Viatour / www.Lucnix.be Never look directly at a solar eclipse as it can burn your eyes and blind you.
Solar Eclipse Every shadow has two parts: • Umbra: the small inner shadow • Penumbra: the larger, outer shadow • Someone in the umbra would see a total eclipse. • Someone in the penumbra would see a partial eclipse.
Solar Eclipses: FYI Because the moon’s distance from the Earth varies, sometimes the moon is too far away to block the entire disk of the sun and you get an annular eclipse (B). Since the moon is moving 3.8cm farther away from the Earth each year, the last total solar eclipse will occur in less than 1.4 billion years from now. A total eclipse occurs somewhere on Earth about every 18 months, but only between 360 – 410 years in a given place.
Lunar Eclipse • Occurs when the Earth comes directly between the sun and the moon • Can only occur during a full moon • The Earth’s shadow on the moon dims it to a dark coppery color. The beginning of the November 2003 lunar eclipse Photo Credit: Oliver Stein
Lunar Eclipse: FYI Why does the moon turn red instead of black during a lunar eclipse? Rays of sunlight bend around the edge of Earth by diffraction and fall within the umbra and on to the moon. The scattering effect of the atmosphere filters out the shorter wavelength leaving primarily red light to fall on the moon. As seen from the moon, the edge of Earth lights up like a sunset-red ring of fire. Credit: Graphic artist Larry Koehn; NASA
Tides • The rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotation of the Earth. • In general, high tide and low tide each occur twice a day. • Because the Earth’s rotation and the moon’s orbit are in the same direction, it take about 24 hours and 50 minutes for the moon to reach the same point in the sky on consecutive days. • Therefore, tides occur at progressively later times each day.
Tides • The moon and the Earth exert a gravitational pull on each other. • The closer an object is to another the stronger the gravitational pull exerted. • The ocean at point A therefore experiences a larger gravitational force than the Earth at point B or the ocean at point C. • Because it experiences a larger attraction, it is pulled away from the Earth, toward the Moon, thus producing the bulge on the moon side.
Tides • The gravitational force of the Moon at point B is larger than that exerted at point C. • The bulge on the far side (C) arises because the Earth is pulled away from the ocean on that side. • Inertia overcomes gravitational pull at C and the ocean is not pulled toward the moon as is B. FYI: The atmosphere bulges kilometers while the solid Earth bulges centimeters.
Tides Low Tide High Tide • The bulges are always roughly aligned with the moon. • As the Earth rotates the bulges move across the surface of the Earth. • A bulge produces a high tide and in between the two bulges is low tide. • A given point on the surface will experience two high and two low tides for each rotation of the planet. High Tide Low Tide Reference: http://csep10.phys.utk.edu/astr161/lect/time/tides.html
Tides: The Sun’s Affect • The gravitational pull of the Sun is less than that of the moon because of its much greater distance away, but it is not insignificant. • When the sun, moon and Earth are roughly aligned (new and full moons) the sun’s gravitational pull works with the moon’s to create the highest tides called spring tides. • At the first and last quarters, when the moon and sun form a right angle with the Earth, the lowest high tides, neap tides, occur.
Tides: Animation http://oceanservice.noaa.gov/education/kits/tides/media/supp_tide06a.html