The Earth exhibits two main motions: • Rotation – the turning, or spinning, of a body on its axis. • Revolution – the motion of a body, such as a planet or moon, along its orbit around some point in space. The Earth also has another very slow motion. • Precession – the wobbling of a body around its axis of rotation.
The main results of the Earth’s rotation (W to E) are day and night. • Takes about 24 hours moving approx. 1000 mph! • Axis of rotation tilted about 23.5 We can measure Earth’s day in 2 ways: • Synodic (Solar) – one complete rotation with respect to the Sun 24 hours. • Sidereal – one complete rotation with respect to distant stars 23 hours, 56 minutes.
Earth revolves around the Sun in a very slight elliptical orbit. (Kepler’s 1st Law) • Takes 365.25 days traveling at an average speed of 67,000 mph! • Average distance of 93 million miles. However, due to its slightly elliptical orbit, Earth’s distance from the Sun varies. • Perihelion – Earth is closest to the Sun. 91 million miles. • Aphelion – Earth is farthest from the Sun. 95 million miles.
Winter Solstice Summer Solstice • December 21 • June 21 • SDR 23.5 S • SDR 23.5 N (Tropic of Capricorn) (Tropic of Cancer) • 10 hrs. day, 14 hrs. night • 14 hrs. day, 10 hrs. night Spring (Vernal) Equinox Fall (Autumnal) Equinox • March 21 • September 21 • SDR Equator • SDR Equator • 12 hrs. day, 12 hrs. night • 12 hrs. day, 12 hrs. night http://astro.unl.edu/classaction/animations/coordsmotion/eclipticsimulator.html
The motion of Earth’s axis • A lot of astronomers link as it traces out a circle on climate change to this. the sky. • Presently, the axis of the Earth points to the star, Polaris Amount of time it takes to (North Star). complete one circle is • In about 13,000 years, it will 26,000 years! point to Vega!
Earth has one natural satellite, the moon (Luna), which also makes an elliptical orbit around the Earth. • Due to this, its distance from Earth varies from time to time. • Perigee – moon is closest to Earth. • Apogee – moon is farthest away from Earth. • The motions of the Earth-moon system constantly change in relation to the Sun, Earth, and moon. This is why the moon appears differently throughout each month.
The lunar phases are caused by the changes in how much of the illuminated (sunlit) side of the moon faces Earth. • Half the moon is always illuminated! The moon produces none of its own light.
1st Phase NEW MOON • None of the moon appears illuminated as viewed from Earth. Waxing – phases during which the lit portion of the moon increases from the RIGHT. Waning – phases during which the lit portion of the moon decreases and only the LEFT hand side remains illuminated.
Thecycle of the moon through all 8 phases takes 29.5 days. • This is one complete revolution with respect to the sun and is called a synodic month. Thetrue period of revolution for the moon is actually 27.3 days. • This revolution is known as a sidereal month, since it is with respect to distant stars. • The reason it takes longer to go through the phases is because as the moon is revolving around Earth, the Earth-moon system is also revolving around the Sun.
Themoon is not only revolving around the Earth, but also rotating upon its own axis. • One complete rotation on its axis takes 27.3 days. • You may recall this number is the sidereal month time. Therefore, the moon revolves around the Earth at the same rate it rotates on its axis. This is called synchronous rotation.
Due to the moon’s synchronous rotation, its surface experiences periods of daylight and darkness lasting about 2 weeks each. This, along with no atmosphere on the moon accounts for the temperature extremes of 127 C (261 F) on the day side of the moon and -173 C (-279 F) on the night side.
Throughout early astronomy, the Greeks realized shadows on certain celestial bodies such as the Earth and moon. When the moon moves in a line directly between Earth and the Sun, it casts a dark shadow on Earth and is known as a solar eclipse. Occur during the new moon phase, as viewed from Earth.
When the Earth is positioned between the Sun and the moon, Earth casts its shadow on the moon, and a lunar eclipse occurs. Occur during the full moon phase, as viewed from the Earth. On average, there are 4 eclipses each year, 2 of each kind (solar and lunar). The zone of full shadow is known as the umbra. The zone of partial shadow is known as the penumbra.
A BSolar Eclipse Lunar Eclipse
Why doesn’t a solar eclipse occur with every new moon and a lunar eclipse occur with every full moon? They would if the orbit of the moon lay exactly along the plane of Earth’s orbit around the Sun. However, the tilt of the moon’s path around Earth is 5 .
During a total solar eclipse, the moon completely blocks out the solar disk of the Sun for up to seven minutes. • Temperature sharply decreases a few degrees during this time. • Visible only to people within the moon’s umbra. A partial eclipse is seen by those in penumbra. • Total solar eclipses are very rare at any location. Next one visible from the United States 8-21-17
During a total lunar eclipse, the moon is completely within Earth’s umbra, but is still visible as a reddish-orange disk. • This is due to Earth’s atmosphere scattering sunlight. Longer, red wavelengths reflect • Visible to anyone on the side of the Earth facing the moon. Total lunar eclipse can last up to almost 2 hours!
Our natural satellite, the moon, is about 1/4 the diameter of Earth. • Much of what we know about the moon comes from the Apollo missions. Six Apollo spacecraft landed on the moon between 1969 and 1972. Gravitational attraction at the lunar surface is 1/6 of that experienced on Earth’s surface. • Example: 150 lb. person on Earth weighs only 25 lb. on the moon.
Most widely accepted theory for the origin of the moon is when the solar system was forming, an object the size of Mars impacted the Earth. • Debris was ejected from this collision and eventually entered orbit around Earth, combining to form the moon. • This is known as the Giant Impact Hypothesis.
Themost obvious feature of the moon’s surface are craters, which are round depressions in the surface of the moon. • Larger craters are about 250 km. in diameter (width of Indiana). • Most craters were produced by the impact of rapidly moving debris or meteoroids from space.
In contrast, the Earth’s surface only has about a dozen easily recognized craters. • Friction with Earth’s atmosphere burns up small Vredefort: 250-300 km. debris prior to making impact with Earth’s surface. Three largest on Earth’s surface include: • Vredefort (South Africa) Sudbury: 200 km. • Sudbury (Canada) • Chicxulub (Mexico) Chicxulub: 170 km.
Most of the moon’s surface is made of densely cratered, light-colored areas known as highlands. • They cover most of the surface on the far side of the moon. • The highlands consist of mountain ranges made from lunar material.
When Galileo first viewed the moon from his telescope, he noticed dark areas, which he thought looked like seas. • We now term these dark, relatively smooth areas on the moon’s surface maria, which is the Latin for “sea”. These beds of old lava flows formed when meteoroids punctured the moon’s surface, allowing magma to bleed out. Long channels called rilles, are associated with maria may be remnants of ancient lava flows.
The first manned mission to successfully land on the moon was the Apollo 11 mission (July 20, 1969) • This mission carried three astronauts: Neil Armstrong, Buzz Aldrin, and Michael Collins. Of those 3, only 2 walked on the moon (Armstrong first, then Aldrin). Collins stayed in the command module. This fulfilled President John F. Kennedy’s dream (speech in 1961).