Topics in Astronomy Text: Modules 11 pages 265 - 270 and 15 pages 397 - 418

Class Dates Assignments

May 2, 5 and 8

Homework AssignmentModule 11 Study Guide Questions 1 - 23 p 283-284Module 16 Study Guide Questions 1 - 25 p 397-422

May 2:

Chapter 11: The Gravitational Force at Work in Our Solar System (Chap 11 p265)

11. 1 Solar System

11.1.1 Sun- Center of the solar system- Largest mass of the solar system - largest gravity- All planets and other objects of the solar system orbit (are satellites) around the sun- What Causes Gravity (p 274)- Brief History of our Solar System (p278)

11.1.2 Planets Inner, Terrestrial (Earth Link), Rocky - Outer planets -

11.1.3 Dwarf Planets -- What about Pluto? (272)

- Dwarf Planet - A dwarf planet, is a celestial body orbiting the Sun that is (1) massive enough to have a round shape because of its own gravity (2) but has not cleared its neighboring region of planetesimals (small asteroids) and (3) is not a satellite (moon).

- Pluto, is the second-largest known dwarf planet in the Solar System (after Eris) and the tenth-largest body observed directly orbiting the Sun. Originally classified as a planet, Pluto is now considered the largest member of a distinct region of solar objects called the Kuiper belt.

11.1.3 Moons

- General Term: A natural satellite or moon is a celestial body that orbits a planets, dwarf planets, and minor planets. Technically, the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy, but it is normally synonymous with moon and used to identify non-artificial satellites of planets, dwarf planets, and minor planets. (There are no known natural satellites of moons.) Two hundred and forty bodies, all in the Solar System, are classified as moons. They include 166 orbiting the eight planets, 4 orbiting dwarf planets, and dozens more orbiting minor planets.

- The Moon: name give to our moon that orbits the earth.

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11.1.4 Asteroids (Asteroid Belt)

- Asteroids, also called minor planets or planetoids, are a class of solid astronomical objects smaller than planets. The term asteroid is generally used to indicate a diverse group of small celestial bodies in the solar system that orbit the Sun. It means "star-like" in Greek, and in English is the most commonly used word for a minor planet, which is the term preferred by the International Astronomical Union. Other languages prefer planetoid (Greek for "planet-like") because it more or less describes what they are. In late August 2006, the IAU introduced the class "small solar system bodies" (SSSB) to include most objects thus far classified as minor planets and comets. At the same time, the class "dwarf planets" was created for the largest minor planets.

- The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets. The asteroid belt region is also termed the main belt to distinguish it from other concentrations of minor planets within the Solar System, such as the Kuiper belt and scattered disk.

11.1.5 Meteoroids, meteors, meteorites (p 268)A meteoroid is a small sand to boulder-sized particle of rocky debris in the Solar system.

A meteor is the visible event that occurs when a meteoroid or asteroid enters Earth's atmosphere and becomes brightly visible. The visible path of a meteoroid that enters Earth's (or another body's) atmosphere is a meteor, commonly called a "shooting star" or "falling star". Many meteors are part of a meteor shower. The root word meteor comes from the Greek meteōros, meaning high in the air.The meteor is simply the visible event rather than an object itself.

A meteorite is a portion of a meteoroid or asteroid that survives its passage through the atmosphere and impact with the ground without being destroyed. Meteorites are sometimes, but not always, found in association with hypervelocity impact craters; during energetic collisions, the entire impactor may be vaporized, leaving no meteorites.

11.1.6 Comets (269)

Comets are small Solar System bodies that orbit the Sun and, when close enough to the Sun, exhibit a visible coma (or atmosphere) and/or a tail — both primarily from the effects of solar radiation upon the comet's nucleus. Comet nuclei are themselves loose collections of ice, dust and small rocky particles, measuring a few kilometres or tens of kilometres across.Comets have a variety of different orbital periods. Short-period comets are thought to originate in the Kuiper Belt, which lies beyond the orbit of Neptune. Long-

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period comets are believed to originate at a very much greater distance from the Sun, in a cloud (the Oort cloud)

Chapter 16: (p 397)

16.1. The StarsA star is a massive, luminous ball of a very hot hydrogen plasma. (Plasma is the state of matter where the electrons become detached form the nucleus).

• A star shines because thermonuclear fusion in its core where hydrogen is fused together to form helium and releases energy that travels across the star's interior and then radiates into outer space.

• The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. Other stars are visible in the night sky, when they are not outshone by the Sun.

• Many stars are in multi-star systems which consist of two or more stars that are gravitationally bound, and generally move around each other in stable orbits.

16.2 The Sun as a star(p 397)

The Sun (Latin: Sol) is the main sequence star at the center of the Solar System; it is the Earth's local star.

1. The Earth and other matter (including other planets, asteroids, meteoroids, comets, and dust) orbit the Sun, which by itself accounts for about 99.8% of the Solar System's mass.

2. Energy from the Sun, in the form of sunlight and heat, supports almost all life on Earth via photosynthesis, and drives the Earth's climate and weather.

3. The Sun is a star, but the Sun is very unique star for supporting life• Single star – most stars are multiple stars• Large but not too large - 330,000 times the size of Earth• Made of H 74 % and He 25% other element 1%

Energy source – nuclear fusion

4. Layers of the sun• Core

-The core of the Sun is considered to extend from the center to about 0.2 solar radii.The core is the only location in the Sun that produces an appreciable amount of heat via fusion: the rest of the star is heated by energy that is transferred outward from the core. All of the energy produced by fusion in the core must travel through many successive layers to the solar photosphere before it escapes into space as sunlight or kinetic energy of particles.

• Radiative zone-The radiation zone is the middle zone in the sun's interior. Energy travels out of the core into the radiation zone. Energy travels through the radiation zone in the form of electromagnetic radiation. The radiation zone is so dense that the waves bounce around. The radiation zone is directly below the convection zone and directly above the core.

• Convective zone

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-In the Sun's outer layer (down to approximately 70% of the solar radius), the solar plasma is not dense enough or hot enough to transfer the heat energy of the interior outward via radiation. As a result, thermal convection occurs as thermal columns carry hot material to the surface (photosphere) of the Sun. Once the material cools off at the surface, it plunges back downward to the base of the convection zone, to receive more heat from the top of the radiative zone. The turbulent convection of this outer part of the solar interior gives rise to a "small-scale" dynamo that produces magnetic north and south poles all over the surface of the Sun.

• Photosphere- The visible surface of the Sun, the photosphere, is the layer below which the Sun becomes opaque (can’t see through) to visible light.

• Atmosphere- The parts of the Sun above the photosphere are referred to collectively as the solar atmosphere. They can be viewed with telescopes operating across the electromagnetic spectrum, from radio through visible light to gamma rays, and comprise five principal zones: the temperature minimum, the chromosphere, the transition region, the corona, and the heliosphere.

• Spots, flares and prominences- Sunspots, which are well-defined surface areas that appear darker than their surroundings because of lower temperatures. Sunspots are regions of intense magnetic activity where convection is inhibited by strong magnetic fields, reducing energy transport from the hot interior to the surface. The magnetic field gives rise to strong heating in the corona, forming active regions that are the source of intense solar flares and coronal mass ejections. The largest sunspots can be tens of thousands of kilometers across.

The number of sunspots visible on the Sun is not constant, but varies over an 11-year cycle known as the Solar cycle. At a typical solar minimum, few sunspots are visible, and occasionally none at all can be seen. Sunspots usually exist as pairs with opposite magnetic polarity. The solar cycle has a great influence on space weather, and is a significant influence on the Earth's climate. Solar activity minima tend to be correlated with colder temperatures, and longer than average solar cycles tend to be correlated with hotter temperatures. In the 17th century, the solar cycle appears to have stopped entirely for several decades; very few sunspots were observed during this period. During this era, which is known as the Maunder minimum or Little Ice Age, Europe experienced very cold temperatures. Earlier extended minima have been discovered through analysis of tree rings and also appear to have coincided with lower-than-average global temperatures.

- A solar flare is a violent explosion in the Sun's atmosphere releasing x rays and gamma rays.

- A solar prominence (also known as a filament) is an arc of gas that erupts from the surface of the Sun. Prominences can loop hundreds of thousands of miles into space. Prominences are held above the Sun's surface by strong magnetic fields and can last for many months. At some time in their existence,

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most prominences will erupt, spewing enormous amounts of solar material into space.

16.3 Nuclear Energy (401)

• Fission - breaking apart of the nucleus to from two substance- Chain reaction requires critical mass – a certain minimum amount to start the chain reaction- Nuclear power – meltdown - safety of storage an issue

• Fusion - 4 H fuse together to become 1 He- 4H →1 He + energy (some of the mass of the 4 H’s is converted to energy according to E=mc2)- 4 H = 4 P+ + 4 e-- 1He = 2 P + +2 e- + 2 N- Note that 2 electrons disappeared into the 2 neutrons but some mass still not accounted for that converts to energy

16.4 Classifying the Stars in the Universe (p 404)

• Done by temperature and spectral color of star which is related to temperature.

Two astronomers, Ejnar Hertzsprung and Henry Russell, independently had the idea of plotting a graph of luminosity (or true brightness) against surface temperature from the spectral color for every star in the sky. The remarkable fact is that the points on the Hertzsprung - Russell diagram are not scattered randomly all over the graph; instead the points are grouped together in three district regions.

Almost every star in the sky falls along the line extending from the upper-left to the lower-right corners of the graph; these are called the main sequence stars. The existence of this main sequence is exactly what we should expect: the hotter the star the brighter it should be and these fall at the lower-right end of the sequence. The Sun is typical of a main sequence star.

Set apart from the main sequence, near the upper right-hand corner of the Hertzsprung-Russell diagram, fall stars that are cool and very bright. Being cool, each square mile of the star's surface emits only a moderate amount of light so they can be bright only if the truly gigantic: they emit mostly reddish light. Almost every reddish-appearing star in the sky is a red giant; Betelgeuse Antares and Arcturus are well-known examples. They are thousands of times larger than the stars in the main sequence; they are the largest stars in the universe.

The third group of dots in the lower left-hand corner of the Hertzsprung- Russell diagram represents stars that are both dim and hot. The only way such a hot star can be so dim is if it is small. These stars are typically about the same size as the Earth, and in view of the blue-white light emitted from their hot, tiny surfaces they are called white dwarfs.

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16.15 Variable Stars (p 409)

A variable star is a star that undergoes significant variation in its luminosity(otherwise known as a star that is subjected to pulsations). In contrast, most stars have little variation in luminosity, such as the Sun, which undergoes relatively little variation in brightness (usually about 0.1% over an 11 year solar cycle).

• Variable stars are of two types:

* stars that are intrinsically variable, that is, their luminosity actually changes, for example because the star periodically swells and shrinks; * eclipsing and rotating variables, where the apparent changes in brightness are a perspective effect.

16.5 Nebula

- A nebula is an interstellar cloud of dust, hydrogen gas and plasma. It is the first stage of a star's cycle.

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16.6 Measuring the distance to the Stars (p 411)

- Parallax Method, more accurately motion parallax, is the change of angular position of two observations of a single object relative to each other as seen by an observer, caused by the motion of the observer. Simply put, it is the apparent shift of an object against the background that is caused by a change in the observer's position.

-- Parallax is often thought of as the 'apparent motion' of an object against a distant background because of a perspective shift, as seen in Figure 1. When viewed from Viewpoint A, the object appears to be closer to the blue square. When the viewpoint is changed to Viewpoint B, the object appears to have moved in front of the red square.

--In astronomy, parallax is the only direct method by which distances to objects beyond the Solar System can be measured. The Hipparcos satellite has used the technique for over 100,000 nearby stars. This provides the basis for all other distance measurements in astronomy, the cosmic distance ladder.

- Apparent Magnitude Method (m) (p 412 -413) , - The apparent magnitude (m) of a celestial body is a measure of its brightness as seen by an observer on Earth, normalized to the value it would have in the absence of the atmosphere. The brighter the object appears, the lower the numerical value of its magnitude. The difference between the apparent (m) and absolute magnitude (M) can be used to solve for the distance.

d = 10 0.2 (m - M + 5)

16.7 Galaxies (p 413)• Galaxy is a massive group (ensemble) of stars and interstellar matter that orbits about a

common center. • Galaxies contain from several million to as many as a trillion stars.• Galaxies have masses between several million and several trillion times that of our Sun. • Their diameters range from a few thousands to several 100,000s "light years". A light year is

the distance light travels in one year (9.6 trillion km, 6 trillion miles). • Typically galaxies are separated by millions of light years distance. Astronomers estimate that

there are about 125 billion galaxies in the universe.

• Types of Galaxies Spiral: Have the form as a spiral

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Lenticular: Lenticular galaxies are disc galaxies. Look like a lens – fatter in the middle but no spiral. A lenticular galaxy is a type of galaxy which is intermediate between an elliptical galaxy and a spiral galaxy in galaxy morphological classification schemes.

Elliptical: Elliptical galaxies have smooth, featureless light-profiles. They range in shape from nearly spherical to highly flattened ellipsoids.

Irregular: They are chaotic in appearance, with neither a nuclear bulge nor any trace of spiral arm structure. Collectively they are thought to make up about a quarter of all galaxies.

16.8 An Expanding Universe (p 414)

• All galaxies seem to be moving away from each other, thus indicating the universe is expanding.

• This has been deduced trough the observation that the more distant a galaxy the redder the light from those galaxies.

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• The Doppler effect show us that the faster an object moves away from you the more the light will be shifted to the red colors.

See Experiment 16.1 An Expanding Universe (p 416)

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Day14: 4.2 Asteroids, Meteoroids - Meteors - Meteorites, and Comets

Asteroids

• Asteroids are rocky and metallic objects that orbit the Sun but are too small to be considered planets.

• They are known as minor planets or planatoids.

• Asteroids range in size from Ceres, which has a diameter of about 1000 km, down to the size of pebbles.

• Sixteen asteroids have a diameter of 240 km or greater.

• Most, however, are contained within a main belt that exists between the orbits of Mars and Jupiter.

• They have been found inside Earth's orbit to beyond Saturn's orbit.

• Some have orbits that cross Earth's path and some have even hit the Earth in times past.

• One of the best preserved examples is Barringer Meteor Crater near Winslow, Arizona.

• If the estimated total mass of all asteroids was gathered into a single object, the object would be less than 1,500 kilometers (932 miles) across or less than half the diameter of our Moon.

• On October 1991 asteroid 951 Gaspra was visited by the Galileo spacecraft and became the first asteroid to have hi-resolution images taken of it. Again on August 1993 Galileo made a close encounter with asteroid 243 Ida. This was the second asteroid to be visited by spacecraft. Both Gaspra and Ida are classified as asteroids composed of metal-rich silicates.

• On June 27, 1997 the spacecraft NEAR made a high-speed close encounter with asteroid 253 Mathilde. This encounter gave scientists the first close-up look of a carbon richtype asteroid. This visit was unique because NEAR was not designed for flyby encounters. NEAR is an orbiter destined for asteroid Eros in January of 1999.

• The largest representatives are 1 Ceres, with a diameter of about 1,003 km (about 623 mi), and 2 Pallas and 4 Vesta, with diameters of about 550 km (about 340 mi). The naming of asteroids is governed by the International Astronomical Union (IAU). After an astronomer observes a possible unknown asteroid, other astronomers confirm the discovery by observing the body over a period of several orbits and comparing the asteroid’s position and orbit to those of known asteroids. If the asteroid is indeed a newly discovered object, the IAU gives it a number according to its order of discovery, and the astronomer who discovered it chooses a name. Asteroids are usually referred to by both number and name.

• About 200 asteroids have diameters of more than 97 km (60 mi), and thousands of smaller ones exist.

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• The total mass of all asteroids in the solar system is much less than the mass of the Moon. The larger bodies are roughly spherical, but elongated and irregular shapes are common for those with diameters of less than 160 km (100 mi). Most asteroids, regardless of size, rotate on their axes every 5 to 20 hours. Certain asteroids may be binary, or have satellites of their own.

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Meteoroids, Meteors and Meteorites

• Meteoroids: Asteroids that are on a collision course with Earth are called meteoroids.

• Meteor: When a meteoroid strikes our atmosphere at high velocity, friction causes this chunk of space matter to incinerate in a streak of light known as a meteor.

• A meteor is a bright streak of light in the sky (a "shooting star" or a "falling star") produced by the entry of a small meteoroid into the Earth's atmosphere. If you have a dark clear sky you will probably see a few per hour on an average night; during one of the annual meteor showers you may see as many as 100/hour. Very bright meteors are known as fireballs

• Meteorite: If the meteoroid does not burn up completely, what's left strikes Earth's surface and is called a meteorite.

• Meteorites are bits of the solar system that have fallen to the Earth. Most come from asteroids, including few are believed to have come specifically from 4 Vesta; a few probably come from comets. A small number of meteorites have been shown to be of Lunar (23 finds) or Martian (22) origin.

• Of all the meteorites examined, 92.8 percent are composed of silicate (stone), and 5.7 percent are composed of iron and nickel; the rest are a mixture of the three materials. Stony meteorites are the hardest to identify since they look very much like terrestrial rocks.

• A very large number of meteoroids enter the Earth's atmosphere each day amounting to more than a hundred tons of material. But they are almost all very small, just a few milligrams each. Only the largest ones ever reach the surface to become meteorites. The largest found meteorite (Hoba, in Namibia) weighs 60 tons.

• The average meteoroid enters the atmosphere at between 10 and 70 km/sec. But all but the very largest are quickly decelerated to a few hundred km/hour by atmospheric friction and hit the Earth's surface with very little fanfare. However meteoroids larger than a few hundred tons are slowed very little; only these large (and fortunately rare) ones make craters.

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Comets

• Comets are small, fragile, irregularly shaped bodies composed of a mixture of non-volatile grains and frozen gases. They have highly elliptical orbits that bring them very close to the Sun and swing them deeply into space, often beyond the orbit of Pluto.

• Comet structures are diverse and very dynamic, but they all develop a surrounding cloud of diffuse material, called a coma, that usually grows in size and brightness as the comet approaches the Sun. Usually a small, bright nucleus (less than 10 km in diameter) is visible in the middle of the coma. The coma and the nucleus together constitute the head of the comet.

• As comets approach the Sun they develop enormous tails of luminous material that extend for millions of kilometers from the head, away from the Sun. When far from the Sun, the nucleus is very cold and its material is frozen solid within the nucleus. In this state comets are sometimes referred to as a "dirty iceberg" or "dirty snowball," since over half of their material is ice. When a comet approaches within a few AU of the Sun, the surface of the nucleus begins to warm, and volatiles evaporate. The evaporated molecules boil off and carry small solid particles with them, forming the comet's coma of gas and dust.

• When the nucleus is frozen, it can be seen only by reflected sunlight. However, when a coma develops, dust reflects still more sunlight, and gas in the coma absorbs ultraviolet radiation and begins to fluoresce. At about 5 AU from the Sun, fluorescence usually becomes more intense than reflected light.

• As the comet absorbs ultraviolet light, chemical processes release hydrogen, which escapes the comet's gravity, and forms a hydrogen envelope. This envelope cannot be seen from Earth because its light is absorbed by our atmosphere, but it has been detected by spacecraft.

• The Sun's radiation pressure and solar wind accelerate materials away from the comet's head at differing velocities according to the size and mass of the materials. Thus, relatively massive dust tails are accelerated slowly and tend to be curved. The ion tail is much less massive, and is accelerated so greatly that it appears as a nearly straight line extending away from the comet opposite the Sun.

Day15: 4.2 Asteroids, Meteoroids - Meteors - Meteorites, and Comets Review

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Day16: The Earth Within the Solar System - Earth's Motions in Relationship to the Sun and Stars

5. The Earth Within the Solar System

5.1 Earth's Motions in Relationship to the Sun and Stars

5.1.1 Rotation

• The earth rotates about its axis once every 24 hours. It is the rotation of the earth that causes the 24 hour day to have a period of daytime and nighttime.

5.1.2 Orbiting

• The earth orbits around the sun once every 365 days. The 365-day orbit combined with the 23.5 degree tilt of the axis on the earth with respect to its orbit case our seasons.

• The tilt of the axis of the orbit cause several things to occur. For the Northern hemisphere, in the winter we look to the south to see the Sun. In the summer it goes more nearly overhead. These results in the winter daylight hours are fewer and the intensity of sunlight is less. The tilt of earth's axis also alters the intensity of sunlight received during different seasons - it is increased during summer and decreased during winter. This occurs because the beam of sunlight, which carries the same amount of energy, is concentrated over a smaller area in the region tilted towards the sun (where it is summer), and is spread over a larger area in the opposite hemisphere (where it is winter). Thus it is colder in the Northern Hemisphere winter even though we are actually a little closer to the sun. Seasons are reversed in the Northern and Southern Hemispheres.

5.1.3 Motion in Relationship to the Milky Way Galaxy

• The solar system is located on the outer edge of the milky way in one of its spiral bands. The solar system along with the earth is orbiting around the center of the galaxy. As the earth rotates and orbits we see different parts of the milky way and the universe when we view the sky.

Day18: The Earth Within the Solar System - Earth's Motions in Relationship to the Sun and Stars - Review

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Day19: Quiz 2 - Section 4: Solar System - Planets and Section 5: Earth's Motions in Relationship to the Sun and Stars

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Day20: The Earth Within the Solar System - Earth - Moon Relationships

5.2 Earth - Moon Relationships

5.2.1 General Characteristics of the Moon

• The Moon is the only natural satellite of Earth. Its has a mass of 7.3522 kg a diameter of 3476 km and orbit is about 384,400 km from Earth.

• The Moon, is the second brightest object in the sky after the Sun. As the Moon orbits around the Earth once per month, the angle between the Earth, the Moon and the Sun changes; we see this as the cycle of the Moon's phases. The time between successive new moons is 29.5 days (709 hours), slightly different from the Moon's orbital period (measured against the stars) since the Earth moves a significant distance in its orbit around the Sun in that time.

• Due to its size and composition, the Moon is sometimes classified as a terrestrial "planet" along with Mercury, Venus, Earth and Mars.

1. The Moon was first visited by the Soviet spacecraft Luna 2 in 1959. It is the only extraterrestrial body to have been visited by humans. The first landing was on July 20, 1969 (do you remember where you were?); the last was in December 1972. The Moon is also the only body from which samples have been returned to Earth. In the summer of 1994, the Moon was very extensively mapped by the little spacecraft Clementine and again in 1999 by Lunar Prospector.

• The Moon has no atmosphere. But evidence from Clementine suggested that there may be water ice in some deep craters near the Moon's south pole which are permanently shaded. This has now been confirmed by Lunar Prospector. There is apparently ice at the north pole as well.

• The Moon's crust averages 68 km thick and varies from essentially 0 under Mare Crisium to 107 km north of the crater Korolev on the lunar far side. Below the crust is a mantle and probably a small core (roughly 340 km radius and 2% of the Moon's mass). Unlike the Earth, however, the Moon's interior is not active. Curiously, the Moon's center of mass is offset from its geometric center by about 2 km in the direction toward the Earth. Also, the crust is thinner on the near side.

• There are two primary types of terrain on the Moon: the heavily cratered and highlands and the relatively smooth and maria. The maria (which comprise about 16% of the Moon's surface) are huge impact craters that were later flooded by molten lava. Most of the surface is covered with regolith, a mixture of fine dust and rocky debris produced by meteor impacts. For some unknown reason, the maria are concentrated on the near side.

• A total of 382 kg of rock samples were returned to the Earth by the Apollo and Luna programs. These provide most of our detailed knowledge of the Moon. They are particularly valuable in

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that they can be dated. Even today, more than 30 years after the last Moon landing, scientists still study these samples.

5.2.2 Ocean Tides and the Moon

• The gravitational forces between the Earth and the Moon cause some interesting effects. The most obvious is the tides.

• The Moon's gravitational attraction is stronger on the side of the Earth nearest to the Moon and weaker on the opposite side.

• Since the Earth, and particularly the oceans, is not perfectly rigid it is stretched out along the line toward the Moon.

• From our perspective on the Earth's surface we see two small bulges, one in the direction of the Moon and one directly opposite. The effect is much stronger in the ocean water than in the solid crust so the water bulges are higher.

• And because the Earth rotates much faster than the Moon moves in its orbit, the bulges move around the Earth about once a day giving two high tides per day. (This is a greatly simplified model; actual tides, especially near the coasts, are much more complicated.)

• 5.2.3 Same Side of Moon Always Faces Earth

• The Moon rotates synchronously with the earth, i.e. it is locked in phase with its orbit so that the same side is always facing toward the Earth. Just as the Earth's rotation is now being slowed by the Moon's influence so in the distant past the Moon's rotation was slowed by the action of the Earth, but in that case the effect was much stronger. When the Moon's rotation rate was slowed to match its orbital period (such that the bulge always faced toward the Earth) there was no longer an off-center torque on the Moon and a stable situation was achieved.

• Actually, the Moon appears to wobble a bit (due to its slightly non-circular orbit) so that a few degrees of the far side can be seen from time to time, but the majority of the far side (left) was completely unknown until the Soviet spacecraft Luna 3 photographed it in 1959. (Note: there is no "dark side" of the Moon; all parts of the Moon get sunlight half the time (except for a few deep craters near the poles). Some uses of the term "dark side" in the past may have referred to the far side as "dark" in the sense of "unknown".

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Day21: The Earth Within the Solar System - Earth - Moon Relationships - Review

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Day22: The Earth Within the Solar System - Earth - Moon Relationships

5.2.4 Phases of the Moon

• As the moon orbits around the earth, we see different moon phases depending on the positions of the earth, moon, and sun. In the diagram below, you are looking down on the north pole of the earth. You can see that the lit half of the moon always faces the sun, but as the moon orbits around the earth, our viewing angle of the lit half changes. The phase that we see when the moon is in a particular position is shown in the box beside the moon's orbit.

• The full moon rises at sunset. It takes the moon 27 days to make a trip around the Earth it slightly more than 29 days between full moons because it takes 27 days for the moon to orbit the Earth with respect to the stars. However, the Earth orbits around the sun as the moon orbits around the Earth, so the moon must travel through an extra distance to make one orbit with respect to the Sun. Thus it takes 29 days for the moon to reach the same position with respect to the Sun.

5.2.5 Solar Eclipses

• It just happens that the Moon and the Sun appear the same size in the sky as viewed from the Earth. And since the Moon orbits the Earth in approximately the same plane as the Earth's orbit around the Sun sometimes the Moon comes directly between the Earth and the Sun. This is called a solar eclipse;

• if the alignment is slighly imperfect then the Moon covers only part of the Sun's disk and the event is called a partial eclipse.

• When it lines up perfectly the entire solar disk is blocked and it is called a total eclipse of the Sun. Partial eclipses are visible over a wide area of the Earth but the region from which a total eclipse is visible, called the path of totality, is very narrow, just a few kilometers (though it is usually thousands of kilometers long). Eclipses of the Sun happen once or twice a year. 5.2.6 Lunar Eclipses of the Moon by The Earth's Shadow

• An eclipse of the Moon (or lunar eclipse) can only occur at Full Moon, and only if the Moon passes through some portion of the Earth's shadow. The shadow is actually composed of two cone-shaped components, one nested inside the other. The outer or penumbral shadow is a zone where the Earth blocks part but not all of the Sun's rays from reaching the Moon. In contrast, the inner or umbral shadow is a region where the Earth blocks all direct sunlight from reaching the Moon.

• Astronomers recognize three basic types of lunar eclipses:

1. Penumbral Lunar Eclipse: The Moon passes through Earth's penumbral shadow. These events are of only academic interest since they are subtle and quite difficult to observe.

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2. Partial Lunar Eclipse: A portion of the Moon passes through Earth's umbral shadow. These events are easy to see, even with the unaided eye.

3. Total Lunar Eclipse: The entire Moon passes through Earth's umbral shadow.

• The reason we don't have a lunuar eclispe one each month is becasue the Moon's orbit around Earth is actually tipped about 5 degrees to Earth's orbit around the Sun. This means that the Moon spends most of the time either above or below the plane of Earth's orbit. And the plane of Earth's orbit around the Sun is important because Earth's shadows lie exactly in the same plane. During Full Moon, our natural satellite usually passes above or below Earth's shadows and misses them entirely. No eclipse takes place. But two to four times each year, the Moon passes through some portion of the Earth's penumbral or umbral shadows and one of the above three types of eclipses occurs.

• When an eclipse of the Moon takes place, everyone on the night side of Earth can see it. About 35% of all eclipses are of the penumbral type which are very difficult to detect, even with a telescope. Another 30% are partial eclipses which are easy to see with the unaided eye. The final 35% or so are total eclipses.

Day23: The Earth Within the Solar System - Earth Universe Relationships

5.3 Earth Universe Relationships

5.3.1 Constellations• Constellations are imagined star pattern as view form earth. We use constellations to help us map

the sky. But it is important to that they myths about them were developed from ancient poets, farmers and astronomers over the past 6,000 years. The useful purpose of the constellations is to help us tell which stars are which. On a really dark night, you can see about 1000 to 1500 stars. Trying to tell which is which is hard. The constellations help us by breaking up the sky into more manageable bits. They can be used as mnemonics, or memory aids.

• A sky atlas of the stars is a amp of the stars and usually show the how the stars appear in terms of the various constellations. There is typically a consistency of how the constellations are drawn in each star atlas. For example in almost every star atlas, you will see Orion drawn with these same lines.

5.3.2 Constellations Sorted by Month Below is a list of all 88 constellations split up into the months when they are best seen in the sky. The months listed assume that you are looking at the sky at 9:00 PM. For every hour later than 9:00, add half of a month. For every hour before 9:00, subtract half a month. The constellations are typically visible for more than just one month, depending on where you are on the Earth. If you need to know exactly when a constellation is visible, check in a star atlas or on a planisphere.

JanuaryCaelum, Dorado, Mensa, Orion, Reticulum, TaurusFebruary

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Auriga, Camelopardalis, Canis Major, Columba, Gemini, Lepus, Monoceros, PictorMarchCancer, Canis Minor, Carina, Lynx, Puppis, Pyxis, Vela, VolansAprilAntlia, Chamaeleon, Crater, Hydra, Leo, Leo Minor, Sextans, Ursa MajorMayCanes Venatici, Centaurus, Coma Berenices, Corvus, Crux, Musca, VirgoJuneBoötes, Circinus, Libra, Lupus, Ursa MinorJulyApus, Ara, Corona Borealis, Draco, Hercules, Norma, Ophiuchus, Scorpius, Serpens, Triangulum AustraleAugustCorona Austrina, Lyra, Sagittarius, Scutum, TelescopiumSeptemberAquila, Capricornus, Cygnus, Delphinus, Equuleus, Indus, Microscopium, Pavo, Sagitta, VulpeculaOctoberAquarius, Cepheus, Grus, Lacerta, Octans, Pegasus, Piscis AustrinusNovemberAndromeda, Cassiopeia, Phoenix, Pisces, Sculptor, TucanaDecemberAries, Cetus, Eridanus, Fornax, Horologium, Hydrus, Perseus, Triangulum

Day 24: The Earth Within the Solar System - Special Characteristics of Earth

5.4 Special Characteristics of Earth

• The following are some special characteristics of the earth and relationship that allow life to exist on earth.

1. Earth- Moon size relationship. The fact the moon is close to the size of the earth stabilizes the orbit and tilt of the rotation.

2. The 23.5 degree tilt of the earth on it axis case the season and helps keep temperature on earth stabilized.

3. Th earth's magnetic filed protect the surface of the earth form the solar wind.

4. The earth has an ozone layer at the other known planets do not have. The ozone layer absorbs harmful ultra violet light rays.

5. The distance from the sun. If the earth were a little closer it would be to hot for life. If were a little further it would be too cold for life.

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6. Position in the galaxy. If the solar system were closer to the center of the galaxy, there would be too much radiation for life to survive.

7. Size and mass of the earth. The amount of gravity at the surface of the earth is very conducive to life as we know it.

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Day 25: The Earth Within the Solar System -Review

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Day 26: Quiz Section 5: The Earth Within the Solar System -Review

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