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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 1

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What is Astronomy? An observational science where people study celestial objects or events. Mostly astronomers get information about celestial object by using the electromagnetic spectrum Why? 

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Fig. 1.COa

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EM spectrum: Gamma rays, x-rays, ultraviolet, blue-red, infrared, microwave, radio Light is this whole thing to physicists and astronomers, visible light is said when talking about blue to red wavelength commonly called light Recently astronomers have been "observing" using elementary particles, Neutrinos, Protons, electrons, and (perhaps someday soon) gravity waves What are some celestial objects or events? Planets, Moons, Comets, Stars, interstellar dust, galaxies, planetary nebula, globular clusters, quasars, white dwarfs, black holes, supernova, the big bang, Red Giants, Asteroids, etc.

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Fig. 1.COb

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Motion is always relative, something is moving with respect to something else. Example: the boat and the pier. 

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The Night Sky To human cultures both past and present everywhere across the earth, the stars formed imaginary patterns, some of those patterns have been passed down to us (mostly from Greek and Roman cultures).  Today we have 88 such patterns, called CONSTELLATIONS.  Some are familiar: Orion, Perseus, Andromeda, and Gemini, and some are not so familiar: Delphinus, Dorado, Camelopardalis, and Ophiuchus.

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Stars that form these patterns or constellations are physically very far from each other and only appear close together from our vantage point within our own galaxy, the milky way.  The milky way, as it is called looks like a thin cloud of light in the sky, that always appears in the same locations relative to the constellations.  This is actually billions of stars that makeup our galaxy seen edge on (we are in a spiral galaxy at a distance of about 2/3 the distance from the center to the edge of the furthest spiral).

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Fig. 1.03a The sickle is the Head of Leo This portion of a constellation is called an asterism.

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Fig. 1.03b

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The stars DO move with their own individual speed and direction (called proper motion), however, they are much too far for this to be observed, with the unaided eye, in one human lifetime.  So the constellations do not noticeably change, in fact, astronomers will often refer to the "Fixed Stars." 

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Fig. 1.04

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Circumpolar stars do not fall below the horizon, their center is celestial pole.  In the northern hemisphere a star currently lies close to the celestial pole: Polaris.  

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Fig. 1.05

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If we identify a particular constellation (or any group of stars) and watch it at the same time night after night, week after week, etc. we notice the gradual movement westward (about 1° per day).  This is due to the earth revolution about the sun.  

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Fig. 1.06

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What we observe is the overall motion of the night sky from east to west, the cause of which is the rotating earth.  

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Angular momentum keeps the rotation earth pointing in the same direction.

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Fig. 1.11

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Motion of the Sun Observer the sun from the same spot day after day, week after week, etc... we notice a few things.    During the day, the sun rises and sets following an arc from east to west across the sky.  If you watched your shadow there is one time of day when it is shortest, this happens when the sun is at its highest point in the sky: noon.  

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Fig. 1.12

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Where does the sun rise and set in between the equinoxes?  Question why is it cold in the winter? 

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Fig. 1.13

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Fig. 1.13a

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Observe your shadow at noon over the course of a year, the length of the shadow will change.  It is longest in the winter, on the day of least sunlight (the sun rises to its lowest height of the year), this day is called the winter solstice.  Your shadow is shortest in the summer, on the day of the most sunlight (the sun rises to its greatest height of the year), this day is called the summer solstice.  In the fall and spring there is a day where the shadows are of equal length, the sun is half way between solstices.  Theses days are called the spring (or Vernal) equinoxes and the fall (or Autumnal) equinoxes.  On the equinoxes the sun rise exactly due east and sets exactly due west. 

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Fig. 1.13b

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Angular Measurement Degrees, Minutes, Seconds  OR Degrees, arcmin, arcsec You know there are 360 degrees in a circle.  The sun and moon are about 0.5 degrees in angular size.  Astronomical measurements are often made in units that are less than one degree.  Divided one degree into 60 equal parts, one of those parts is called an arcmin.  The moon is about 30 arcmin across.  Further divide one arcmin into 60 equal parts, one of those parts is an arcsec. 

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In Astronomy when two features can be distinguished from each other it is called Resolution, a telescope is said to have a certain Resolving power.  On a good night, very large ground based telescopes have a resolution of about 0.5 arcsec, this is considered excellent "seeing." The stars travel in arcs across the sky every night because of the rotation earth, what direction are we rotation?   They travel about 15 degrees per hour.  This makes sense because 15x24=360 If we identify a particular constellation (or any group of stars) and watch it at the same time night after night, week after week, etc. we notice the gradual movement westward (about 1° per day).  This is due to the earth rotation about the sun.   Motion is always relative, something is moving with respect to something else.  Example: the boat and the pier. 

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The sun moves east with respect to the stars over the course of a year.  The path the sun is called the ecliptic.  Precession of the equinoxes.  If you watch a spinning top as it slows down, you may notice that it wobbles, that wobble is called precession.  The earth can be thought of as a giant spinning top, it is slowing down--very very slowly.  As it does, it undergoes precession. For us spring occurs when the sun is in Pisces.  But this was not always the case (during the time of the Greeks it was when the sun was in Aries.  Also, the celestial pole wonders in a great circle, it takes about 26,000 years to complete one cycle. 

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Fig. 1.14

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The motion of the planets: The planets (like the sun and moon) move from the west toward the east against the backdrop of stars.  They move along the path called the ecliptic (this path passes through the signs of the zodiac). However, things are not quite so simple, the five naked-eye planets (as well as Uranus, Neptune, and Pluto) undergo a radical westward or RETROGRADE MOTION for short time then return to their eastward path.  The planets seem to wonder in the heavens: Planet is the Greek word for wonderer.   The number of days a planet undergoes retrograde motion varies slightly from cycle to cycle.  Each planet undergoes retrograde motion for different durations.  

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Fig. 1.15

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Mercury and Venus orbit the sun within the earth's orbit, therefore from our vantage point they are never seen very far from the sun.  Even though they are planets they are referred to as "morning star" or "evening star" depending on whether they are seen in just before dawn, or just after twilight.  When the reach their maximum angle from the sun they are said to have reached Maximum ELONGATION.  As an "evening star" they reach Max. Eastern Elongation (the planet is as far east of the sun as it is going to get).  And, as a "morning star" they reach Max. Western Elongation (the planet is a far west of the sun as it is going to get).  Mercury and Venus begin their retrograde motion when the reach Max. Eastern Elongation.  They move westward past the sun and appear as morning stars.   Conjunction means that two (or more) celestial objects appear close together in the sky.  They don't have to be right on top of each other for a conjunction to occur.  For example: the moon passes close to Venus (within a few degrees or so), the moon and Venus are said to be in conjunction.  

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Fig. 1.16

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Motion of the Moon  Like the sun and stars, it rises in the east and sets in the west.   However, the moon (like the sun) moves from the west toward the east against the backdrop of stars.  It moves along the constellations of the zodiac (this is where the ecliptic is).  You can observe the moon move eastward in a single night, each hour it moves about 30 arcmin. If you note its position with respect to bright stars, then observe it a few hours later, you will see the movement eastward.   The moon's angular speed through the zodiac is much faster than the sun's.  The sun takes a year to complete its cycle.  But the moon does it in about 27 days, 13 complete cycles per year. 

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The Phases of the moon: New moon: the moon is not visible, it is within a few degrees of the sun Waxing Crescent First quarter: the moon is half full (the west facing half), it is 90° east of the sun Waxing Gibbous Full moon: the entire face of the moon is reflecting sunlight, it is 180° east of the sun. Waning Gibbous Last quarter: the moon is half full (the east facing half), it is 90° west of the sun Waning Crescent South Carolina State Flag It takes 29.5 days to complete one cycle of phases (almost a month). Many cultures both past and present used or use this lunar cycle for their calendar.  

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Fig. 1.17

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When the moon or a planet actually blocks out the light from a star, or the moon blocks out the light from a planet, an OCCULTATION has occurred.  When Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto are undergoing Retrograde Motion, they reach a point in their orbit when they are separated by an angle of  180° from the sun.  The planet rises just as the sun sets.  This is called OPPOSITION.  The sun and moon can never display retrograde motion. Why?  And, Mercury and Venus can never be in opposition to the sun.  WHY? 

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Fig. 1.18

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Eclipses of the sun and moon The moon and sun have about the same angular diameter as seen from the earth (about 30 arcmin).  Solar Eclipse: The moon's shadow falls on the earth Close to new moon if the moon happens to be aligned with the ecliptic (this happens at two locations in the moon's orbit) the moon will block the sun's light.  A total solar eclipse is a rare event for a particular location on the surface of the earth.  The moon's shadow is only a few hundred km wide, if you are not directly in the path you will not see a total eclipse.  Just off the path of the moon's shadow you will see a partial eclipse, this is where the moon blocks only a portion of the sun's light.  Think about the surface of the earth, where are most solar eclipses likely to occur?  

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Fig. 1.19

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Lunar Eclipse: The moon passes through the earth's shadow During a full moon, if the moon happens to be aligned with the ecliptic, the moon will pass through the earth's shadow.  There can be a partial or total eclipse of the moon  depending on the alignment of sun, earth and moon.  Unlike a solar eclipse, just about everyone on the dark side of the earth can see a lunar eclipse.   The outer part of the shadow is called the Penumbra and the inner part is called the Umbra.  When the moon is in the Umbra of of the earths shadow we see a total lunar eclipse, while in the Penumbra the moon is partially eclipsed.   Note: because the atmosphere of the earth bends light (like a lens) and red light is bent the most, a total lunar eclipse can look slightly reddish. 

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Fig. 1.20

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Why do we not see eclipses every month?  The orbit of the moon is tilted by about 5 degrees with respect to the ecliptic.  Eclipses are only possible when the moon crosses the ecliptic (the path of the sun across the sky), in fact, that's how the ecliptic got it's name.   Scientific Models: Two important features, it must be able to explain what is observed, and predict what is to be observed. 

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Kepler’s 3 laws of planetary motion: 1. the planets travel around the sun in ellipses, with the sun at one focus. 2. planets travel fastest when they are closest to the sun and slowest when furthest away from the sun. The square of the period of a planets orbit is directly proportional to the cube of its semi-major axis. P2 (in years)=a3 (in AU)

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