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The Sky Chapter 2

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Learn how scientists divide the sky into constellations Learn origin of constellations and their names Understand and apply the concept of magnitude Understand the celestial sphere Understand what causes the seasons Describe the motion of the planets Learn about astrology (!!) Goals:

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I. The Stars A. Constellations B. The Names of the Stars C. The Brightness of Stars D. Magnitude and Intensity II. The Sky and Its Motion A. The Celestial Sphere B. Precession III. The Cycles of the Sun A. The Annual Motion of the Sun B. The Seasons Outline

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IV. The Motion of the Planets A. The Moving Planets B. Astrology V. Astronomical Influences on Earth's Climate A. The Hypothesis B. The Evidence Outline (continued)

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Daily Motion in the Sky (SLIDESHOW MODE ONLY)

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Constellations In ancient times, constellations only referred to the brightest stars that appeared to form groups, representing mythological figures.

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Constellations (2) Today, constellations are well-defined regions on the sky. The International Astronomical Union lists 88 official constellations with clearly defined boundaries

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Constellations (3) The stars of a constellation only appear to be close to one another Usually, this is only a projection effect. The stars of a constellation may be located at very different distances from us. An asterism is a part of a constel- lation. (The Big Dipper is part of Ursa Major)

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Constellations (4) Stars are named by a Greek letter ( ) according to their relative brightness within a given constellation + the possessive form of the name of the constellation: Orion Betelgeuze Rigel Betelgeuse = Orionis Rigel = Orionis

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The Magnitude Scale First introduced by Hipparchus (160 - 127 B.C.): Brightest stars: ~1 st magnitude Faintest stars (unaided eye): 6 th magnitude apparent visual magnitude = unaided eye from earth absolute magnitude = from about 33 light years away 1 st mag. stars appear 100 times brighter than 6 th mag. 1 mag. difference is a factor of 2.512 in apparent brightness (larger magnitude => fainter object!)

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Betelgeuse Rigel Magnitude = 0.41 mag Magnitude = 0.14 mag The Magnitude Scale (Example) Magn. Diff.Intensity Ratio 12.512 22.512*2.512 = (2.512) 2 = 6.31 5(2.512) 5 = 100 For a magnitude difference of 0.41 0.14 = 0.27, we find an intensity ratio of (2.512) 0.27 = 1.28.

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The Magnitude Scale (2) Sirius (brightest star in the sky): m v = -1.42 Full moon: m v = -12.5 Sun: m v = -26.5 The magnitude scale system can be extended towards negative numbers (very bright) and numbers > 6 (faint objects):

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The Celestial Sphere Celestial equator = projection of Earths equator onto the c. s. North celestial pole = projection of Earths north pole onto the c. s. Zenith = Point on the celestial sphere directly overhead Nadir = Point on the c.s. directly underneath (not visible!)

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The Celestial Sphere (2) From geographic latitude l (northern hemisphere), you see the celestial north pole l degrees above the horizon; l 90 o - l Celestial equator culminates 90 l above the horizon. From geographic latitude l (southern hemisphere), you see the celestial south pole l degrees above the horizon.

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Celestial Sphere (SLIDESHOW MODE ONLY)

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The Celestial Sphere (Example) The Celestial South Pole is not visible from the northern hemisphere. Horizon North Celestial North Pole 37.7 0 South 52.3 0 Horizon San Francisco: l 37.7 Celestial Equator

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The Celestial Sphere (3)

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Apparent Motion of The Celestial Sphere Circumpolar constellations never rise or set, but appear to rotate counterclockwise around the north celestial pole (Polaris). star trails video

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Apparent Motion of The Celestial Sphere (2)

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Precession (1) Gravitys pull from the Sun and Moon does the same to Earth. The resulting wobbling of Earths axis of rotation takes about 26,000 years and is called precession. At left, gravity is pulling on a slanted top, causing it to wobble around a vertical axis.

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Precession (2) As a result of precession, the celestial north pole follows a circular pattern on the sky, once every 26,000 years. We are lucky to live at a time when a fairly bright star (Polaris, magnitude 2) is near the north celestial pole. It will be closest to Polaris ~ A.D. 2100. ~ 12,000 years from now, it will be close to Vega in the constellation Lyra.

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The Sun and Its Motions Earths rotation is causing the day/night cycle. (rotation - turning of a body about an axis)

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The Sun and Its Motions (2) The Suns apparent path on the sky is called the Ecliptic. (Another definition: the Ecliptic is the projection of Earths orbit onto the celestial sphere.) Due to Earths revolution around the sun, the sun appears to move through the zodiacal constellations. (revolution - motion around a point located outside a body)

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Constellations in Different Seasons (SLIDESHOW MODE ONLY)

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The Seasons Earths axis of rotation is inclined by 23.5 (from a right angle to plane of orbit) which causes the seasons.

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The Seasons (2) Seasons are not related to Earths distance from the sun. In fact, Earth is slightly closer to the sun in (northern- hemisphere) winter than in summer. Light from the sun Steep incidence Summer Shallow incidence Winter The Seasons are only caused by a varying angle of incidence of the suns rays.

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Seasons (SLIDESHOW MODE ONLY)

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The Seasons (3) Northern summer = southern winter Northern winter = southern summer Notice the angle of suns rays on each hemisphere during each solstice

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Shadow and Seasons (SLIDESHOW MODE ONLY)

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The Seasons (4) The number of daylight hours differ each season: summer solstice = most hours winter solstice = least hours equinoxes = equal hours Equinox also means sun crosses celestial equator: summer sun north of c.e. winter sun south of c.e. ecliptic video

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The Seasons (5) Earths distance from the sun has only a very minor influence on seasonal temperature variations. Earth is 1.7% closer to sun in the northern winter than in the northern summer. Sun Earth in July Earth in January Earths orbit (eccentricity greatly exaggerated)

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The Motion of the Planets The planets are orbiting the sun almost exactly in the plane of the Ecliptic. Jupiter Mars Earth Venus Mercury Saturn The Moon is also orbiting Earth in almost the same plane of the Ecliptic.

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The Motion of the Planets (2) All outer planets (Mars, Jupiter, Saturn, Uranus, Neptune and Pluto) generally appear to move eastward along the Ecliptic. The inner planets Mercury and Venus can never be seen at large angular distance from the sun and appear only as morning or evening stars. retrograde video

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The Motion of the Planets (3) Mercury appears at most ~28 from the sun. It can occasionally be seen shortly after sunset in the west or before sunrise in the east. Venus appears at most ~46 from the sun. It can occasionally be seen for at most a few hours after sunset in the west or before sunrise in the east. Morning star - planets visible before sunrise. Evening star - planets visible in the evening sky

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Astrology Zodiac - an 18 degree wide band centered on the ecliptic (the highway the planets follow). Astrologers divide this band into 12 segments named for constellations along the ecliptic. These are the signs of the zodiac. Your horoscope shows the location of the sun, moon, and planets among the zodiacal signs with respect to the horizon at the moment of a persons birth.

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constellation asterism magnitude scale apparent visual magnitude (m v ) celestial sphere horizon zenith nadir north celestial pole south celestial pole celestial equator north point south point east point west point angular distance angular diameter circumpolar constellation precession rotation revolution ecliptic vernal equinox summer solstice autumnal equinox winter solstice evening star morning star zodiac horoscope New Terms