Slides Night Sky

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    Science is Seeing

    Naked Eye

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    Constellation

    Its a regionof the sky,within official borders

    set in 1928 by the IAU.

    Often recognizableby a pattern, asterism,

    or grouping of stars.

    There are 12 zodiacconstellations, 88 in all

    parts of the sky.

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    Origin of Constellations

    Most official constellation names come from antiquity. Some

    constellations in the southern hemisphere were named byEuropean explorers in the 17th& 18thcenturies.

    The patterns of stars have no physical significance! Stars that

    appear close together may lie at very different distances.

    Modern astronomers

    use them as landmarks,

    but their significancetoday is cultural, not

    really practical except as

    memnonics to the sky.

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    The Celestial Sphere

    The sky above looks like adomea hemisphere

    If we imagine the skyaround the entire Earth,

    we have the concept ofthe celestial sphere.

    This is a 2-dimensionalrepresentation of the sky

    Because it represents ourview from Earth, we placethe Earth in the center ofthis sphere.

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    The Celestial Sphere

    1. Celestial Equator (Earths rotation)

    2. Ecliptic (Earths orbit of the Sun)

    3. Milky Way (disk of our galaxy)

    3 Planes:

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    Measuring Angles

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    Objects in the sky can be measured with two angles: the

    altitudeabove the horizon, and the azimuth around the

    horizon. Zenith is straight overhead and the line joining

    the zenith to due south and due north is the meridian.

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    Daily Motion

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    Diurnal Motion

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    Diurnal Motion

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    Earths Rotation

    (explaining, for example, why KSC is in Florida not Maine)

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    Diurnal Motion

    N & S celestial poles

    the points in space directly above the Earths poles

    celestial equator

    the line in space directly above the Earths equator

    sidereal day = 23 hours 56 min 4.1 sec

    solar day = 24 hours

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    Celestial Sphere

    Celestial Sphere Rotation

    Celestial Sphere Rotation

    Celestial Sphere

    Star A

    Star B

    1

    1

    3

    2

    2

    4

    4

    3 Horizon

    Due to the Earths rotation, stars rise in the east and set in

    the west. They move in circles around the celestial pole,

    the projection of the Earths pole into space. The angle of

    the celestial pole above the horizon is the local latitude.

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    Solar vs. Sidereal Day

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    Solar vs. Sidereal Day

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    Annual Motion

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    Annual Motion

    ecliptic

    the apparent path of the Sun through the sky

    equinox

    where the ecliptic intersects the celestial equator

    solstice

    where the ecliptic is farthest from the celestial equator

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    In the northern hemisphere, the Sun rises due east and

    sets due west on the equinoxes(equal day and night). It

    rises and sets north of due east and west on the summer

    solsticeand south of due east and west on winter solstice.

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    North

    Star

    365 days

    1 day

    Aries

    Pisces

    Taurus

    Gemini

    Cancer Leo

    Virgo

    Libra

    Scorpius

    Sagittarius

    Capricornus

    Aquarius

    Due to the Earths orbit of the Sun, the constellations all

    migrate through the night sky each year. Each star rises

    and sets four minutes earlier each day. Constellations on

    the path traveled by the Sun (the ecliptic) are the zodiac.

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    Earths Orbit

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    Seasons

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    Seasons

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    When the Sun is high in the sky during the day, the number of

    daylight hours is greater and the amount of direct sunlight received is

    greater. This results in Summer.

    The tilt of the

    Earths spin axis

    causes the angleof illumination of

    the Sun at noon

    vary over a year.

    When the mean

    illumination angle

    is shallower, the

    heating effect is

    less, when larger,

    the heating effect

    is greater.

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    Summer in the N

    = Winter in the S

    Note: the Earths

    orbit is slightly

    elliptical but theeffect is too small

    to cause seasons

    (theyre opposite

    in the N and S)

    Winter in the N =

    Summer in the S

    A l

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    Analemma

    If the Earth had a circular orbit and no axial tilt,

    the analemma would be a point. Tilt gives the

    vertical size, eccentricity gives lobe asymmetry,

    speed variation in orbit give horizontal extent.

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    Precession

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    Annual Motion

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    Annual Motion

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    Annual Motion

    zodiac

    the constellations which lie along the ecliptic

    solar year = 365d 5h 48m 45s

    SEASONSare caused by the 23.5 tilt ofthe Earths axis with respect to the ecliptic,

    notthe distance from the Earth to the Sun!

    (= 365.24219 days)

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    Moon

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    Lunar Motion

    Phases of the Moon:

    How long does it take the Moon to complete 1 cycle?

    sidereal period

    time it actually takes the Moon to orbit the Earth

    = 27.3 days

    synodic period

    time between one full moon and the next

    = 29.5 days

    Ph f th M

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    Phases of the Moon

    Ph f th M

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    Phases of the Moon

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    Animation at http://antwrp.gsfc.nasa.gov/apod/image/9911/lunation_ajc.gif

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    The Moon is

    always half litby the Sun but

    we see varying

    proportions of

    the lit part

    from Earth

    depending on

    where the

    Moon is in its

    orbit of Earth.

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    Lunar Motion

    When the Moons orbit intersects the ecliptic(called the node):

    at new moon solar eclipse

    at full moon

    (everyone on the night side of Earth can see it)

    lunar eclipse

    (you must be in Moons shadow to see it)

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    Eclipses

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    Eclipses occur ONLY when the Moon crosses the plane of the Earths

    orbit around the Sun and ONLY during the NEW or FULL phases.

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    When the Earths shadow hits the Moon we have a .

    Lunar Eclipse

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    Lunar Eclipse

    When Earth blocks most of the sunlight fromilluminating the Moons surface for several

    hours when in the full moon phase

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    When the Moons shadow hits the Earth, we have a .

    Solar Eclipse

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    Solar Eclipse

    Annular Eclipse

    Partial Eclipse

    Total Eclipse

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    Pl t

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    Planets

    Planets in the Sky

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    Planets in the Sky

    Inferior Planets Mercury

    Venus

    Superior Planets

    Mars

    Jupiter Saturn

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    Conjunction

    Maximum Elongation

    Opposition

    Same position as Sun

    Planet at farthest point from Sun

    Planet is 180 from Sun

    inferior

    superior

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    The planets orbit the Sun in nearly a plane, so they are always

    seen near the ecliptic, the apparent path of the Sun in the sky.

    Planets that are inferior have interior orbits to the Earth so are

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    Planets that are inferiorhave interior orbits to the Earth, so are

    always seen relatively close to the Sun in the sky. Planets that

    are superiorhave exterior orbits to the Earth, so can be seen in

    any position along the ecliptic.

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    Mars Retrograde

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    Mars Retrograde

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    As was realized during the Copernican Revolution, retrograde motion

    occurs when the Earth overtakes a slower moving outer planet.

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    Discover for Yourself

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    Discover for Yourself