Slides Night Sky
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Transcript of 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