coordinates and celestial sphere
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UNIT 1: PATTERNS IN THE SKYASTRONOMYHILLGROVE HIGH SCHOOL
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First, what is astronomy?
What is astronomy?
The study of celestial objects and thephysical universe
What is science?
Science is the pursuit of knowledgeand understanding of the naturalfollowing a systematicmethodology based on evidence.
What is astrology?A belief in the supposed influences of
the stars and planets on humanaffairs and terrestrial events
M17, 5500 ly away, 100 ly across
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Don’t write this down, but
appreciate subtleties !
If you expect to see flames in the telescopeand aliens flying by….you will bedisappointed !
The closest star to us (other than the sun) is
approximately 4 years away if you weretraveling at the speed of light.
Anticipation of next question: No, we can’ttravel at the speed of light.
Even looking through a relatively largetelescope, one needs to appreciate subtlefeatures (color, brightness, “fuzzy”galaxies etc. )
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This is Mars through a really nice
telescope and a CCD camera
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How many stars can one see on a clear night ?
Approximately 3000.
What is the closest star to Earth?
The sun…after that, ~ 4 light years away
What is a light year?
the distance traveled by a beam of light in one year(~9.5 x 1013 km)
That’s about 6 trillion miles, and it would take asatellite ~20,000 years to travel one light year
So, we are not going to another star anytime soon
What about time travel?
Watch more Star Trek
On what horizon do stars rise? Set?
Rise from general east- Set in the general westdirections (Around Polaris)—the North Star
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COORDINATES:ALTITUDE AND AZIMUTH
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Coordinate systems
Coordinates a set of numbers used to locate
something
Terrestrial coordinateslatitude and longitude
Equator
0 latitude
Prime meridian
0 longitude
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Angular Measurements
Degree = a circle’s divided into360 degrees
minute of arc = 1/60 of a degree
second of arc = 1/60 of a minute
(…. Or 1/3600 of a degree)
Hold finger out at arm’s length =~2 degrees
Hold fist out at arm’s length =~10 degrees
Moon and sun are only 0.5 °(stays constant--if it looksbigger...it’s an illusion)
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Horizon system
Horizon systemlocating a star/planet using altitude
(height) and azimuth (around)
Zenith the point directly above you in the sky
Horizon where the ”land” or treeline meets the sky
Altitude
from 0º on horizon to 90º at zenith
Azimuth from 0º at North and going around 360º
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Horizon system
Problems...
1. Stars change positionover the course of thenight because of rotationof Earth—stars rise andset like the sun
2. Stars occupy different
positions in the sky atdifferent points on theearth (N vs. Shemispheres)
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Notice that the position of stars change over the course of afew minutes or hours because Earth is spinning
(the yellow streaks are fairies and fireflies)
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THE SEASONS
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The Seasons
Earth’s axis of rotation is inclined vs. the normal to its
orbital plane by 23.5°, which causes the seasons.
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The Seasons
They are not related to Earth’s distance from the
sun. In fact, Earth is slightly closer to the sun in(northern-hemisphere) winter than in summer.
Light fromthe sun
The Seasons are a result of how direct the
sunlight is that strikes Earth.
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The Seasons
Northern summer = more direct light, sun higher insky, daytime sunlight is longer
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The Seasons
Northern winter = less direct light, sun lower in sky,daytime sunlight is shorter
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What is the ecliptic?
Definition #1the daily path of
the sun in thesky
Definition #2
the path of the sunon the celestial
sphere over thecourse of theyear
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How the path of the sun changesover the course of the year(this is at 40 degrees latitude)
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COORDINATES:THE CELESTIAL SPHERE
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Celestial Sphere
Celestial Sphere
Imaginary sphere onto which allstars are projected(imaginary sphere of what wesee)
North Celestial Pole
Point on celestial sphere directlyabove North Pole of earth
Polaris
The North Star because it ispractically located on theNorth Celestial Pole
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Celestial Sphere
The Celestial sphere is
an easier way to locatestars because they are“fixed” on this map
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Celestial SphereRight ascension (RA)
like longitude; RA is anangular measurement
around sphere(measured in hours)
Declination (dec)
like latitude; declinationis an angularmeasurement above
or below celestialequator
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If you are standing at the North
Pole, your view of the celestialsphere looks like this…
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Diurnal Circles from N. Pole
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If you are standing at the equator,
your view of the celestial spherelooks like this…
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Diurnal Circles from Equator
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If you are standing in Atlanta, your
view of the celestial sphere lookslike this…
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Diurnal Circles from mid-latitude
Circumpolar stars
group of stars that donot set below horizon
(they go aroundPolaris)
Latitude and Polaris
The angle of Polarisabove your horizon isthe same as yourlatitude in degrees.
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Sidereal Day vs. Solar Day
Sidereal Day
The time it takes a star to line backup with Earth
23 hours, 56 minutes, 4 seconds (~4
minutes shorter than a 24 hr day)
Therefore stars rise
~ 4 minutes earlier each night
Mean Solar Day
=24 hrs (avg. time for Sun to return tosame position)
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Precession
The Sun’s gravity is doing the same to Earth.
The resulting “wobbling” of Earth’s axis of rotation around the
vertical w.r.t. the Ecliptic takes about 26,000 years and iscalled precession.
At left, gravity is pulling on a slanted top.Wobbling around the vertical.
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Precession
As a result of precession, the celestial north
pole follows a circular pattern on the sky,once every 26,000 years.
It will be closest toPolaris ~ A.D. 2100.
There is nothingpeculiar about Polaris
at all (neitherparticularly bright nor
nearby etc.)~ 12,000 years from
now, it will be close toVega in the
constellation Lyra.
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POSITIONING OFTHE PLANETS IN
THE SKY
mercury
venus
mars
jupiter
moon
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Notice that the
planets lie along the
ecliptic because the
solar system is in thesame plane (a flat
disc)
Imagine the sun here below horizon
in west
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The positions of the stars change over the course of theyear because the Earth orbits the Sun…
Off-seasonconstellations are
“up” during the
daytime
Notice that between
June and August,
the Sun is directly
“in” Gemini
Notice that June to
August, Sagittarius
is prime viewing
during summer
nights
Th M ti f th Pl t
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The Motion of the Planets
The planets are orbiting the sun almostexactly in the plane of the Ecliptic.
Venus
Mercury
The Moon is orbiting Earth in almost thesame plane (Ecliptic).
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Motion of the Planets The Sun, Moon and planets can always be found in
a Zodiac Constellation on the ecliptic.
Planets "wander" across the celestial sphere andthrough the stars in two ways:
1. Direct Motion - normal eastward movement of planets
2. Retrograde Motion - occasional westward movementof planets
Th M ti f th Pl t
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The Motion of the Planets
Mercury appears at most~28° from the sun.
It can occasionally beseen shortly after sunsetin the west or before
sunrise in the east.
Venus appears at most~46° from the sun.
It can occasionally beseen for at most a fewhours after sunset in thewest or before sunrise in
the east.
Pl t th t NOT i ibl
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Planets that are NOT visible
Planets may not bevisible at nightbecause of where they
might be in their orbits.
What if Jupiter werehere?
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Retrograde Motion
Retrograde motion occurs
when an inner planet passes by an outside planet and the
outside planet appears to go
“backwards” in the sky for
a few weeks (this doesn’thappen in a night)
Click for Animation
The movement is
apparent — Mars does NOTreally move like that in
space. The position of mars
shifts against the backdrop
of distant stars.
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PHASES OF THEMOON AND
ECLIPSES
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Phases of the Moon
Phases of the moonoccur because wesee only a portion ofthe lit side of themoon
Half the moon isalways lit and halfthe moon is always
dark, but we mightonly see a portion ofthe lit side
The Phases of the Moon
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The Phases of the Moon
New Moon First Quarter Full Moon
Evening Sky
The Phases of the Moon
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The Phases of the Moon
Full Moon Third Quarter New Moon
Morning Sky
The Phases of the Moon
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The Phases of the Moon• The Moon orbits Earth in a sidereal period of 27.32 days.
27.32
daysEarthMoon
Fixed direction in space
The Phases of the Moon
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The Phases of the Moon
• The Moon’s synodic
period (to reach the same
position relative to the
sun, i.e., the same lunar
phase) is 29.53 days (~ 1
month).
Fixed direction in space
Eart
hMoo
n
Earth orbits around Sun =>
Direction toward Sun changes!
29.53
days
Lunar Eclipses
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Lunar Eclipses
partial shadow
is the
Penumbra,
and a zone of
full shadow is
the Umbra
If the entire surface of the moon enters the
Umbra, the lunar eclipse is total.
Solar Eclipses
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Solar Eclipses
The sun appears approx. as large in the sky (same
angular diameter ~ 0.50) as the moon.
When the moon passes in front of the sun, the moon can cover
the sun completely, causing a total solar eclipse.
Conditions for Eclipses
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Conditions for Eclipses
A solar eclipse can only occur
if the moon passes a node near
new moon.
The moon’s orbit is inclined against the ecliptic by ~ 50.
A lunar eclipse can only occur
if the moon passes a node near
full moon.
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