Transcript 8 Jan, 2008 - Tj.H`s Git

SCI238 W08
Lecture 1:The Sky
star trails over Vienna
8 Jan, 2008
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SCI238/W08 – Course Topics:
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8 Jan, 2008
The Sky
Early Astronomy
Tools: gravity and radiation
Solar System
Stars and the Interstellar Medium
Galaxies
Cosmology
ET Life
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Today’s Lecture
 details, structure…
 info sheet hand-out, webpage …
 this week’s events
 today’s topic: naked eye astronomy
 stars and constellations
 rotation of the Earth and “sky coordinates”
 phases of the moon
 the ecliptic: path of the Sun + planets in the sky
 seasons
 time and the calendar
 precession
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Math?
 no calculus required – but some may be shown
 basic arithmetic, powers of ten, simple
logarithms (e.g. log-log plots)
 100 = 102 → log 100 = 2
 if 5 = a log 100 then 5/a = 2 and a=2.5
 109 / 104 = 105 (≠ 109/4 !!! )
 101/2 = 100.5 = √10 = 3.162277669….
 some geometry and trigonometry (lots of
triangles!), simple algebra, …
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This week’s events:
the Earth: at perihelion Jan 2, 6:00pm EST
the Moon: New on Tues Jan 8
Venus: visible low in east before sunrise; brightest
“morning” star
Mars: is visible all night, rises at sunset
Jupiter: not visible
Saturn: rises at 10pm
Quadrantid Meteor shower: peaked Wed Jan 4
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What do you know about the
night sky? …not know?
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Why???
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Naked Eye Astronomy
 what can you determine about the Universe
around us – with your eyes alone? …no
telescope, photographs …perhaps with a
time-keeping device …a protractor?
 much was known about the Universe –
especially about our Solar System – before
the telescope was invented…
 most of what we can learn comes from making
notes of what we see… especially: where
things are in the sky and how they move
 observe for yourself
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The Night
Sky
as seen from
Australia..
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What are constellations?
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Orion:
•striking pattern
•bright stars
•different colours
•gaseous nebula
•star clusters
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the star
pattern
constellation
“drawing”
the “real”
picture
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Motions in the Sky
 daily rotation of sky (of Earth, really!)
 motion of Moon from one day to next,
and change in “phases”
 motion of Sun from one day to the next
 hard to notice, because when the Sun is up you
can’t see the stars; but the Sun is in a (slightly)
different part of the sky (with respect to the
stars) every night… how can you tell that this is
true?
 motion of planets
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Stars move “around the sky”
cause?
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Rotation of Earth:
<-> apparent rotation of the sky
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Stars appear to lie on a great
“celestial sphere” around Earth
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Earth’s rotation makes the celestial sphere
appear to rotate about us (from E-W)
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Earth’s rotation
also makes the stars
appear to move
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what time is it? … at different
places…
“pole-on” view
“time of day”
is also due to
Earth’s axial
rotation
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Location in the sky
• how do we identify the location of things in
the sky, on the Celestial Sphere?
…need a (spherical) coordinate system of some
kind.
• on Earth: use longitude and latitude…(angles
with respect to the centre of the Earth,
with 0° longitude at Greenwich, England and 0°
latitude at the Equator.
• also, how do we measure angles in the sky?
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latitude = N/S angular distance
longitude = E/W angular distance
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Greenwich
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Latitude (cross-section of Earth)
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Measuring Positions in the Sky
 angular position… measured in degrees,
(arc)minutes, (arc)seconds
 1 degree = 60 arcmin = 3600 arcsec …or… 1° = 60′ = 3600″
 coordinate system for sky => longitude and
latitude on Earth, extended out into space
 latitude → declination
 longitude → Right Ascension… ?? but sky “moves
(once every 24 hours); stars constantly change
longitude…. need to fix the sky coordinate.
 Right Ascension: measured in hrs, min, sec; arbitrary
fixed zero point in the sky (originally = position of
the Sun at the instant Spring starts)
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Declination
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The sky seen from a fixed position on the Earth
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The Celestial
andEarth
“You” (your
local
viewlatitude
of the sky)
• the
sky we Sphere
see on
varies
with
• NCP altitude = observer’s latitude
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Stars move “around the sky”
cause?
latitude determines altitude of “centre”
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Sky rotation at the Equator
Sept. 14 2007,
Montlaux, France.
Stars near the
celestial equator
make almost straight
lines.
Note Venus rising on
the left, a satellite
leaving the frame at the
top left.
477 consecutive 30 second
exposures over 4.3 hours
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Positions of objects in “your” sky
How high in “your” sky will
an object be?
• we are at a latitude of
44o. On 21 Dec the Sun is
at a declination of -23.5o
(would be seen directly
overhead at noon at
latitude of -23.5o).
• how high in the sky
(altitude) is the Sun at
noon?
• altitude = 180 – 44 – 23.5
- 90 = 22.5o
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the Moon
and its
phases
what causes
the phases
of the
Moon?
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The Sun’s Position in the Sky
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Sun’s path is through the constellations of the zodiac
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The path of each
of the planets
across the sky is
along a line that
is very close to
the path followed
by the Sun over
the course of a
year
-> the “ecliptic”
the orbits of all
of the planets
are in very nearly
the same plane in
the sky.
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Ecliptic = Sun’s path through the Sky (actually the “reflex”
or negative or opposite of the Earth’s motion around the
Sun); it is not in the same plane as the equator – which is set
by the rotation of the Earth. These two planes (ecliptic and
equator) are inclined to each other by 23 ½ degrees Why?
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it is this inclination of Earth’s rotation axis to
its orbital plane that is the primary cause of
our seasons
Why?
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Time of day and the seasons: Dec 22
Arctic Circle
Tropics
noon
Equator
3 pm
Antarctic Circle
6 pm
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Time of day and the seasons: Mar. 22
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Next Lecture
 more “sky” – calendar,
precession, eclipses etc.
 early astronomy – knowledge of
SS, motions, scale
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