in ancient times the sky was not well understood!...
TRANSCRIPT
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Astronomy is the oldest science!
In ancient times the sky was not well understood! Eclipses
Comets
Bad Omens?
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The Ancient Greeks
Laid the foundations of modern science
The Scientific Method
Our ideas must always be consistent with our observations!
In science observations are made with our five senses
Observations↓
Hypotheses “Ideas”↓
Predictions↓
Experiments
Hypotheses must be modified or even thrown out if they are found not
to consistent with observations!
Reproducibility
Valid experiments are those that are repeatable by other scientists
working independently
At the turn of the 20th century the astronomer Percival Lowell claimed to
see canals on Mars!
Observations could not be reproduced by other scientists!
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Hypotheses that have been verified
experimentally and shown to be reliable are then
collected together to form theories
and laws
They are also subject to further
testing
Laws
Summaries of observed behavior without explanation as to why the
behavior occurs
Laws predict WHAT will happen!
Example:Kepler’s Laws of Planetary Motion
Theories (Models)
Explanations of observed behavior
Theories explain WHY something happens (and will also allow a
prediction of what will happen!)
Example:Einstein’s Theories of Relativity
Non-scienceAnything that cannot be measured
with the 5 senses
Example:
religious belief
Science without religion is lame, religion without science is blind
Albert Einstein
Astronomy is unique among the sciences!
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Geology
Chemistry
Botany
Common link?
Direct Measurement!
Astronomy
The Universe is huge!
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How can we know anything about the Universe when we can’t make
direct measurements?
By collecting and analyzing the information carried across the
Universe by light!
light = electromagnetic (EM) radiation
Carries information
Not just visible
Very fast – travels at 300,000 km/s
Numbers in Astronomy
Astronomy is a Physical Science –it uses numbers to express things!
Angles
1 circle = 360 degrees (º), 1º = 60 arc minutes (‘), 1’ = 60 arc seconds (“)
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Angles are subdivided just like time!
Average time to run a marathon:
4 hours 32 minutes 8 seconds
Separation between two stars in the sky:
4 degrees 32 arc minutes 8 arc seconds(4º 32’ 8”)
Angular Separation
Angular and Linear Sizes
Sun and Moon have same angular size (½º) but different linear sizes!
In astronomy we have to deal with both very large and very small numbers….
Distance to nearest star
40000000000000000 meters
Size of a hydrogen atom
0.0000000001 meters
Scientific ‘Powers of 10’ Notation
10y
10 is called the base
y is called the exponent – it tells us how many times to multiply 10 by itself
101 = 10
102 = 10 x 10 = 100
103 = 10 x 10 x 10 = 1,000
104 = 10 x 10 x 10 x 10 = 10,000
etc.
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At the end of a large number is a decimal point, even though it is not normally written:
40000000000000000.
Converting Large Numbers into Scientific Notation
Move decimal point y places to the left:
Large # → # between 1-10 x 10y
40000000000000000. 4 x 1016
Negative Exponents
10-y
10-1 = 1 / 10 = 0.1
10-2 = 1 / 10 x 10 = 0.01
10-3 = 1 / 10 x 10 x 10 = 0.001
10-4 = 1 / 10 x 10 x 10 x 10 = 0.0001
etc.
Converting Small Numbers into Scientific Notation
Move decimal point y places to the right:
Small # → # between 1-10 x 10-y
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0.0000000001 1 x 10-10
Write the following numbers in scientific notation:
a. 55000
b. 480
c. 0.000005
d. 0.00014
e. 0.00785
f. 670000
Converting from Scientific Notation to Standard Notation
Move decimal point in opposite direction!
Fill spaces with zeros!
1.234 x 108 123400000
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Write the following as standard numbers:
a. 1.2 x 104
b. 8.25 x 10-2
c. 4 x 106
d. 5 x 10-3
Astronomical Distances – normal units not meaningful!
The Solar System
1 Astronomical Unit (AU) = average Sun-Earth distance
= 150 million km = 1.5 x 108 km
Sun-Mercury = 5.79 x 107 km = 0.39 AU
Sun-Neptune = 4.5 x 109 km = 30 AU
The Stars – much further!
Closest star (excluding Sun) > 270,000 AU away!
The Light Year
1 light year (ly) = distance light travels in one year at 3 x 108 m/s
= 9.46 x 1012 km = 63,240 AU
The light year is NOT a unit of time!
Proxima Centauri – the nearest star
Distance = 3.87 x 1013 km = 4.22 ly
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Light Travel Time
Light year = how many years light takes to travel from the
object
Example
Light takes 4.22 years to travel from the nearest star!
Significant delays!
Result
We see the nearest star not as it is now but how it was 4.22 years
ago!
Astronomy should also be considered a branch of history!
Observing the Sky
The Celestial Sphere
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The Constellations – star patterns
Some well known constellations
Which constellation is this?
Ursa Major the great bear
Which constellation is this?
Orion the Hunter
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What about this one?
Leo the Lion
Cygnus the Swan
Canis Major the Hunting Dog
Sagittarius the archer Scorpius the scorpion
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Constellations Today
88 constellations cover the sky, each
with its own boundary
State Boundaries
Constellations of the Zodiac
12 Constellations are special!
Astronomy is NOT Astrology!
Motions of the Sky
The sky appears to be moving around the Earth!
Really it is the Earth that is moving!
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The Earth’s Rotation and Revolution Diurnal Motion due to the Earth’s rotation
Star Trail Photography
As the Earth revolves around the Sun, the constellations that can be seen a night shift!
Northern Summer Constellations Northern Winter Constellations
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Celestial Coordinates
Celestial Reference Points
GP = Geographical PolesGE = Geographical Equator
Terrestrial Coordinates
Los Angeles: lat 34ºN, long 118ºW
Declination and Right Ascension
Celestial Navigation
Polaris is close to the North Celestial Pole
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Stars appear to move in circles about the celestial poles!
Polaris appears stationary
Circumpolar Stars: stars which a
permanently above the horizon from a
particular location
Finding Polaris in the sky
Polaris and navigation
Polaris from Los Angeles
Polaris from London
Special Places
North Pole (lat 90º N): Polaris is directly overhead (the zenith)
Equator (lat 0º): Polaris is on our horizon
South Pole (lat 90º S): Polaris is directly beneath our feet (the nadir)
Polaris is permanently below the horizon in the Southern Hemisphere
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At what latitude was this picture taken?
Precession
The location of the North Celestial Pole slowly shifts over a 26,000 year period!
Polaris is currently close to the NCP but will slowly shift away with time!
The location of the South Celestial Pole also shifts over the same time period!
There is currently no bright star close to the SCP but there will be in the future!
The Earth’s Axis Tilt
This is caused by the Earth’s axis of
rotation precessing like a top
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Precession is due to the pull of the Sun and the Moon’s gravity on the bulge of the Earth!
The Seasons
Seasonal Variations Seasonal Variations
1. Changing temperatures
Seasonal Variations
1. Changing temperatures
2. Changing day lengths
The seasons are due to the axis tilt of the Earth, NOT its changing distance from the Sun!
Seasons in N and S hemispheres are inverted!
If the Earth had no axis tilt there would be NO seasons!
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The Sun’s rays are more concentrated in the Summer and less concentrated in the Winter!
The Sun rises and sets in different directions during the year leading to varying day lengths!
Mar 21, Sep 21: Sun rises due E, sets due Wday = night
Vernal Equinox (Mar 21)Autumnal Equinox (Sep 21)
N Summer: Sun rises NE, sets NWday > night
longest day Jun 21 (Summer Solstice)
N Winter: Sun rises SE, sets SWnight > day
shortest day Dec 21 (Winter Solstice)
The Phases of the Moon
Properties of the Moon
1. Revolves around the Earth
2. Orbital period ~ 4 weeks = one month
3. Seen because it reflects light from the Sun
4. ALWAYS one hemisphere illuminated and one hemisphere in darkness
The Synodic Month
Every 29.5 days the Moon goes through a
cycle of phases
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As the Moon moves around the Earth the Sun-Earth-Moon Angle changes
As the moon moves around the Earth, this angle changes causing us to see
differing amounts of the illuminated
hemisphere!
S-E-M Angle Illumination Seen
Phase
0º 0% New
0-90º 0-50% Waxing Crescent
90º 50% 1st Quarter
90-180º 50-100% Waxing Gibbous
180º 100% Full
180-270º 50-100% Waning Gibbous
270º 50% 3rd Quarter
270-360º 0-50% Waning Crescent
Relationship between Sun-Earth-Moon Angle and Phase
How do you tell whether the Moon is waxing or waning?
When the Moon trails the Sun in the sky (is East of it) we have a waxing phase
When the Moon is ahead of the Sun in the sky (is West of it) we have a waning phase
When the moon is close to the horizon it appears orange and larger than normal
The Moon appears red close to the horizon for the same reason that the Sun appears red close to the horizon
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Atmospheric Scattering
The Moon is not really larger close to the
horizon!
It is an optical illusion due to having
landmarks close to the horizon to compare its
size to!
A Blue Moon
The cycle of lunar phases is 29.5 days which is slightly shorter than the average calendar month
If a new moon occurs right at the beginning of a month it is possible for a second full moon to occur
in the same month
A second full moon seen in the same month is called a Blue Moon
Next Blue Moon: August 31st 2012
Eclipses
The Two Types of Eclipses
Lunar – the Moon passes into the Earth’s shadow at Full Moon (S-E-M angle 180º)
Solar – the Earth passes through the Moon’s shadow at New Moon (S-M-E angle 0º)
The orbits of the Earth and Moon are not in the same plane!
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At most new and full moon’s the shadow miss each other – no eclipse!
Eclipses occur only when the Sun, Moon and Earth are aligned along the Line of Nodes
There a minimum of 4 and a maximum of 7 eclipses per year
Lunar Eclipses
Moon passes into Earth’s shadow at full moon
There are a minimum of two and a maximum of five lunar eclipses each year
The Shape of the Earth’s Shadow
During a total lunar eclipse the Moon becomes completely engulfed in the Earth’s umbra
The progression of a total lunar eclipse
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The Moon turns orange during totality due to light bent by the Earth’s atmosphere into its shadow!
Maximum duration of total eclipse: 1 hr 42 min
Other Types of Lunar Eclipses
During a partial lunar eclipse the Moon is only partially engulfed in the Earth’s umbra
A partial lunar eclipse
During a penumbral lunar eclipse the Moon misses the umbra and only enters the penumbra
A penumbral lunar eclipse
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Summary
Next lunar eclipse visible from Southern California:
December 20th 2010 (total)
Solar Eclipses
Earth passes through Moon’s shadow at new moon
There are a minimum of two and a maximum of five solar eclipses each year
The Moon’s Shadow on the Earth
The Moon’s shadow traces out an eclipse path on the Earth as it rotates. The eclipse path is very narrow, having
a width of only 250 km
An Eclipse Diagram
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A total solar eclipse is seen if you on the eclipse path (i.e. in the umbra of the Moon’s shadow)
Totality – maximum duration only 7½ min!
A Partial Solar Eclipse
Seen in the penumbra of the Moon’s shadow
The Elliptical Orbit of the Moon
Annular Eclipse:
Moon too small to completely block Sun
if eclipse occurs at apogee
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The times of eclipses can be easily predicted!
Next solar eclipse visible from Southern California:
May 20th 2012 (partial)
Next total solar eclipse not until May 3rd 2106!