Transcript ASTR 330: The Solar System Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
Lecture 2:
Finding Our Place in Space
~ and ~
A Historical Perspective on
Astronomy
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
The Night
Sky…
“An', as it blowed an' blowed,
I often looked up at the sky
an' assed meself the question
-- what is the stars, what is
the stars?”
from Juno and the Paycock,
by Sean O’ Casey
…what
sorts of
things do
we see?
Picture credit: Wally Pacholka AMS
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Constellations
• What do we mean by the term ‘constellation’?
• A constellation (meaning ‘stars together’) is a
pattern of stars on the sky, popularly recognized to
form the shape of a person, animal or object: e.g.
Orion the hunter, Ursa Major the Great Bear, Libra the
scales, etc.
• The stars comprising a constellation are only
apparently clustered together in one place: they may
actually be at greatly varying distances from us.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Orion:
The
Hunter
Figure credit: wikipedi.org
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
The Orion Nebulae
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Nebulae In Orion
“Pillars of Creation” in M42,
The Great Orion Nebula
(picture: Hubble Space Telescope)
M43 Horsehead Nebula
(picture: Angle-Australian Telescope)
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Apparent
Motion of The
Sun and Stars
(Circumpolar Stars Movie)
(Sirius Diurnal Motion Movie)
Movie credit: Rick Pogge, Ohio State
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Solar ‘motions’
• The Sun appears to travel from east to west
across the sky each day.
• The path the sun takes across the sky
changes during the year:
• Higher in the summer → longer days.
• Lower in the winter → shorter days.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
The Ecliptic
• apparent solar path on sky:
Figure credit: David P. Stern, Code 695, NASA GSFC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Seasonal Changes
• Some important dates:
• 21st June = northern summer solstice:
 longest day (northern hemisphere)
 shortest day (southern hemisphere)
• 21st December = northern winter solstice:
 shortest day (northern hemisphere)
 longest day (southern hemisphere)
• 21st September & 21st March … what?
 equal length day and night:
spring and fall equinoxes.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Extremes of Day and Night
• The extremes of solar motion occur at
the poles:
• 24 hours daylight in mid-summer.
• 24 hours darkness in mid-winter.
• What sort of seasonal variation in the
length of day would we expect at the
equator?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
The Zodiac
• The Sun appeared to
pass through 12 of the
original constellations
in the sky over the
course of one year,
hence your ‘sun-sign’
depending on which
day you were born.
• Actually, the Sun now passes through
13 constellations, and at different dates
from the ‘official’ astrological ones!
• The 12 signs were
mostly animals, hence
the ‘zodiac’ from the
same Greek etymology
as ‘zoo’.
Picture credit: [email protected]
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Phases Of The Moon
• The Moon is observed to change in
appearance over the course of about 30 days:
Why?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lunar Phases: Explanation
• The position of
the Moon,
relative to the
Sun (lighting) and
Earth (viewer)
determines
whether we see
the sunlit side,
shadow side, or
somewhere in
between.
Picture credit: wikipedia.org
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Eclipses
• The Sun, Moon and Earth all lie nearly in
the same plane.
• Also, by coincidence, the apparent size, or
angular diameter of the Moon and Sun as
seen from the Earth are about the same.
• As the Moon goes round the Earth, what
happens when:
• The Moon comes between the Earth and Sun?
• The Earth comes between the Moon and Sun?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Solar Eclipse
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Total Solar Eclipse
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Views Of Solar Eclipses...
Below shows a sequence of shots as the eclipse unfolds…
The solar corona (‘crown’) –
the outer part of the sun’s
atmosphere, and normally
invisible – is spectacularly
revealed at totality. Many solar
astronomers use this
opportunity to study the
corona.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
“Bailey’s Beads”
What could be causing the ‘beading’ effect?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lunar Eclipse
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lunar Eclipse Views…
Why does the moon not
disappear completely at
totality?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Announcements 9/5/06
• Revised Office hours:
Conor Nixon:
11-12 Tuesdays and 2-3 Thursdays
Room CSS 0225
KwangHo Park: 11-12 Mondays and Wednesdays
Room CSS 0224
• Yellow forms & enrollment.
• Textbooks.
• Homework #1 out today. Due back 9/12/06.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
The Moon and the Calendar
• The near 30-day cycle of lunar phases gives
us the basic period of one ‘monath’, or month.
• We can then simply divide the year (solar
cycle) into 12 months (lunar cycles), with some
extra days being added to some of the months.
• We already know where the idea of ‘day’
comes from…
… but where does the 7-day week come from??
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Wandering Stars
• In the night sky, there are five points of light visible to
the naked eye, which move, like the Sun and Moon,
and unlike the stars in the fixed constellations.
• The Greeks used the term ‘wanderer’ from which we
get the word ‘planet’.
• These acquired the names of different gods in
different cultures: we use the Roman names:
swift Mercury the messenger god, bright Venus
goddess of love, red Mars god of war, kingly Jupiter,
and slow-moving Saturn, god of time.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Planets, Astrology, Alchemy
• Over time, the seven ‘heavenly bodies’ became associated with
more than just deities:
Planet
Symbol
Metal
Weekday
(latin)
Sun

Gold
Sunday
Moon

Silver
Monday
Mercury
Mercury
Wednesday
Venus
Copper
Friday
Mars
Iron
Tuesday
Jupiter
Tin
Thursday
Saturn
Lead
Saturday
Symbol graphics from astro.uvic.ca
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Motions of the planets
• The planets wandered through the same 12
constellations of the zodiac as the Sun and Moon.
• Mercury and Venus stayed close to the Sun and so
were always seen at dawn or dusk (hence the ‘Evening
Star’). The others could be seen during the night as
well.
• However, unlike the Sun and Moon, these planets
exhibited curious ‘retrograde motion’ – periodic
reversals of direction.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Retrograde Motion
The apparent motion of Mars, Jupiter and Saturn was erratic,
showing retrograde loops in their forward motion.
Picture: www.mhhe.com
(Mars Retrograde Movie)
Movie credit: Rick Pogge, Ohio State
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Geocentric System
• To the ancient Greeks, the circle was the most perfect geometric
figure. The Sun, Moon and planets were thought to be perfect,
unchanging bodies circling a stationary Earth – a geocentric universe.
There was little reason to doubt this hypothesis.
• However, an explanation for retrograde
motion and also the periodic variations in
brightness of the planets was needed. This
was provided by a system of circles within
circles: known as epicycles.
• The triumph of ancient astronomy was
the Ptolemaic system of epicycles (after
Claudius Ptolemy, right, 2nd century AD)
which endured for over 1000 years!
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Aristotle’s Geocentric Universe
Picture credit: phys.utk.edu
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Ptolemy’s Epicycles
Picture and animation credit: phys.utk.edu
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Nicholaus Copernicus (1473-1543)
• Copernicus was the reluctant revolutionary who overthrew the
geocentric universe.
• In a book published at the end of his life, he
proposed that a much simpler model of the
universe was possible, if we assume that the
Sun is at the center and the Earth and other
planets circled around it.
• This heliocentric model was a heretical
view in the 16th century!
• In fact, Aristarchus of Samos had proposed
a heliocentric model around 200 BC, but
Aristotle’s view won (back then)! What were
the objections?
Picture: Univ. St. Andrews
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Copernican Heliocentric Universe
Picture credit: phys.utk.edu
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Retrograde motion in the Copernican system
Much fewer
epicycles
were
needed.
Animation credit: phys.utk.edu
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
The Ecliptic: revisited
• the Earth’s view of the Sun changes over the year:
Figure credit: David P. Stern, Code 695, NASA GSFC
• definition: 1 astronomical unit (AU) is the average distance from
the Earth to the Sun (150 million km, 93 million miles).
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Johannes Kepler (1571-1630)
• Kepler took Copernicanism a step further. By analyzing very precise
astronomical observations made by his mentor, Tycho Brahe (15461601), he realized that circles were hopeless for fitting the data.
• Kepler’s
genius was to fit the motions
of Mars using an elliptical orbit, with the
Sun at one focus:
Picture credit: phys.utk.edu
Picture: Univ. St. Andrews
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Kepler’s Laws (1)
1. Law of Orbits: Each planet moves in an elliptical orbit
about the sun, with the Sun at one focus of the ellipse.
a = semi-major axis
e = eccentricity
Ra = aphelion distance
Rp = perihelion distance
Picture credit: gsu.edu
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Kepler’s Laws (2)
2. Law of Areas: An imaginary line connecting the Sun with a
planet sweeps out equal areas in equal times as the planet
moves about the sun.
Picture credit: gsu.edu
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Kepler’s Laws (3)
3. The Law of Periods: The square of the period of
any planet is proportional to the cube of the semimajor axis of orbit.
T2 = K a3
Picture credit: gsu.edu
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Galileo Galilei (1564-1642)
• Galileo provided the first crucial evidence that the Copernican solar
system was physical reality, not just a useful aid to calculation.
• Galileo enthusiastically took to the
new tool of science: the telescope,
and turned it on the sky. He soon
found:
1. The four large moons of Jupiter, a
mini-solar system, in itself.
2. The phases of the planet Venus,
similar to the moon.
3. The rings of Saturn.
Picture: Univ. St. Andrews
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Isaac Newton (1642-1727)
• Newton was a temperamental genius who founded multiple areas of
classical physics: including optics, gravitation, and mechanics.
• His three laws of motion (see textbook)
form the basis of mechanics.
• Newton crucially realized that the force
which holds the Moon in its orbit about
the Earth is the same force causing an
apple to fall to the ground.
• Newton was hence able to deduce the
famous inverse-square law of gravity, and
prove Kepler’s laws.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Newton’s Law of Universal Gravitation
“The gravitational force between two objects is
proportional to product of their masses and inversely
proportional to the square of the distance between them”.
GM1M 2
F
2
R
• This centrally-acting force opposes the tendency of the planets
to continue in straight line motion, and holds them in orbit about
the sun.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Newton’s Cannon and orbits
• Newton proposed a ‘thought experiment’ in which a cannon was
fired from a mountaintop, at progressively greater and greater speeds.
• The ball falls further and further from the mountain, and eventually
‘misses’ the Earth altogether!
• Today, we know
it is possible to
do as Newton
imagined and to
send objects into
orbit…
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Blast-Off!
(STS 108 Launch Movie)
Picture and movie credit: NASA KSC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Escape Velocity
• Orbital velocity depends on the altitude of the orbit:
• LEO – Low Earth Orbit (200 km) requires 8 km/s
• GEO – Geo-stationary Earth Orbit requires about 10 km/s
• At a velocity of 11.2 km/s, known as escape velocity, the
spacecraft can escape the Earth’s gravity and go into solar orbit.
• This is the minimum velocity required for spacecraft to reach
other planets.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Numerical Examples from Chapter 1
• Question
2: What is the revolution period of a hypothetical
planet that orbits the Sun at half the distance of Mercury?
Kepler’s third law states that T2 = Ka3, where T is the orbital period and
a is the semi-major axis of orbit. If T is in years and a in AU (distance
from Sun to Earth) then K=1. So T2 = a3.
Now, Mercury orbits the Sun at a=0.39 AU. So, a for the proposed planet
is 0.195, and a3 = (0.195)3 = 7.41 x 10-3.
Finally, T = √(7.41 x 10-3) = 0.086 years.
• Of one that has twice the distance from the Sun as Pluto?
(distance of Pluto = 39.48 AU). You do it!
Answer: 702 years!
Quick method: rearrange formula to T=√(a3)=a1.5
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Numerical Examples from Chapter 1
• Question 5: A spacecraft on a trajectory from the Earth to
Saturn follows an ellipse with perihelion at the Earth’s orbit (1
AU) and aphelion at Saturn’s orbit (9 AU). If the semi-major axis
of the ellipse is 5 AU what is the time required for the trip from
the Earth to Saturn?
The key here is to realize that the spacecraft is in an orbit about the
Sun, although an elliptical one. Kepler’s third law is applied again, to
calculate the period, T = √(a3) = √(53) = 11.2 years.
But, this is the time for one complete orbit. We only need the time from
perihelion to aphelion, or half the total time, = 5.6 years.
• Using similar reasoning, find the trip time to Mars (1.5 AU).
Answer: ½ T = ½ √(a3) = ½ √(1.253) = 0.7 years.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz - Summary
1. What is a constellation?
2. Define ecliptic and zodiac, and explain the relation between them.
3. What causes a solar eclipse? A lunar eclipse?
4. What is meant by a geocentric universe? A heliocentric one?
5. Which is the odd one out: 1 day, 1 week, 1 month, 1 year?
6. Give a major contribution to astronomy by each of the following:
Aristotle, Ptolemy, Copernicus, Kepler, Galileo, Newton.
7. What are Kepler’s three laws of planetary motion?
Dr Conor Nixon Fall 2006