Download Lecture 1: The Scale of the Cosmos

Midterm Review
Please press “1” to test your transmitter.
What is the scientific notation for
the number of kilometers in an AU
1 Astronomical Unit (AU) = Distance Sun – Earth =
150 million km = 150,000,000 km
1)
2)
3)
4)
5)
1,000,000150 km
100,000,0001.5 km
1.5*106 km
1.5*107 km
1.5*108 km
What is the Local Group?
1. The Solar System
2. The Earth-moon system
3. The cluster of stars to which our Sun
belongs
4. The cluster of galaxies to which our
Milky Way belongs
5. A rock band from Athens, OH
The Local Group of Galaxies
Galaxies usually don’t exist alone, but in clusters of galaxies
Distance to the nearest large galaxies:
several million light years
July is in which season in
South Africa?
1.
2.
3.
4.
5.
Spring
Summer
Fall
Winter
Eclipse Season
The Seasons
• The Seasons are only caused by a varying angle of
incidence of the sun’s rays.
• They are not related to the Earth’s distance from the sun.
• In fact, the Earth is slightly closer to the sun in (northernhemisphere) winter than in summer.
Steep incidence
→ Summer
Ecliptic
Shallow incidence
→ Winter
Light from
the sun
How long is the period of the
precession of Earth’s axis?
1.
2.
3.
4.
5.
1 year
11 years
3,000 years
26,000 years
10 billion years
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 to
Polaris ~ A.D. 2100.
~ 12,000 years from now,
it will be close to Vega in
the constellation Lyra.
There is nothing peculiar about Polaris at all
(neither particularly bright nor nearby etc.)
What is the moon’s synodic
orbital period?
1. The period from new moon to new moon.
2. The period of one orbit around Earth, in a
fixed reference frame (e.g., a distant star).
3. The period of one orbit around the sun.
4. The moon’s period of rotation.
5. The period of one orbit around the center
of the Milky Way.
The Phases of the Moon
Fixed direction in space
29.53 days
Earth
Moon
Earth orbits around Sun =>
Direction toward Sun changes!
• The moon’s synodic
period (to reach the
same position relative
to the sun) is 29.53
days (~ 1 month).
What are spring tides?
1. High tides occurring in the spring.
2. Low tides occurring in the spring.
3. Tides during which the tidal forces of the
moon and the sun cancel each other out.
4. Tides during which the tidal forces of the
moon and the sun add to each other.
5. Tides during which tidal forces are
enhanced by a spring.
Spring and Neap Tides
Spring tides
The Sun is also
producing tidal effects,
about half as strong as
the Moon.
• Near Full and New
Moon, those two effects
add up to cause spring
tides
• Near first and third
quarter, the two effects
work at a right angle w.r.t.
each other, causing neap
tides.
Neap tides
What is Stongehenge?
1. An ancient greek script with astronomical
significance, dating back about 2,500 years.
2. An ancient Maya site with astronomical
significance, dating back about 1,500 years ago.
3. An ancient monument in England with
astronomical significance, dating back to about
3,000 B.C.
4. Probably the first stone building known to be
built by mankind.
5. A stone monument built by the earliest known
population of homo sapiens in southern Africa.
Stonehenge
• Constructed 3000 – 1800
B.C. in Great Britain
• Alignments with locations of
sunset, sunrise, moonset and
moonrise at summer and
winter solstices
• Probably used as calendar.
How does the Copernican model (correctly)
explain the retrograde (westward) loops of
the outer planets’ motion?
1.
2.
3.
4.
5.
With epicycles on which the planets move, and which
themselves move in circles around the Earth.
By proposing that the Earth wobbles back and forth, giving
the impression of the planets moving in loops.
By proposing that the planets orbit imaginary planets, which
themselves orbit Earth, thereby giving the impression of
moving in loops.
By realizing that both Earth and all other planets orbit the sun,
and Earth passing the outer planets on the inside naturally
gives the impression of the outer planet moving in loops.
Nonsense: The Copernican model made no predictions
concerning the motions of the planets.
The Copernican “Universe”:
New (and correct) explanation for
retrograde motion of the planets:
Retrograde
(westward)
motion of a
planet occurs
when the Earth
passes the
planet.
This made
Ptolemy’s
epicycles
unnecessary.
How many moons did Galileo
discover around Jupiter?
1.
2.
3.
4.
5.
1
2
4
8
16
Major discoveries of Galileo:
• Moons of Jupiter
(4 Galilean moons)
(What he really saw)
• Rings of Saturn
What he really saw: Two little
“moons” on both sides of Saturn!
According to Kepler’s Third
Law, which planet has the
shortest orbital period?
1.
2.
3.
4.
5.
Mercury
Venus
Earth
Mars
Jupiter
Kepler’s Laws of Planetary Motion
1. The orbits of the planets are ellipses with the sun at
one focus.
2. A line from a planet to the sun sweeps over equal
areas in equal intervals of time.
3. A planet’s orbital period (P) squared is proportional to
its average distance from the sun (a) cubed:
Py2 = aAU3
“A body at rest remains at rest; a body in
uniform remains in uniform motion, unless
acted upon by a force” – This is …
1.
2.
3.
4.
5.
Einstein’s Theory of Special Relativity
Einstein’s Theory of General Relativity
Newton’s First Law
Newton’s Second Law
Newton’s Third Law
Newton’s Laws
1. A body at rest stays at rest, a body
in motion stays in uniform motion,
unless acted upon by a force.
2. Force F and acceleration a are
related to the mass m through
F = m*a
3. For every force there is an equal
and opposite force acting on the
body exerting the force.
Also: Law of mutual gravitation:
F = G*M*m/d2
Which of the following is NOT a
prediction of Special Relativity?
1.
2.
3.
4.
5.
Time dilation (moving clocks measure less time).
Length contraction (moving objects appear shortened
along the direction of motion).
Doppler shift (light from a approaching light source is
shifted to higher frequencies = shorter wavelengths).
Light from a moving light source propagates at a faster
speed than the standard speed of light (300,000 km/s).
Relativistic aberration (light from a moving light source is
beamed in the direction of motion).
Effects of Special Relativity
• Time dilation: Fast moving objects
experience less time.
• Length contraction: Fast moving objects
appear shortened.
• The energy of a body at rest is not 0.
Instead, we find
E0 = m c2
• Relativistic aberration: Distortion of angles
• Relativistic Doppler shift: Change
of wavelength (color) of light.
Which of these colors represents the
highest frequency of light?
1.
2.
3.
4.
5.
Green
Blue
Red
Orange
Yellow
Wavelengths and Colors
4000 Å = 400 nm
7000 Å = 700 nm
7.5*1014
(f = 4.3*1014 Hz)
(f =
Hz)
Different colors of visible light correspond
to different wavelengths.
Which of these wavelength ranges
probe temperatures around 1 million K?
1.
2.
3.
4.
5.
Radio
Infrared
Visible Light
X-rays
Gamma-rays
The Electromagnetic Spectrum
Wavelength
109 oK
106 oK
Need satellites
to observe
103 oK
High
flying air
planes or
satellites
1 oK
Frequency
Temperature
What determines the nature of an atom
(i.e., whether it is Hydrogen, Helium,
Carbon, …)?
1. The number of protons in the nucleus.
2. The number of neutrons in the nucleus.
3. The number of neutrons + protons in the
nucleus.
4. The number of neutrons + protons + electrons
making up the atom.
5. The ratio of protons to neutrons in the atom.
Atomic Structure
• An atom consists of
an atomic nucleus
(protons and
neutrons) and a
cloud of electrons
surrounding it.
• Almost all of the mass is
contained in the nucleus,
while almost all of the
space is occupied by the
electron cloud.
Different Kinds of Atoms
• The kind of atom
depends on the
number of protons
in the nucleus.
• Most abundant:
Hydrogen (H),
with one proton
(+ 1 electron).
• Next: Helium (He),
with 2 protons (and
2 neutrons + 2 el.).
Helium 4
Different
numbers of
neutrons ↔
different
isotopes