Midterm Review

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What is the scientific notation for the number of kilometers in an

AU1 Astronomical Unit (AU) = Distance Sun – Earth =

150 million km = 150,000,000 km

1) 1,000,000150 km2) 100,000,0001.5 km3) 1.5*106 km4) 1.5*107 km5) 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

Distance to the nearest large galaxies:Distance to the nearest large galaxies: several million light yearsseveral million light years

The Local Group of GalaxiesGalaxies usually don’t exist alone, but inGalaxies usually don’t exist alone, but in clusters of galaxiesclusters of galaxies

July is in which season in South Africa?

1. Spring

2. Summer

3. Fall

4. Winter

5. 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 (northern-

hemisphere) winter than in summer.

Light from the sun

Steep incidence → Summer

Shallow incidence → Winter

Ecliptic

How long is the period of the precession of Earth’s axis?

1. 1 year

2. 11 years

3. 3,000 years

4. 26,000 years

5. 10 billion years

PrecessionAs 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

• The moon’s synodic period (to reach the

same position relative to the sun) is 29.53 days (~ 1 month).

Fixed direction in space

Earth

Moon

Earth orbits around Sun => Direction toward Sun changes!

29.53 days

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 TidesThe 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.

Spring 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. With epicycles on which the planets move, and which

themselves move in circles around the Earth.

2. By proposing that the Earth wobbles back and forth, giving the impression of the planets moving in loops.

3. By proposing that the planets orbit imaginary planets, which themselves orbit Earth, thereby giving the impression of moving in loops.

4. 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.

5. 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:

This made Ptolemy’s epicycles

unnecessary.

Retrograde (westward) motion of a

planet occurs when the Earth

passes the planet.

How many moons did Galileo discover around Jupiter?

1. 1

2. 2

3. 4

4. 8

5. 16

Major discoveries of Galileo:

• Moons of Jupiter

(4 Galilean moons)

• Rings of Saturn

(What he really saw)

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. Mercury

2. Venus

3. Earth

4. Mars

5. 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 = aAU

3

“A body at rest remains at rest; a body in uniform remains in uniform motion, unless

acted upon by a force” – This is …

1. Einstein’s Theory of Special Relativity

2. Einstein’s Theory of General Relativity

3. Newton’s First Law

4. Newton’s Second Law

5. 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. Time dilation (moving clocks measure less time).

2. Length contraction (moving objects appear shortened along the direction of motion).

3. Doppler shift (light from a approaching light source is shifted to higher frequencies = shorter wavelengths).

4. Light from a moving light source propagates at a faster speed than the standard speed of light (300,000 km/s).

5. Relativistic aberration (light from a moving light source is beamed in the direction of motion).

Effects of Special Relativity

• 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.

• Time dilation: Fast moving objects experience less time.

Which of these colors represents the highest frequency of light?

1. Green

2. Blue

3. Red

4. Orange

5. Yellow

Wavelengths and Colors

Different colors of visible light correspond to different wavelengths.

4000 Å = 400 nm

(f = 7.5*1014 Hz)

7000 Å = 700 nm

(f = 4.3*1014 Hz)

Which of these wavelength ranges probe temperatures around 1 million K?

1. Radio

2. Infrared

3. Visible Light

4. X-rays

5. Gamma-rays

The Electromagnetic Spectrum

Need satellites to observe

Wavelength

Frequency

High flying air planes or satellites

Temperature109 oK 106 oK 103 oK 1 oK

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.

Helium 4

Different numbers of neutrons ↔ different isotopes

• Most abundant: Hydrogen (H), with one proton (+ 1 electron).

• Next: Helium (He), with 2 protons (and 2 neutrons + 2 el.).