Midterm 1Review

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What is the name of the galaxy cluster containing our own galaxy?

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1. The Solar System

2. The Milky Way

3. The Solar Group

4. The Local Group

5. The Local Cluster1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

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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

Clusters of galaxies are grouped intoClusters of galaxies are grouped into superclusterssuperclusters..Superclusters formSuperclusters form filamentsfilaments and and wallswalls around around voidsvoids..

The Universe on Very Large Scales

The Astronomical Unit (AU) is defined as

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1. The time it takes for the moon to orbit once around the Earth

2. The distance between the Earth and the moon

3. The distance between the Earth and the sun

4. The diameter of the Earth5. The size of the solar

system

9.5*109 equals

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1. 10995

2. 95,000,000,000

3. 9,500,000,000

4. 950,000,000

5. 951,000,000,000

Orbits in the Solar System

Plu

toN

eptu

neUra

nus

Saturn

Jupi

terM

ars

(Distances and times reproduced to scale)

Asteroids

Comets

~ 50 AU

Which planet’s orbit is the most highly inclined against the ecliptic?

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1. Mercury

2. Venus

3. Jupiter

4. Uranus

5. Pluto

Retrograde rotation Tipped over by more than 900

Mercury and Pluto: Unusually highly inclined orbits

Planetary Orbits and Rotation

Over the course of the night, stars are rising in the … and setting in the …

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1. North / South

2. East / West

3. South / North

4. West / East

5. East / East

Apparent Motion of the Celestial Sphere

Geo

grap

hic

Latit

ude l

Which of the following values of visual magnitudes seem correct to you?

1. A

2. B

3. C

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Object Magnitude

A B C

Sun 26.5 -26.5 -26.5

Full Moon 12.5 -12.5 -12.5

Venus 4 -4 5

Sirius 1.4 -1.4 0

The magnitude scale system can be extended towards negative numbers (very bright) and

numbers > 6 (faint objects):

Sirius (brightest star in the sky): mv = -1.42

Full moon: mv = -12.5

Sun: mv = -26.5

Which of the following is the longest time?

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1. The Earth’s orbital period2. The moon’s sidereal

orbital period3. One precession cycle4. The moon’s synodic

orbital period5. The Saros cycle

Precession (I)

Gravity is pulling on a slanted top. => Wobbling around the vertical.

The Sun’s gravity is doing the same to the Earth.

The resulting “wobbling” of the Earth’s axis of rotation around the vertical

w.r.t. the Ecliptic takes about 26,000 years and is called precession.

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

Conditions for Eclipses (II)

Eclipses occur in a cyclic pattern.

→ Saros cycle: 18 years, 11 days, 8 hours

Spring and Neap 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.

Spring tides

Neap tides

A Total Lunar Eclipse (II)

A total lunar eclipse can last up to 1 hour

and 40 min.

During a total eclipse, the moon has a faint, red

glow, reflecting sun light scattered in

the Earth’s atmosphere.

There are ~ 1 – 2 lunar eclipses per year.

An annular solar eclipse can occur when …

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1. The Earth is near apogee and the moon is near perihelion.

2. The Earth is near aphelion and the moon is near perigee.

3. The Earth is near perihelion and the moon is near apogee.

4. The Earth is near perigee and the moon is near aphelion.

5. Both the Earth and the Moon are near perihelion.

Earth’s and Moon’s orbits are slightly elliptical:

Sun

Earth

Moon

(Eccentricities greatly exaggerated!)

Perihelion = position closest to the sun (January)

Aphelion = position furthest away from the sun (July)

Perigee = position closest to Earth

Apogee = position furthest away from Earth

Kepler’s 2nd Law states that …

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1. The acceleration of a body is proportional to the force exerted on it, divided by the mass of the body.

2. To every force there is an equal and opposite force balancing it.

3. The planets are moving around the sun on ellipses with the sun in one focus.

4. The planets are moving on circular epicycles, which, in turn, are orbiting around the sun on perfect circles.

5. A virtual line from the sun to any planet swipes over equal areas in equal amounts of time.

A line from a planet to the sun sweeps over equal areas in equal intervals of time.

Fas

t Slow

Kepler’s 2nd Law

History of Astronomy – A one-slide overview

Ptolemy: Copernicus:

Earth in the center (geocentric Universe)

perfect circles

epicycles

Sun in the center (heliocentric Universe)

perfect circles

no epicycles

Galilei:First detailed telescope observations: Moons of Jupiter; rings of Saturn; phases of Venus; sunspots; surface features on the moon -> Support of Copernican model

Kepler:

1. Elliptical orbits

2. Non-uniform velocities

3. Inner planets orbit faster than outer planets

Newton:

1. No force => uniform, straight motion

2. Force = mass*acceleration

3. action = reaction

The Electromagnetic Spectrum

Need satellites to observe

Wavelength

Frequency

High flying air planes or satellites