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PRS questions(before exam 1)

Note: PRS questions from thelast lecture before the exam willbe posted on Tuesday, Sept. 30)

There are at least 100,000,000,000galaxies in the universe. They exhibit two

odd behaviors:1. The vast majority of galaxies are

moving away from us.2. The more distant the galaxy, the

faster it is moving away from us!PRS question. True or false: this meansthat there was a huge explosion, and sinceall galaxies are moving away, we are closeto the center of the explosion.

1. TRUE 2. FALSE

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The Milky Way: a disk-like objectLeft side Right side

From this picture, we can conclude that the inner galaxy is on1. The right side2. The middle3. The left side

The Milky Way• Our Galaxy is shaped like a disk.• Our solar system is in that disk.• When we look at the Milky Way in the sky, we arelooking along that disk, i.e., at the disk edge-on

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The Milky Way: a disk-like objectBut what are those darkpatches and streaks?Answer: dust and gasin space

Also, why is it bulged in one direction?Answer: because the disk is bulged in that direction.

The current model of our galaxy…

Scientific model: a concept/representation/tool usedto think about how something works.

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PRS question. True or false:Constellations are important inmodern astronomy.

1. TRUE 2. FALSE

Constellations.

A Constellation• Most official constellation names come from antiquity. Some southern

hemisphere constellations were named by European explorers in the 17th

& 18th centuries.• The patterns of stars have no physical significance! Stars that appear

close together may lie at very different distances.

Modern astronomersmainly use constellationsas landmarks, but theirusefulness is pretty limited.

is just a region of the sky

Constellations were usefulin prehistoric times to keeptrack of the year, e.g., whento plant or harvest the crops.

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PRS Question:Modern astronomers use

constellations for which of thefollowing purposes?

1. For specifying the locations of objects in the sky2. For naming celestial objects3. For studying the ways that stars interact and

evolve4. For determining when to plant crops and when

to harvest5. All of the above6. None of the above

Yes, occasionally…(not very oftenthough)

For the most part,constellations are notused in most modernastronomy

PRS question:If you travel north from theUS into Canada, how willPolaris (the North Star)

change?1. It will be brighter.2. It will be dimmer.3.It will be higher in the sky.4. It will be lower in the sky.5. It will be exactly the same.

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Polaris, the “North Star”

Straight upat “A”

Straight upat position “B”

Suppose that “A”is the North Pole.The Earth rotatesaround the NorthPole. At this place,Polaris would bestraight up. It wouldbe as high in the skyas it can be.

At “B”, Polaris would bejust barely above thehorizon. The angle between “straight up”and Polaris would belarge.

PRS Question:Most stars rise and set, butsome never rise or set (the

circumpolar stars).Is this statement universally

true for any observer onEarth?

1. Yes. At any place, some stars will becircumpolar, and some will rise/set.

2. No.

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Consider an observer atthe North or South Pole:

At noon, a star ishere.

The observercan easily seeit at noon.

At midnight, thestar is here.

The observercan still easilysee the star.

At the North or South Pole, all stars are circumpolar.Nothing rises or sets!! (We would not say “most starsrise and set” if we lived at the North Pole.)

Seasons are due to the tilt of the earth’s axis.

PRS QUESTION: Suppose that the tilt of the Earth’s axis ofrotation were 90 degrees instead of 23.5 degrees, and the NorthPole faces the Sun in June. HOW MANY HOURS OF SUNLIGHTWOULD WE HAVE IN AMHERST IN DECEMBER?

Imagine this axis of rotation instead.

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Day length= 24 hours

Day length= 12 hours

Day length= 0 hours

In this imagined Solar System, the amount of daylight in Amherst at different times of year would be as indicated…

PRS question. We are made from a variety ofelements; carbon is a particularly importantexample.True or false: most of the atoms from whichwe are made were once deep inside the Sun.

1. TRUE 2. FALSEYes, the Sun creates heavier elements in its interior.However, those elements remain stuck deep inside.Something must happen to the Sun to get the stuff out!

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At aphelion, does a planet move faster,slower, or the same speed as it does at

perihelion?1. Faster2. Slower3. The same

speed

PRS Question

Kepler’s Third Law3. The ratio of the cube of a planet’s average

distance from the Sun to the square of itsorbital period is the same for each planet.

a3 / P2 = 1 a3 = P2

First correct mathematical formulas in astronomy!

(a must be in AU and P in years)

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Most asteroids are located betweenMars and Jupiter. If an asteroid has an

average distance of 3AU from theSun, how long does that asteroid take

to orbit the Sun (approximately)?1. 0.5 years2. 1 year3. 2 years4. 5 years5. 50 years

Kepler’s 3rd law: P2 = a3

• a = 3 AU• a3 = 3 x 3 x 3 = 27• P2 = 27, so P = square root (27) = √27• √27 is roughly equal to √25• √25 = 5 years

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Phases of the Moon PRS question. This month, the new moon was on September29th. When will the next full moon occur?

1. September 29th2. October 1st3. October 7th4. October 15th5. Halloween

PRS QuestionIf the Moon is relatively far from the Earth so

that its umbra does not reach the Earth’ssurface, someone directly below the umbra will

observe:1. a penumbral lunar eclipse2. a partial lunar eclipse3. a partial solar eclipse4. an annular eclipse5. no eclipse