Midterm 2 - ASTR 207

Who proposed the Sun-centered model

Copernicus

Who used a model to determine layout of
solar system (planetary distances
in AU)

Copernicus

What was wrong with Copernicus’s model of the solar system?

It was no more accurate than the Ptolemaic model in predicting planetary positions, because it still used perfect circles

Who Compiled the most accurate (one arcminute) naked eye measurements ever made of planetary positions

Tycho

Who could not detect stellar parallax, and thus still thought Earth must be at center of solar system (but recognized that other planets go around Sun)

Tycho

Who Hired Kepler, who used their observations to discover the truth about planetary motion

Tycho

Who first tried to match Tycho's observations with circular orbits, but an 8-arcminute discrepancy led him eventually to ellipses

Kepler

What is an ellipse

a term used to describe the shape of a planets orbit

What is Keplers first law of planetary motion?

The orbit of each planet around the Sun is an ellipse with the Sun at one focus.

What is Keplers second law of planetary motion?

As a planet moves around its orbit, it sweeps out equal areas in equal times.

What does Keplers second law mean for planetary speed in reference to its closeness to the Sun?

a planet travels faster when it is nearer to the Sun and slower when it is farther from the Sun

What is Keplers third law of planetary motion?

More distant planets orbit the Sun at slower average speeds, obeying the relationship p2 = a3
p = orbital period in years
a = avg. distance from Sun in AU

Why didn’t scientific thinkers immediately accept the Copernican model of the solar system?

Copernicus’s model did not make noticeably better predictions than the Earth-Centred model and they were reluctant to throw out the E.C. model because it had been around for so many years

How was careful observation of the sky used in early cultures?

to determine the seasons, decide when to plant crops and to naviagate on long voyages

True or False:

Tycho measured the parallax of a supernova and showed that it was further away than the moon

True

true or false

tycho measured the parrallax of a comet and showed that it was further away thn the moon

true

true or false

tycho measured the parralx of stars, showing that the Earth orbits the Sun

false

true or false tycho measured the positions of the planets with unprecedented accuracy, making it possible for Kelper to determine their orbits

true

Who solidified the Copernican Revolution?

Galileo

What were the major objections to the Copernican view.

1. Earth could not be moving because objects in air would be left behind.

2. Non-circular orbits are not "perfect" as heavens should be

3. If Earth were really orbiting Sun, we'd detect stellar parallax

Who thought that all objects naturally come to rest.

Aristotle

How did Galileo show Earth could be moving?

Galileo showed that objects will stay in motion unless a force acts to slow them down (Newton's first law of motion).

Who thought he had measured stellar
distances, so lack of parallax seemed to rule out
an orbiting Earth

Tycho

Who showed that there was a parallax by showing stars must be much farther than Tycho thought — in part by using his telescope to see the Milky Way is countless individual stars

Galileo

If stars were much farther away, then lack of
detectable ______ was no longer so troubling.

parallax

Who saw four
moons orbiting Jupiter,
proving that not all
objects orbit Earth.

Galileo

Who’s observations of phases of Venus
proved that it orbits the Sun and not Earth

Galileo

How can we distinguish science from non-science?

The idealized scientific method

What is the idealized scientific method based on?

proposing and testing hypotheses

hypothesis = educated guess

the idea that modern science seeks explanations for observed
phenomena that rely solely on natural causes is a:

hallmark of science

True or false

A scientific model cannot include divine intervention

True

What is the second Hallmark of Science

Science progresses through the creation and
testing of models of nature that explain the
observations as simply as possible.
(Simplicity = "Occam's razor")

What is the third Hallmark of Science

A scientific model must make testable
predictions about natural phenomena that would
force us to revise or abandon the model if the
predictions do not agree with observations.

A scientific theory must

• Explain a wide variety of observations with a
  few simple principles, AND

• Must be supported by a large, compelling
  body of evidence.

• Must NOT have failed any crucial test of its
  validity.

True or False

Galileo measure the parallax of a comet, supporting the Sun-centred model

False

True or False

Galileo observed mountains on the moon in support of the sun centred model

True

Why do many scientists consider Galileo the origionator of modern science?

He emphasized how important it is to test ideas through experiment

Define a Sidereal day

Earth rotates once
on its axis in 23
hours, 56 minutes,
and 4.07 seconds.

Define a Solar Day

The Sun makes one
circuit around the
sky in 24 hours.

Why is a solar day is longer than a sidereal day by
about 1/360

Earth moves about 1° in
orbit each day.

Define a sidereal month )

Moon orbits Earth in 27.3
days. Earth and Moon travel 30° around Sun
during that time (30°/360° = 1/12

Define a synodic month

A cycle of lunar phases; takes
about 29.5 days, 1/12 longer than a sidereal
month.

Define a sidereal year

Time for Earth to complete one
orbit of Sun

Define a tropical Year

Time for
Earth to complete one
cycle of seasons.

True or False

Tropical year is about 20
minutes (1/26,000)
shorter than a sidereal
year because of
precession.

True

Planetary periods can be measured with respect to ____ (sidereal) or to apparent position of ____
(synodic).

stars, sun

The Difference between
a planet's orbital
(sidereal) and
synodic period
depends on

how far
planet moves in 1
Earth year.

On rare occasions, an inner planet will be perfectly aligned with Sun during inferior conjunction, this is called

a transit across Sun's surface

Apparent solar time depends on

the position of the Sun in
the local sky

Length of an apparent
solar day changes during
the year because

Earth's
orbit is slightly elliptical

Mean solar time is based
on

the average length of a
day

What is the average time at
which Sun crosses
meridian

12 pm

What is a local definition of
time.

Mean solar time

Define the analemma

it illustrates
position of Sun with
respect to mean solar time

How does mean solar time compare with standard
time if you are at the far eastern edge of a time
zone?

The mean solar time is later than standard
time.

Define Universal time

the mean
solar time at 0° longitude

What is also known as Greenwich Mean Time
(GMT) because 0° longitude is defined to pass
through Greenwich, England

Universal Time

What is the standard time used for astronomy and
navigation around the world.

Universal Time

Why do we have leap year?

Because a tropical year is 365.25 days, we
need to add an extra day every 4 years so that
the seasons remain synchronized with the
calendar.

True or False

For precise synchronization, years divisible by
100 (e.g., 1900) are not leap years unless they
are divisible by 400 (e.g., 2000).

True

How long is a tropical year in days?

365.25

What happens to the moment of spring equinox in a
leap year?

It comes earlier than usual in the calendar
year.

Sidereal day (Earth's rotation with respect to
stars) is ___ minutes shorter than a solar day.

4

Sidereal month (27.3 day orbit of moon) is
____ then synodic month (29.5 day cycle of
phases)

shorter

Tropical year (cycle of seasons) is ____ minutes
shorter than sidereal years (time to orbit Sun).

20

Local solar time is based on:

the Sun’s position

Mean solar time is defined:

locally based on the
average solar day.

Standard time is defined with respect to

time zones

what is the difference between the lunar sidereal month and the lunar synodic month

the synodic month is 29 ½ days long and the sidereal month is shorter

When a superior planet is located on the opposite side of the Sun from Eart, what is its position called?

conjunction

(T/F) Mean solar time is based on the average length of the solar day

true

(T/F) Apparent solar time measures noon when the Sun is on the meridian

True

(T/F) Mean solar time is sometimes ahead of apparent solar time and sometimes behaind apparent solar time

True

(T/F) Mean solar time and apparent solar time are always within about 4 minutes of each other

False

The difference between a tropical year and a sidereal year is due to

the precession of Earth’s rotation axis

Speed

Rate at which object moves

Veolcity

Speed and direction

Acceleration

Any change in velocity, units of speed/time (m/s2)

According to the theory of the acceleration of gravity, All falling objects accelerate at ____ rate (not counting friction of air resistance

the same

Momentum =

mass x velocity

Net force

changes momentum, which
generally means an acceleration (change in
velocity)

Angular momentum

Rotational momentum of a spinning or orbiting
object

mass

the amount of matter in an object

weight

the force that acts upon an object

True or False

There is NO gravity in space

False

What is weightlessness due to in space?

a constant state of
free-fall

Who realized the same
physical laws that operate
on Earth also operate in
the heavens
– one universe

newton

Who discovered laws of motion
and gravity

Newton

what is newtons first law of motion

An object
moves at constant
velocity unless a net
force acts to change its
speed or direction.

What is Newtons second law of motion

Force = mass x acceleration

• or Force = rate of change in momentum

What is Newtons third law of motion?

For every force, there is always an equal and
opposite reaction force.

What did Newton discover laws of?

Motion and Gravitation

Objects continue at constant velocity
because of

conservation of momentum

True or False

The total momentum of interacting objects cannot change unless an external force is acting on them

True

What do interactinf objects exchange through equal and opposite forces?

momentum

Angular momentum =

mass x velocity x radius

What is torque

an external twisting force

The angular momentum of an object cannot
change unless a(n)
is acting on it.

torque

What keeps a planet (Earth) rotating and orbiting the sun

Earth experiences no twisting force as it orbits
the Sun, so its rotation and orbit will continue
indefinitely.

Why do objects rotate faster as the shrink in radius?

Angular momentum conservation