AstronomyExam1Studyguide

In six months you'll be able to see a different set of stars in the night sky because..

the earth has moved. We are only able to view objects visible when the Sun is on the opposite side of Earth. After six months, Earth will have completed half its orbit about the Sun and the visible night sky will be in the opposite direction from a half y

A full moon is exactly _______ in position to the sun

opposite

If it is 6:00 P.M. where you live and you see a full Moon, it is:

Rising from the Eastern horizon

The average distance between the center of the Sun and the center of Earth is defined as an

astronomical unit

A line running from due north to due south through a point directly overhead is called:

Meridian

When the Sun crosses the meridian at your location on Earth, it is:

local noon

Residents of the Southern Hemisphere cannot see the stars around the north celestial pole because:

Earth itself is in the way. In the Southern Hemisphere, the south celestial pole is visible but the north pole is under the horizon, hidden by the earth under our feet.

The tilt of Earth's axis affects the seasons by:

changing the inclination of sunlight upon your location on Earth as the year progresses. During the season of summer, sunlight strikes Earth face-on while in winter the surface of Earth is more inclined relative to the sunlight direction, so therefore les

If Earth's axis were tilted twice as much as it currently is, we would experience:

larger changes in seasons. The greater the tilt of Earth, the more directly the polar areas of the planet will be exposed to the Sun during summer seasons. This should increase the intensity of heating and therefore trigger greater changes on our planet d

The celestial sphere helps us to communicate a star's:

The celestial sphere helps us specify the direction of a star on a spherical map which completely surrounds Earth.

Using the aberration of starlight, we can measure:

Starlight appears to change direction slightly due to Earth's orbit about the Sun. Careful observations show that the position of a star in the sky makes a small loop during the course of a year.

The moon takes ___ days to orbit around the Earth

27.23

From our perspective, the moon goes through its phases in 29.53 days because

The Moon's sidereal period is shorter than the cycle for its phases because the Moon must travel further to reach the same position with respect to the Sun. Earth has had time to move in its orbit about the Sun during the 27.3-day sidereal period of the M

When we compare a solar eclipse to a lunar eclipse, we find that:

the solar eclipse is visible over less of Earth's surface than a lunar eclipse.

from the equator, stars at night will seem to

rise in the east, move overhead, and then set in the west.

A lunar eclipse is possible:

The Moon's orbit is not exactly in the same plane as Earth's orbit about the Sun. An eclipse is possible only when the line on which the two orbits intersect also passes through the position of the Sun.

Is there any location on Earth where the entire sky is potentially visible within a 24-hour period (ignoring the fact that some stars can't be seen during the daytime)?

At the equator one can see all the way to the north and south celestial poles since they will be on the horizon. Throughout a 24-hour period, the entire sky will pass over.

We only see one side of the Moon because:

the Moon rotates on its axis once per revolution around Earth.

During the vernal equinox, a person on the equator will experience sunlight:

On the day of the vernal equinox, the Sun's position is on the celestial equator. Every location on Earth sees the Sun for 12 hours and experiences the night sky for 12 hours.

Our modern calendar system accounts for the precession of the equinoxes, so that:

Our calendar is based on a tropical year which is 365.24 solar days long. This requires the addition of an extra day every leap year so that the seasons occur during the same months year after year.

A sky observer in the Southern Hemisphere would:

In the Southern Hemisphere an observer would always see the south celestial pole and furthermore would see the sky moving around the pole in a clockwise direction, as opposed to the counter-clockwise rotation viewed by observers in the northern hemisphere

A measurement of an object's inertia is made via its

Mass is the property of matter, which is the degree to which an object resists changes in its motion.

Knowing Earth's orbital speed and its distance from the Sun, one can calculate

the mass of the sun. Kepler's third law applied to circular orbits shows that the square of the period of an orbit is equal to the cube of the radius of the orbit multiplied by a constant that includes the mass of the Sun.

Who is responsible for Newton's first law?

Galileo

The gravitational force between Earth and yourself depends upon

The attractive force of gravity between two objects is proportional to the product of their masses.

Astronauts in orbit around Earth feel weightless because

The astronauts (and their spacecraft) are falling around the Earth together. They never land on the Earth's surface because the Earth curves beneath them. They feel weightless, but really the Earth still exerts a force on them and keeps them in a state of

Jupiter takes longer to move around the Sun than Earth does because

Kepler's third law states there is a simple mathematical relationship between the orbital period of a planet and the size of the orbit. This relationship means that if a planet is farther from the Sun, it must have a longer period of orbit.

Escape velocity" refers to

the velocity required for an object to escape a planet's gravity.

If the Earth were twice as far from the Sun as it is now, the gravitational force between the Earth and the Sun would be

The force of gravity is inversely proportional to the square of the distance between the two objects. If the distance between the Earth and the Sun is doubled, the force of attraction will be only one fourth as great.

Kepler's second law, the law of equal areas, states that

In order for a line between a planet and the Sun to trace out equal areas in equal time intervals, the planet must move faster when nearer the Sun. When the distance to the Sun is smaller, the speed of the planet must be greater.

Newton's law of universal gravitation predicts the empirical laws that __________ articulated.

Kepler

According to Newton's first law, a space probe traveling through interplanetary space will

maintain its motion without any outside force. Unless acted upon by an outside unbalanced force, Newton's first law states that an object at rest will remain at rest, and an object in straight line motion will remain on that path.

An ellipse has no more than __ foci

2

Gravity is responsible for

Gravity causes matter to be gathered into planets and it causes objects to orbit each other.

What was Tycho Brahe's major role in the history of modern astronomy?

He gathered a vast amount of accurate planetary observations. Tycho Brahe collected much observational data before telescopes were used in astronomy. Kepler used Tycho's data to discover his laws.

Bound orbits take what kind of path?

elliptical. Bound orbits are closed curves called ellipses. Unbound orbits are either parabolas or hyperbolas.

Why do forces always occur simultaneously in pairs?

Every time an object exerts a force on another, an equal and opposite force is exerted by the second object on the first.

Copernicus is famous for reviving what ancient Greek idea?

The Earth orbits the Sun.

Which is a type of force: weight or mass?

weight

The time it takes a planet to complete its orbit is determined by

it's distance from the Sun. Kepler's third law

If two objects are droppe d from the same height, what will most likely take place?

Both objects will land at the same time.

What is Kepler's first law of motion?

the orbit of each planet is an ellipse with the sun at one focus

Are all planet orbits circular?

No

What is Kepler's second law of motion?

planet's sweep out equal areas in equal times

What is Kepler's 3rd law?

the square of the period of the planet's orbit, measured in years, is equal to the cube of the semimajor axis of the planet's orbit, measured in astronomical units.

what is the escape velocity of Earth?

28,000 km/hr

What is an orbit?

when an object is in free fall, constantly falling toward the Earth without actually hitting the ground

How long does it take for the sun to make one full circuit on the ecliptic?

one year

How did the Greeks know the Earth was round?

The shadow of the Earth during a lunar eclipse was round. Ships seemed to sink as they sailed away

You see the sun directly overhead on June 21st. What is your latitude?

23.5 degrees North

Can Venus ever be directly overhead at midnight? why or why not?

No. If Venus were directly overhead at midnight, it would be on the opposite side of the Earth from the sun. but it orbits inside Earth's orbit so that can never happen.

At what time is the moon highest in the sky?

midnight

It is March 21st the spring equinox. The sun at noon is at an altitude of 75 degrees above the Southern horizon. What is your latitude?

15 degrees North.

The gravitational force between two objects is 100 Newtons. The distance between them is now increased by a factor of five. Now what is the gravitational force between them?

If the distance increases by a factor of five, then the force of gravity is weaker by a factor of 25 (inverse-square law). So the force is 4 newtons.

Is the gravitational force of the Sun on the Earth greater than, equal to, or smaller than the gravitational force of the Earth on the Sun?

Newton's 3rd law: they are equal

If the moon is at or close to it's full moon phase today, what will it's phase be in Australia? (180 degrees of longitude away)

Full

Why don't we see a solar eclipse every full moon?

Because the plane of the moon is tipped at 5 degrees

One half of the moon's surface is never visible to the Earth even though the Moon orbits the Earth once every 27 days. What does this tell you about the rotation period of the Moon?

The moon must rotate once every 27 days.

The tilt of Jupiter's rotation axis is only 3 degrees. If Earth's axis had this tilt, how would it affect our seasons.

Seasons would be much milder.

Does an astronomer working at the South pole ever see stars rise or set? Explain why or why not.

No. Being at the South pole is like being at the North pole. The south celestial pole is directly overhead, and the celestial sphere rotates around it. So all stars move horizontally, and neither rise nor set.

Describe in one sentence one major contribution to astronomy of ptolemy

epicycles to explain retrograde motion, geocentric universe

Describe in one sentence one major contribution to astronomy of copernicus

first realistic heliocentric model

Describe in one sentence one major contribution to astronomy of Tycho

very precise observations of planets and stars, especially mars

Describe in one sentence one major contribution to astronomy of of Galileo

sunspots, Jovian moons, phases of Venus, invention of telescope

Describe in one sentence one major contribution to astronomy of Kepler

abandoned circles, introduced elipses. Kepler's 3 laws.

What is retrograde motion?

Planets move along celestial sphere, then stop, reverse direction for a while, then stop and reverse again.

How did ptolemy explain retrograde motion?

epicycles

How did Copernicus explain retrograde motion?

Earth moves on an inside track and overtakes the other planet

The Earth and Moon both travel in ___________ around the _________

elliptical orbits, center of gravity

If you are standing at the Earth's North Pole, which of the following will be directly overhead?

the north celestial pole

Ancients noticed patterns in the motion of the objects in the sky, which led to the development of units of time like hours, days, weeks, months, and years. Which of the following moves the least in the sky, regardless of the time period being considered?

Polaris

If you observe Polaris to be 55 degrees above the horizon, you are at a latitude of approximately

55 degrees

In the Northern Hemisphere, summertime is warmer than wintertime because

the sun is visible for more hours, sunlight is more concentrated on the ground

If there is a waning gibbous moon visible in Chicago, that night in Australia there will be a

waning gibbous moon

You observe the Moon rising at 6 pm around sunset. Its phase is

full

You observe the Moon rising at 3 am, a few hours before sunrise. Its phase is

between third quarter and new

If you see a full Moon at midnight, about how long will it be until there is a new Moon?

2 weeks

If the Moon were twice as large in diameter as it is, we would have total solar eclipses

more often than now but not every month

Galileo used his observations of the changing phases of Venus to demonstrate that

Venus follows an orbit around the Sun rather than around the Earth

A major objection to the heliocentric model not resolved until the development of high-quality telescopes was that

the stars did not exhibit parallax

A planet is retrograde motion

shifts westward with respect to the stars

If the distance between two bodies is increased by a factor of 4, the gravitational force between them is ______ by a factor of ________

decreased; 16

What's the formula for surface gravity?

g = GM/R^2

Measuring an object's speed in orbit

V = sqroot(GM/d)

Measuring an object's mass using orbital motion

M = V^2d/G

What's Newton's third law of motion?

When two objects interact, they create equal and opposite forces on each other

The law of gravity

F = GMm/d^2

how do you find acceleration?

a = v/t or a = F/m

What were some flaws of Copernicus?

insisted that planetary orbits were circles

How many times per year are the length of the day and night equal and how much time separate these events?

twice per year, every 6 months

What is the phase of the moon during a solar eclipse?

new

Approximately how many times does the moon orbit on its axis during one complete orbit around earth?

1

If you are standing at the Earth's North Pole, which of the following will be at its Zenith?

the north celestial pole

During a solar eclipse

the Moons shadow falls on the Earth

What are some of the contributions to Astonomy from the Ancient Greeks?

Determined the Earth was round, measured the size of the Earth, found the relative size of the earth and moon, and suggested a heliocentric model of the solar system.

What is the definition of acceleration?

change in velocity

What is the ecliptic?

It's the line one the celestial sphere where the planets, the moon, and the Sun are found in the sky.

What formula do you use to find out how long it takes light to get from the sun to Earth?

t = d/v (d is AU and v is speed of light)

Why did early Greek astronomers reject the heliocentric model?

couldn't observe parallax

Why doesn't the moon have an atmosphere?

The escape velocity is very low on the moon

Galileo was the first to observe the phases of

Venus

If you double the distance between two bodies, the force of gravity between them drops to

one fourth

The dates of the equinox are

March 20th and September 22nd

The dates of solstice are

June 21st and December 21st

Every _________ years, the Earth's rotation axis precesses to new direction, causing the North Star to move.

20 thousand

How come eclipse seasons don't always remain the same?

Because of precession - shift in the moon's orbit

What is sidereal time?

How long it takes for a celestial object, like a star, to return to the same position in the sky after the Earth rotates. The sidereal day is 23 hours 56 minutes and 4 seconds