OpenStax - Astronomy 1/e

1. 5/5 points | Previous Answers OSAstro1 4.E.002. My Notes

[This question has multiple parts and problem types. It includes a numeric part, multiple-choice drop-down distractors, and an essay box for student input. It tests students on the what and the why, asking students in the essay portion to explain their understanding of the concept—in this case specifically, "Why does longitude have no meaning at the North and South Poles?"]

What is the latitude (in degrees) of the North Pole?

°

What is the latitude (in degrees) of the South Pole?

°

Why does longitude have no meaning at the North and South Poles?

Score: 1 out of 1

Comment:

†This work is a derivative of a text by OpenStax, used under CC BY by Cengage, Inc.

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2. 2/2 points | Previous Answers OSAstro1 4.E.014. My Notes

[This is a multiple-choice question that gives students a customized learning experience, since it has randomized variables in red.]

What fraction of the Moon's visible face is illuminated during third quarter phase?

one quarter one half three quarters fully illuminated

Why is this phase called third quarter?

This phase only occurs during the third quarter of the month. Only three quarters of the Moon is visible. This phase only occurs during the third quarter of the year. The Moon is three quarters of the way around its orbit.

†This work is a derivative of a text by OpenStax, used under CC BY by Cengage, Inc.

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3. 2/2 points | Previous Answers OSAstro1 4.E.034. My Notes

[This is a multiple-choice, qualitative-reasoning question.]

Describe what an observer at the crater Copernicus would see while the Moon is eclipsed on Earth.

They would see the Sun, but not the Earth. They would see the Earth fully cover the Sun. They would see the Earth fully illuminated by the Sun. They would see a small, dark spot pass along the face of the Earth.

What would the same observer see during what would be a total solar eclipse as viewed from Earth?

They would see the Sun, but not the Earth. They would see the Earth fully cover the Sun. They would see the Earth fully illuminated by the Sun. They would see a small, dark spot pass along the face of the Earth.

†This work is a derivative of a text by OpenStax, used under CC BY by Cengage, Inc.

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4. 3/3 points | Previous Answers OSAstro1 4.E.038. My Notes

[This question is a fill-in-the-blank numeric problem.]

What is the right ascension and declination (in degrees) of the Sun at noon on the summer solstice in the Northern Hemisphere?

right ascension

° declination

°

†This work is a derivative of a text by OpenStax, used under CC BY by Cengage, Inc.

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5. 1/1 points | Previous Answers OSAstro1 4.E.041. My Notes

[This is a multiple-choice, qualitative-reasoning question.]

Regions north of the Arctic Circle are known as the "land of the midnight Sun." Explain what this means from an astronomical perspective.

Due to the tilt of the Earth's axis, during some periods of the year, the day length in these regions is a full 24 hours. Due to the colder temperatures and thinner atmosphere, the Moon appears to be almost as bright as the Sun. In these regions, the sun rises around 6 p.m. and sets around 6 a.m. Due to the tilt of the Earth's axis, these regions only see the sun for a brief period around midnight.

†This work is a derivative of a text by OpenStax, used under CC BY by Cengage, Inc.

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6. 1/1 points | Previous Answers OSAstro1 4.E.051. My Notes

[This is a multiple-choice question that gives students a customized learning experience, since it has randomized variables in red.]

If a star rises at 10:30 p.m. tonight, approximately what time will it rise three months from now?

†This work is a derivative of a text by OpenStax, used under CC BY by Cengage, Inc.

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7. –/1 points seedshorizons14 3.li.ql.001.defective My Notes

[This is an engaging Quick Lesson animation video, complete with text transcript. Students are guided through a concept, or series of concepts, in a brief animation video and then asked to summarize what they've learned in an essay box so you can gauge their comprehension.]

Consider the following video.

Summarize what you learned in this video.

This answer has not been graded yet.

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8. 3/3 points | Previous Answers seedsfoundations14 3.tut.001.defective My Notes

[This is a step-by-step tutorial that coaches the student through every step of the most essential of astronomical problems. These highly structured, scaffolded learning activities give strong support to the student with emerging or dormant quantitative-reasoning skills.]

The Moon appears to be 0.51° wide in the sky. If the Moon's diameter is 3,480 km, how far away is it from the Earth in kilometers?

Part 1 of 3

We can use the small angle approximation to relate the physical size and distance of objects in the sky to their angular size.

angular diameter

2.06 ✕ 10⁵ arc seconds/radian

=

d

D

Part 2 of 3

If the Moon is 0.51° across on the sky, we need to first figure out how big that is in arc seconds. (Give the number of arc seconds in one degree, and then give the angular diameter in arc seconds.)

1 degree = N_as arc seconds

= 3600

3600

arc seconds

θ_as = θ_deg · N_as arc seconds/degree

θ_as =

1836

1840

arc seconds

Part 3 of 3

How far away (in km) is the Moon (3,480 km linear diameter) from the Earth if it has this angular diameter on the sky?

Solving for D in the small angle formula gives us:

D_km =

d_km · 2.06 ✕ 10⁵ arc seconds/radian

θ_as

D_km =

390000

3.90e+05

km

You have now completed the tutorial.

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9. 8/8 points | Previous Answers seedsfoundations14 3.tut.002.defective My Notes

[This is a step-by-step tutorial that coaches the student through every step of the most essential of astronomical problems. These highly structured, scaffolded learning activities give strong support to the student with emerging or dormant quantitative-reasoning skills.]

If the Earth is 1.50 ✕ 10⁸ km from the Sun, will the Moon (linear diameter 3,480 km) completely cover the Sun during a solar eclipse if it is 383,000 km from the Earth?

Part 1 of 4

We can use the small angle approximation to relate the physical size and distance of objects in the sky to their angular size:

angular diameter

2.06 ✕ 10⁵ arc seconds/radian

=

d

D

where d is the linear diameter of the object and D is its distance from Earth.

Part 2 of 4

During a solar eclipse, the Earth is 1.50 ✕ 10⁸ km from the Sun. What is the Sun's angular diameter in degrees if its linear diameter is 1.39 ✕ 10⁶ km?

First, we need to know how many arc seconds are in 1 degree.

1 degree = 3600

3600

arc seconds

Solving the small angle formula for the angular diameter and converting to degrees gives us:

angular diameter =

d_km(2.06 ✕ 10⁵ arc seconds/radian)

D_km

angular diameter =

1390000

1.39e+06

km

(2.06 ✕ 10⁵ arc seconds/radian)

150000000

1.50e+08

km

θ_as = 1909

1908.9

arc seconds

θ_deg = 0.53

0.5303

°

Part 3 of 4

If the Moon is 383,000 km from the Earth, we can use the small angle formula to determine the angular size in degrees.

angular diameter =

d_M‐km(2.06 ✕ 10⁵ arc seconds/radian)

D_M‐km

θ_M‐as =

1872

1871.7

arc seconds

θ_M‐deg =

0.52

0.5199

°

Part 4 of 4

In this configuration, would the Moon completely cover the Sun?

Yes

No

You have now completed the tutorial.

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10. 2/2 points | Previous Answers SeedsFoundations14 VAL.5.001. My Notes

Welcome to Lab 5: Planetary Geology

Even though the planets in our solar system have different physical properties in terms of radius, mass, and density, they often have similar characteristics with respect to planetary geology. By thoroughly studying the geology of Earth, we can leverage that knowledge to understand conditions on other planets.

Images of the four terrestrial planets side by side:

Mercury,

Venus,

Earth,

and Mars.

This lab will focus on the geology of Earth, since this is the planet that we know the most about. We will study the techniques that are used to learn about Earth's inner structure, and the processes that have caused Earth to evolve since it formed. While studying the terrestrial planets, our approach will utilize comparative planetology, noting the similarities and differences of these planets.

Learning Objectives ▲
After conducting this Virtual Astronomy Laboratory, the learner will be able to …
1. LO5.1 … explain how the surface of Earth gives clues to the structure of the interior.
2. LO5.2 … describe the different regions of the interior of Earth.
3. LO5.3 … list the lines of evidence (on Earth) favoring the theory of plate tectonics.
4. LO5.4 … compare and contrast the surfaces and interiors of the other terrestrial planets with those of Earth.
Seismic Waves ▲
Waves generated by earthquakes travel through Earth and carry information about the interior to the surface.

We can learn about the interior of Earth by studying the transmission of seismic waves through Earth. These waves are most often produced by earthquakes, but can also be caused by impacts or explosions.

Seismologists identify two different types of relevant seismic waves:
- P waves (pressure or compression waves) are longitudinal waves. This means that the medium through which the wave travels moves back and forth in the same direction the wave itself is traveling. (Sound waves are longitudinal waves.)
- S waves (shear waves) are transverse waves, meaning that the medium moves at right angles to the wave's direction of propagation. (Radio waves are transverse waves; so is the wriggle that you make when you and a friend hold opposite ends of a rope and one of you shakes it up and down.)
Generate and observe each type of wave using the simulator below.
You need an iFrame capable browser to view this.
When an earthquake occurs, both P waves and S waves propagate through Earth. Both types of waves may undergo reflection and refraction at the boundaries of layers that have different densities. Both waves also refract (or bend) as they travel through Earth; this is due to the increase in density with depth.

In general, P waves move faster than S waves. Another difference between the two is that pressure (compression) waves can travel through either solid or liquid material, whereas shear waves cannot propagate through liquids.

Seismograms are collected from all over the world when earthquakes occur. They allow us to study whether or not both types of waves have been detected. They also reveal the relative arrival times of the two types.

Regions Within Earth ▲

The interior of Earth is divided into four concentric layers: the crustThe outermost solid shell of a rocky planet or planetoid, the mantleThe layer of dense rock and metal oxides that lies between the molten core and the surface of Earth, the outer (or liquid) coreThe fluid layer of dense metals inside Earth, below the mantle and above the solid core, and inner (or solid) coreSolid metallic material at the very center of Earth.

From P and S wave measurements and other data, seismologists can identify four distinct regions of Earth's interior. From the center outward, they are:

Solid (Inner) Core: a hot region, largely composed of nickel and iron, that extends from the center of Earth to a radius of about 1,200 km.
Liquid (Outer) Core: a liquid nickel- and iron-rich region that extends from a radius of 1,200 km to about 3,500 km.
Mantle: a lower-temperature region primarily composed of silicates, which are less dense than nickel and iron. The mantle extends halfway through Earth, almost to Earth's surface.
Crust: the brittle, low-density outer layer of Earth, 20 to 70 km thick (depending on location). The crust also is composed mainly of silicates.

Region	Percent Mass	Temperature	Density
Crust	0.5%	500 K	2.5 g/cm³
Mantle	67.0%	3,000 K	4.5 g/cm³
Outer Core	30.8%	5,200 K	10.9 g/cm³
Inner Core	1.7%	5,700 K	12.9 g/cm³

Four layers of Earth are shown as concentric filled circles. From outside in the layers are:

crust,

mantle,

outer core,

and inner core.

Assessment: Scanning an Unknown Planet
This simulation allows you to measure seismic waves on an unexplored planet in order to determine the structure of the interior.

To use it, click on each of the three probes and place them at different distances from the "top" of the planet. Note that a vertical bar appears on the graph whenever you move one of the probes. This shows how far the probe lies from the planet's "north pole," in units of degrees of arc (180° is the "south pole" at the "bottom" of the globe).

Next, click the Make Waves button to create a burst of seismic waves from the top of the planet.

As the probes detect the S waves from this burst, their arrival times will appear as functions of delta, the angular distance from the pole.

Move the probes to three new delta values and repeat the process. Do this a few more times, until you have probed at least 12 different positions ranging from delta = 0 to delta = 180.

If no data point appears at a delta value where a probe is located, that doesn't necessarily mean you've made an error. Rather, the seismic wave may have failed to reach that position. (Based on what you read under the heading Seismic Waves above, can you guess why?)

Once you have completed the graph, click on the Next Page button (→) to interpret your data. Instructions continue below.
You need an iFrame capable browser to view this.
Once you have turned to Page 2 of the Unknown Planet Interior activity, note the Core Radius and S Wave Velocity sliders; your data (+ symbols); and a red line, tilting upward—the predictions of a mathematical model for wave propagation through the unknown planet.

Adjust the Core Radius slider until you mark off the area where you were unable to detect seismic waves. The existence of this shadow zone implies that a liquid core exists at the center of the planet, and the size of this zone determines the radius of this fluid zone.

Now adjust the S Wave Velocity slider until the red curve lies as close as possible to your data points. The S wave velocity that gives the best fit is your measurement of the wave speed within the planet's solid portion. Wave speeds can help planetary geologists determine the density of the solid material.

(a)

The Core Radius that best fits my data is km.

(b)

The S Wave Velocity that best fits my data is km/s.

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11. 4/4 points | Previous Answers SeedsFoundations14 VAL.5.002. My Notes

12. 5/5 points | Previous Answers SeedsFoundations14 VAL.5.003. My Notes

Comparative Planetology

Because of the similarities in the ways that the terrestrial planets formed, we can use our knowledge of the Earth's geology to learn about the geology of other rocky planets.

The planets in our solar system formed 4.6 billion years ago, from a swirling disk of material known as the solar nebulaThe rotating cloud of gas and dust from which the Sun, the planets, and minor bodies such as asteroids and comets all formed.. As the solar nebula cooled, gases condensed into solid grains of material that gradually combined into larger structures in a process known as accretionThe accumulation of small particles or bits of matter, resulting in the formation or growth of a larger body..

The diagram below lists many of the evolutionary processes that terrestrial planets and large, rocky moons experience. Although time is always increasing as you move down the chart, the time scale is different for each body. The importance of each labeled process also varies for each body.

A large vertical arrow points down, and evolutionary processes of terrestrial planets and large, rocky moons are listed to the right. In descending order the processes are:

Accretion

Heating, Differentiation

Solid Crust Forms

Bombardment by Solar System Debris

Volcanic Flooding

Possibility of Plate Tectonics

Solidification of Mantle

Cold Interior, Geologically Dead

Images of Mercury and the Earth are approximately to the left of "Volcanic Flooding". Images of Mars and Venus are approximately to the left of "Solidification of Mantle".

The remainder of this lab will broadly survey each of the terrestrial planets and the Moon, looking for evidence of these evolutionary processes and comparing them to the Earth.

Cooling and Size ▲

The interiors of smaller planets cool faster than those of larger planets, and the cooling rate directly affects each planet's interior structure.

Each of the terrestrial planets had a certain amount of heat energy at the time of its formation, and each has gained some additional energy because of the decay of radioactive elements. Both of these heat sources depend on the volume of material present—that is, on the size of the object, from center to edge.

The surface area from which the object can cool (radiate away this heat) also increases with size. However, as the size of an object increases, its volume grows more rapidly than its surface area. Consequently, small solar-system objects have cooled much more rapidly than larger ones.

Use the interactive calculator below to visualize the ratio of volume to area for a planet-sized sphere. Note that this ratio has dimensions of length (kilometers in this case). For every square kilometer of surface, a large spherical body will contain more cubic km of material—and each of those carries a fixed amount of thermal energy that ultimately will find its way into outer space.
You need an iFrame capable browser to view this.
Our Moon cooled very rapidly because of its small size. Measurements made by NASA's Lunar Prospector suggest that the Moon has a very small metallic core. This core is solid out to 440 km from the center and is partially molten at radii between 440 and 700 km. This notion of a smaller core is consistent with the fact that the Moon has no global magnetic field.
The Moon and Mercury ▲
- The lunar highlandsHigh-elevation lunar landscapes dominated by craters and mountains. were formed by meteor impacts, not plate tectonics.
- The mariaLow-elevation lunar landscapes dominated by smooth fields of hardened lava bearing relatively few craters., on the other hand, were formed by outflows of lava, implying that the Moon's interior was warmer at some point in the past than it is now
- Because the Moon is small, it has cooled off too quickly to possess convection currents.
- Mercury is nearly as small as the Moon, and the two bodies share similar surface features.
A photograph of Mercury.

You could be excused for thinking that the photo above is of Earth's Moon: its rugged mountains and bright, round craters look eerily familiar. Yet this is an image of our solar system's innermost planet, Mercury, taken in 2009 by the MESSENGER spacecraft. Neither object has enough mass to have retained an atmosphere, which would slow down, deflect, or break up many of the small solid objects that carved out the craters. Because of this, Mercury and our Moon both provide a valuable record of our solar system's early history, when orbiting chunks of rubble were much more numerous than they are today.

Let's study the Moon first. There are two very distinct types of terrain on the Moon. The light-colored areas are known as highlands. They consist largely of low-density, light-colored rocks. These materials rose to the surface when all was molten rock, and then solidified. The image below shows a typical highlands landscape. Because they solidified before the time of the Late Heavy Bombardment—a temporary surge in the impact rate, about 4 billion years ago—the highlands are heavily cratered.

A photograph of a heavily-cratered highlands landscape of the Moon.

While highlands cover essentially all the Moon's far side (the side that never faces Earth), the near side is dominated by dark areas known as maria (Latin for "seas"). These are huge, flat plains of hardened lava. When this lava flowed onto the surface of the Moon three billion years ago, the low-lying basins, produced by very large impacts, were filled in. However, the lava never reached the highlands.

The distribution of the craters tells us a lot about the history of the Moon. Note how the amount of cratering in the highlands is much greater than that in the maria. The highlands preserve the record of an earlier time, when the solar system was filled with debris. By the time the maria formed, much of the debris from planet formation had been swept up by the planets or ejected from the solar system. As a result, the maria are only sparsely cratered.

In the view from Apollo 15's Lunar Module shown below, the Command and Service Module assembly appears in front of the Sea of Fertility (Mare Fecunditatus).

A photograph from Apollo 15's Lunar Module that shows the Command and Service Module assembly in front of the Sea of Fertility.

Mercury is very similar to the Moon in that it has both extensive cratering and maria. It also cooled rapidly, due to its small size (2,400 km, compared to the Moon's 1,700 km). Another consequence of these bodies' small sizes is the lack of an atmosphere: neither had enough gravity to hold onto one.

Mercury's rapid cooling gave rise to some very interesting geologic features. Known as lobate scarps, these are giant, curved cliffs, as high as 3 km and hundreds of kilometers long. They formed when Mercury's crust wrinkled because of rapid cooling, much like the drying out of a raisin. (The image below shows a scarp curving upward across the lower-right quadrant, casting a shadow where the planet's surface descends abruptly.)

An image of Mercury's surface that shows a scarp curving upward across the lower-right quadrant, casting a shadow where the planet's surface descends abruptly.

Although Earth has the highest density (5.5 g/cm³) of any planet in the solar system, this was achieved over time, through gravitational compression. Earth's uncompressed density would be about 4.0 g/cm³. Mercury's density of 5.3 g/cm³ has been achieved with very little compression and likely implies a large iron core.

The Mariner 10 spacecraft detected a magnetic field when it flew by Mercury decades ago. Although 200 times weaker than the Earth's magnetic field, Mercury's is stronger than any of the other terrestrial planets' magnetic fields. This provides additional indirect evidence for an iron core.
Venus and Mars ▲
- Mars and Venus both show evidence of geologically recent volcanism.
- Neither Mars nor Venus shows evidence of plate tectonics, however.
- Mars and Venus both have retained atmospheres, but the former is much colder and less dense than Earth's, whereas the latter is much hotter and denser.
Mars is intermediate in size between the Moon and Mercury (on the small side) and Earth and Venus (on the large side). One would therefore expect Mars to have been internally active for an intermediate length of time as well.

An image of Olympus Mons, the largest of the volcanoes in the Tharsis region on Mars. A superposition of the outline of the state of Arizona, USA is superimposed on the image to show that Olympus Mons is approximately equal in area to that state.

There is considerable evidence of past volcanic activity on Mars, but evidence of recent activity can be seen only in the Tharsis region, where there are several huge volcanoes. These are all shield volcanoes created by hot spots poking through the crust. (The Hawaiian island chain is a row of shield volcanoes created as the Pacific Plate slides across a hot spot beneath the Pacific Ocean.)

On Earth, crustal plates don't stop moving long enough to allow a truly gigantic shield volcano to grow. However, Mars has never had plate tectonics. In addition, the crust of Mars must also be considerably thicker than Earth's in order to support the weight of these large volcanoes.

The scarcity of cratering on the volcano Olympus Mons (shown above) indicates that it has been active within the last 200 million years. Although fresh volcanic activity has not been observed on Mars, there is no reason to assume that it won't occur again in the Tharsis region.

Most of what we know about Venus comes from the Magellan space probe, which created radar maps of almost the entire surface. Its maps resolve features as small as football fields. The image below shows the planet's surface, as mapped by Magellan's radar. (The color is not intended to show what the surface would look like to the human eye. Rather, it was chosen to highlight differences in texture.)

An image of Venus highlighting differences in texture.

Very few impact craters are found on Venus. It appears that all crater evidence was erased about half a billion years ago, in an episode of volcanic resurfacing. Models of the interior suggest that Venus may undergo brief periods of intense volcanic activity every couple of hundred million years, with more calm periods in between. (Venus has more than 500 volcanoes with diameters greater than 20 km, and many smaller ones as well.)

Venus shows no evidence of a crust broken into plates. In fact, the crust motion appears to be largely vertical. Most of the planet's geological features have been created by rising plumes of magma that distort the crust.

Assessment: Comparing Rocky Worlds

Consider the following table.

Planet or Satellite	Relative Mass (Earth = 1.00)	Atmosphere	Crater Density	Volcanism	Plate Tectonics
Earth's Moon	0.012	No	high	No	No
Mercury	0.055	No	high	No	No
Mars	0.107	Yes	low	Yes	No
Venus	0.816	Yes	low	Yes	No
Earth	1.000	Yes	low	Yes	Yes

(a)

The presence of an atmosphere appears to

the number of craters on the surface of a planet or satellite.

(b)

A planet or satellite is likelier to retain an atmosphere if it has a

mass.

(c)

A planet or satellite is likelier to exhibit volcanism if its mass is

.

(d)

Of the five bodies listed above, only one exhibits plate boundaries and continental drift. Which is it?

(e)

The planet that exhibits these signs of plate tectonics also has the

.

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OpenStax - Astronomy 1/e (Homework)

Current Score : 36 / 37

Due : Monday, January 28, 2030 00:00 EST

Instructions

Assignment Submission

Assignment Scoring

Welcome to Lab 5: Planetary Geology

Earth: A Restless World

Plate Tectonics Explorer

Comparative Planetology

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WebAssign

Welcome, demo@demo

OpenStax - Astronomy 1/e (Homework)

Current Score : 36 / 37

Due : Monday, January 28, 2030 00:00 EST

Instructions

Assignment Submission

Assignment Scoring

Welcome to Lab 5: Planetary Geology

Earth: A Restless World

Plate Tectonics Explorer

Comparative Planetology