Question 1

Which of these planets fits the following description: "a solid, cool surface, with occasional dust clouds and a thin CO₂ atmosphere"? A)Venus B)Jupiter C)Mars D)Mercury

Accepted Answer

This description matches that of Mars, which has a solid surface, occasional dust storms, and a thin atmosphere primarily composed of carbon dioxide. Venus has a thick and hot atmosphere, Jupiter is a gas giant and does not have a solid surface, and Mercury has a hot and barren surface with no atmosphere.

Question 2

A friend who says that he is an astronomer claims that he was outside at midnight a few weeks ago looking at Mercury. What should be your response?
A)"Oh, you must have been in Australia or South America since Mercury can be seen at midnight only from the southern hemisphere."
B)"You must be mistaken, because Mercury NEVER appears in our midnight sky."
C)"Really! Have you just purchased a new telescope? Mercury can be seen at midnight only through a telescope."
D)"Congratulations, you have been fortunate enough to see Mercury on a very rare occasion."

Accepted Answer

Mercury is never visible at midnight because it is an inferior planet, orbiting closer to the Sun than Earth does. Therefore, it is only seen in the early morning or evening sky, near the horizon, not at midnight.

Question 3

When Mercury is at greatest elongation it is sometimes at a more favorable position for viewing and sometimes at a less favorable position for viewing. What can change from one occurrence of greatest elongation to the next to cause this difference?
A)the eccentricity of Mercury's orbit
B)the inclination of Mercury's orbit relative to the ecliptic
C)the position of Mercury in its orbit
D)the direction in space of the semimajor axis of Mercury's orbit

Accepted Answer

The position of Mercury in its orbit determines whether it will be at a more favorable or less favorable position for viewing during its greatest elongation. If it is closer to the Sun in its orbit, it will be at a less favorable position for viewing because it will be too close to the Sun and difficult to observe. If it is farther from the Sun in its orbit, it will be at a more favorable position for viewing because it will be easier to observe without being too close to the Sun. The eccentricity of Mercury's orbit, the inclination of its orbit relative to the ecliptic, and the direction of its semimajor axis do not have a significant effect on its position during greatest elongation.

Question 4

Mercury's orbital period around the Sun is 88 days, and its synodic period (the time from one inferior conjunction to the next) is 116 days. How often do we observe a transit of Mercury across the face of the Sun?
A)It occurs every 88 days.
B)It occurs every 116 days.
C)It occurs much less frequently than once every 116 days because Mercury's orbit is inclined 7° from the plane of the ecliptic.
D)We never see a transit of Mercury because it always crosses the face of the Sun when it is on the opposite side from Earth.

Accepted Answer

Transits of Mercury occur when Mercury passes directly between the Earth and the Sun, so that we see it as a small black dot moving across the face of the Sun. Since Mercury's orbital plane is inclined 7° from the plane of the ecliptic (the plane of Earth's orbit around the Sun), transits can only occur when Mercury is near the points where its orbital plane intersects the plane of the ecliptic. This only happens twice a year, around May 8 and November 10, but not all of these transits are visible from Earth. In addition, the Earth must be in the right position in its orbit to see the transit, which only happens about 13-14 times per century. Therefore, transits of Mercury are much less frequent than once every 116 days.

Question 5

When Mercury is at its least value of greatest elongation it is seen 18° from the Earth-Sun line. How long before sunrise (or after sunset) will Mercury be visible at this time?
A)18 minutes
B)a little less than 1 hour
C)2 hours
D)4 hours

Accepted Answer

When a planet is at its greatest elongation, it is at its farthest point from the Sun as seen from Earth. This means it can be visible in the sky before sunrise or after sunset, depending on its position. Since Mercury has an orbital period of only 88 days, it is always relatively close to the Sun and is never visible at midnight. When Mercury is at its least value of greatest elongation, it is seen 18° from the Earth-Sun line. The angle between the Sun and the horizon at sunrise or sunset is usually about 8°, so we can use basic trigonometry to calculate that Mercury will be visible roughly 18/(360/24) = 1 hour before sunrise (or after sunset), which corresponds to choice B.

Question 6

When Mercury is seen 18° from the Earth-Sun line, it will be visible a little less than 1 hour after sunset. What is the most important value in determining this time?
A)Mercury's rotation period about its axis
B)Mercury's sidereal period
C)Earth's rotation period about its axis
D)Earth's sidereal period

Accepted Answer

The most important value in determining the time when Mercury is visible is Earth's rotation period about its axis, as this determines the timing of sunset and therefore the time when Mercury will be visible. The other values listed (Mercury's rotation period about its axis, Mercury's sidereal period, and Earth's sidereal period) are not directly relevant to determining the timing of Mercury's visibility after sunset.

Question 7

Suppose you observe Mercury at each of the following phases and you extrapolate the full angular diameter from the part that you can see. At which phase will the angular diameter appear greatest?
A)full
B)first quarter
C)third quarter
D)crescent

Accepted Answer

Mercury's angular diameter appears greatest during its crescent phase because it is closest to Earth at this time, making it appear larger despite the smaller illuminated portion visible.

Question 8

Why are there are so few solar transits of Venus across the Sun's face as it revolves in its orbit and passes through inferior conjunction?
A)The orbital plane of Venus is inclined at 3.4° to the ecliptic plane.
B)The synodic period of Venus is very long because of the relative orbits of Earth and Venus.
C)Venus revolves around the Sun in a retrograde direction compared to the other terrestrial planets.
D)Venus rotates on its axis in a retrograde direction compared to the other terrestrial planets.

Accepted Answer

The orbital plane of Venus is inclined at 3.4° to the ecliptic plane, which means that most times when Venus passes through inferior conjunction it will be either above or below the Sun from our perspective, and not directly in front of it. This is why solar transits of Venus are relatively rare events.

Question 9

Why does Venus experience fewer transits of the Sun than does Mercury?
A)The orbit of Venus has a larger incline angle to the ecliptic than does the orbit of Mercury.
B)Mercury is closer to the Sun and thus orbits more frequently.
C)The retrograde rotation of Venus inhibits transits of the Sun.
D)The large eccentricity of Venus's orbit results in fewer transits.

Accepted Answer

Mercury is closer to the Sun, so it orbits the Sun more frequently than Venus, leading to more opportunities for transits.

Question 10

There are several reasons the surface of Mars is more easily seen from Earth than any other planet's surface. Which of the following is NOT one of those reasons?
A)relative proximity to Earth
B)high mountain ranges casting strong shadows and providing high-contrast images
C)visible high in the sky at midnight when at opposition
D)a thin, almost cloudless atmosphere

Accepted Answer

The visibility of Mars' surface from Earth is not significantly enhanced by high mountain ranges casting strong shadows. The primary factors that make Mars' surface more easily seen include its relative proximity to Earth, its position high in the sky at midnight when at opposition, and its thin, almost cloudless atmosphere, which allows for clearer views of the surface. High mountain ranges on Mars, like Olympus Mons, do not directly contribute to the ease of observation from Earth in the context of the reasons listed.

Question 11

Mars is best viewed from Earth when it is at opposition, but some occasions are more favorable than others. Why is this?
A)Mars has an elliptical orbit, and favorable oppositions occur when Mars is near perihelion in its orbit, and hence closest to Earth.
B)Mars has an elliptical orbit, and favorable oppositions occur when Mars is near aphelion in its orbit, and hence closest to Earth.
C)Mars's orbit is inclined at a significant angle to the ecliptic, so favorable oppositions occur when it is crossing the ecliptic plane while near opposition.
D)Even though Mars moves in a circular orbit, the orbit of Earth is elliptical, and so favorable oppositions occur when Earth is near perihelion.

Accepted Answer

Mars has an elliptical orbit, and favorable oppositions occur when Mars is near perihelion in its orbit, and hence closest to Earth.

Question 12

We have seen that our best Earth-bound views of Mercury and Venus occur when these planets are at their greatest elongations. Where is Mars when we have our best Earth-bound view of it?
A)greatest western elongation
B)greatest eastern elongation
C)conjunction
D)opposition

Accepted Answer

Like Jupiter and Saturn, Mars is best seen during opposition, when it is directly opposite the Sun in the sky and relatively close to Earth. During opposition, Mars is at its brightest and easiest to observe from Earth.

Question 13

When we view Mars at its most favorable opposition, a Martian would view Earth at:
A)most favorable opposition.
B)least favorable opposition.
C)superior conjunction.
D)inferior conjunction.

Accepted Answer

When we view Mars at its most favorable opposition, it means that Mars is in opposition to the Sun and Earth is directly between Mars and the Sun. Therefore, from Mars' perspective, Earth would be at inferior conjunction, which is when Earth is closest to the Sun and appears smallest in the sky. This is the least favorable opposition for Earth when viewed from Mars.

Question 14

How was Mercury's unusual spin-orbit coupling discovered?
A)Earth-based telescopes watched the movement of surface features on Mercury as it rotated.
B)It was determined by watching the phases of Mercury as observed from Earth.
C)It was not observed until a spacecraft was put into a permanent orbit around Mercury.
D)Radar waves were bounced off Mercury from an Earth-based radio telescope, and the Doppler shift of the reflections was measured.

Accepted Answer

Radar astronomy was crucial in discovering Mercury's spin-orbit coupling. By measuring the Doppler shift of radar waves bounced off Mercury, astronomers were able to determine that Mercury rotates three times on its axis for every two orbits it makes around the Sun. This was a significant discovery that corrected earlier assumptions about Mercury's rotation period.

Question 15

To what does the phrase "synchronous rotation" for an astronomical object (e.g., planet or moon) refer?
A)It has a rotation rate that is precisely maintained (e.g., 23 h 56 m 4.096 s for Earth).
B)For an object in an elliptical orbit, the rotation rate increases and decreases to match the changes in its orbital speed.
C)It completes precisely one rotation around its own axis for every orbit (1-to-1 spin-orbit coupling).
D)It has any rotation period that is in simple proportion to its orbital period (1-to-1 spin-orbit coupling, 3-to-2 spin-orbit coupling, etc.).

Accepted Answer

Synchronous rotation refers to when an astronomical object completes precisely one rotation around its own axis for every orbit. This is also known as 1-to-1 spin-orbit coupling. The other options listed are not accurate definitions of synchronous rotation.

Question 16

The reason the temperature on the dark side of Mercury is warmer than originally expected is that:
A)Mercury's large iron core conducts heat through the planet.
B)Mercury does not rotate synchronously with its orbital period.
C)several very active volcanoes on Mercury, produced by tidal stresses from the Sun, produce excess heat.
D)winds in Mercury's tenuous atmosphere carry heat from the daytime side to the night side.

Accepted Answer

Mercury does not rotate synchronously with its orbital period, allowing the dark side to be heated by residual heat from the daytime side.

Question 17

If you are on Mercury and the time is noon (Sun directly overhead), what time of day will it be 1 Mercurian year later (after Mercury has orbited the Sun once)? (See Figure 11-3.)
​
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A)It is noon.
B)It could be any time, because Mercury rotates independently of its revolution.
C)It is midnight.
D)It is just after sunset.

Accepted Answer

The answer of If you are on Mercury and the...

Question 18

If you are on Mercury and the time is noon (Sun directly overhead), how many times will Mercury rotate on its axis before the Sun is again directly overhead? (See Figure 11-3.)
​
 ​
A)one
B)two
C)three
D)58 2/3

Accepted Answer

The answer of If you are on Mercury and the...

Question 19

Mercury obeys Kepler's third law a³/T² = 1. What is T in this equation? A)Mercury's synodic period (in days) B)Mercury's synodic period (in years) C)Mercury's sidereal period (in days) D)Mercury's sidereal period (in years)

Accepted Answer

Kepler's third law relates the orbital period of a planet to its distance from the Sun. In this equation, T represents the time it takes for Mercury to complete one orbit around the Sun, which is its sidereal period, measured in years.

Question 20

Mercury rotates once on its axis with respect to the distant stars in 58 2/3 days and has a sidereal period of 88 days. A star is overhead to an observer on Mercury at midnight. How many times will Mercury orbit the Sun before this star is again directly over the observer's head at midnight? (Draw a diagram to help with this exercise.)
A)It will never again reach this specific position.
B)one
C)two
D)three

Accepted Answer

Let's first calculate how many times Mercury rotates on its axis in 88 days (its sidereal period).

Rotation per day = (1 rotation / 58 2/3 days) = (3 rotations / 176 days)
Rotations in 88 days = 88 * (3/176) = 1.5 rotations

This means that after one full orbit around the Sun (which takes 88 days), Mercury will have rotated 1.5 times on its axis with respect to the distant stars.

Now, let's consider the star that is overhead at midnight. After one orbit around the Sun (88 days), Mercury will have completed 1.5 rotations on its axis, but the star will have completed 2 rotations on its own axis (since it is much further away and has a much longer period).

So, in order for the star to be overhead at midnight again, Mercury must complete another 1/2 rotation on its own axis (since 1.5 rotations + 1/2 rotation = 2 rotations).

Therefore, the star will be overhead again after Mercury completes two full orbits around the Sun (since each orbit takes 88 days, and we need to wait for the star to complete two rotations on its own axis).

So the answer is choice C, two orbits.

Quiz 11: Mercury, Venus, and Mars: Terrestrial, yet Unique

Which of these planets fits the following description: "a solid, cool surface, with occasional dust clouds and a thin CO₂ atmosphere"?

A friend who says that he is an astronomer claims that he was outside at midnight a few weeks ago looking at Mercury. What should be your response?

When Mercury is at greatest elongation it is sometimes at a more favorable position for viewing and sometimes at a less favorable position for viewing. What can change from one occurrence of greatest elongation to the next to cause this difference?

Mercury's orbital period around the Sun is 88 days, and its synodic period (the time from one inferior conjunction to the next) is 116 days. How often do we observe a transit of Mercury across the face of the Sun?

When Mercury is at its least value of greatest elongation it is seen 18° from the Earth-Sun line. How long before sunrise (or after sunset) will Mercury be visible at this time?

When Mercury is seen 18° from the Earth-Sun line, it will be visible a little less than 1 hour after sunset. What is the most important value in determining this time?

Suppose you observe Mercury at each of the following phases and you extrapolate the full angular diameter from the part that you can see. At which phase will the angular diameter appear greatest?

Why are there are so few solar transits of Venus across the Sun's face as it revolves in its orbit and passes through inferior conjunction?

Why does Venus experience fewer transits of the Sun than does Mercury?

There are several reasons the surface of Mars is more easily seen from Earth than any other planet's surface. Which of the following is NOT one of those reasons?

Mars is best viewed from Earth when it is at opposition, but some occasions are more favorable than others. Why is this?

We have seen that our best Earth-bound views of Mercury and Venus occur when these planets are at their greatest elongations. Where is Mars when we have our best Earth-bound view of it?

When we view Mars at its most favorable opposition, a Martian would view Earth at:

How was Mercury's unusual spin-orbit coupling discovered?

To what does the phrase "synchronous rotation" for an astronomical object (e.g., planet or moon) refer?

The reason the temperature on the dark side of Mercury is warmer than originally expected is that:

If you are on Mercury and the time is noon (Sun directly overhead) , what time of day will it be 1 Mercurian year later (after Mercury has orbited the Sun once) ? (See Figure 11-3.)

If you are on Mercury and the time is noon (Sun directly overhead) , how many times will Mercury rotate on its axis before the Sun is again directly overhead? (See Figure 11-3.)

Mercury obeys Kepler's third law a³/T² = 1. What is T in this equation?