Astronomy Quiz: Moon Phases
20 questions · exam conditions
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Moon PhasesQuestion 1 of 20

If the Moon is observed to be at an elongation of 135 degrees east of the Sun, what is its approximate phase?

Waxing Crescent
First Quarter
Waxing Gibbous
Waning Gibbous
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Astronomy Quiz

Astronomy Quiz: Moon Phases

Practice Moon Phases in Astronomy with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Moon Phases, giving you a quick way to practice the rules, question types, and explanations that matter most for Astronomy.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

If the Moon is observed to be at an elongation of 135 degrees east of the Sun, what is its approximate phase?

  1. Waxing Crescent
  2. First Quarter
  3. Waxing Gibbous (correct answer)
  4. Waning Gibbous
Explanation: Elongation is the angular separation between the Sun and the Moon as seen from Earth. A First Quarter Moon is at 90° east elongation, and a Full Moon is at 180° elongation. An elongation of 135° east falls between these two phases, which corresponds to a waxing gibbous Moon. A waning gibbous would have a western elongation.

Question 2

A crescent Moon hangs low in the west just after sunset. Its bright limb points toward

  1. the zenith point
  2. the rising Sun
  3. the setting Sun (correct answer)
  4. the north horizon
Explanation: The Moon's bright limb always points toward the Sun, because sunlight illuminates the side of the Moon facing it. Just after sunset, the Sun is below the western horizon, so a crescent Moon low in the west has its bright limb aimed at the setting Sun. The tempting mistake is thinking the bright edge points upward toward the zenith, but the source of light is the Sun, not the sky overhead.

Question 3

A Moon is 135° east of the Sun in the sky. Its phase is

  1. waning gibbous phase
  2. waxing gibbous phase (correct answer)
  3. first-quarter phase
  4. the full Moon phase
Explanation: At 135 degrees east of the Sun, the Moon is more than 90 degrees past new but not yet opposite the Sun at 180 degrees. Being east of the Sun means its illuminated side is growing, so it is waxing gibbous. First-quarter is only at 90 degrees east, and full is at 180, so both are too far from this position.

Question 4

A waxing gibbous Moon is on the meridian at 9 PM. When did it rise?

  1. around 3 PM (correct answer)
  2. around 9 AM
  3. around noon
  4. around 6 PM
Explanation: The Moon reaches the meridian roughly halfway between its rise and its set, about 6 hours after rising. If it is on the meridian at 9 PM, it must have risen around 3 PM. That fits a waxing gibbous Moon, which rises in the afternoon. The tempting wrong answer is around 6 PM, but that is when a full Moon rises, not a waxing gibbous.

Question 5

At 10 PM, a nearly full Moon sits low on the eastern horizon. Its phase is

  1. waning gibbous phase (correct answer)
  2. waxing gibbous phase
  3. the full Moon phase
  4. first quarter phase
Explanation: A nearly full Moon low in the east at 10 PM has just risen. The full Moon rises at sunset, so by 10 PM it would already be well above the horizon. A waxing gibbous rises before sunset and would also be higher. A waning gibbous rises after sunset, which matches a nearly full Moon just appearing in the east at 10 PM.

Question 6

At sunset, a first-quarter Moon is on the meridian. One week later at sunset, where is it?

  1. not visible at all
  2. still on the meridian
  3. setting in the west
  4. rising in the east (correct answer)
Explanation: One week after first quarter the Moon is full. A full moon is opposite the Sun in the sky, so it rises at sunset rather than setting or being on the meridian. Therefore you see it low in the east, just rising. The tempting mistake is to picture sunset as meaning it sets in the west, but that is the Sun's side, not the full moon's.

Question 7

The diagram shows the Earth-Moon system viewed from above the North Pole, with sunlight coming from the right. An observer on Earth sees the Moon in the phase shown in the photograph. At which position is the Moon in its orbit?

  1. Position A
  2. Position B
  3. Position C (correct answer)
  4. Position D
Explanation: The photograph shows a waning crescent, which means only a sliver of the Moon is lit, and the illuminated portion is decreasing. This phase occurs just before a New Moon. Looking at the diagram, sunlight comes from the right. Position D is the Full Moon (fully illuminated face seen from Earth). As the Moon orbits counter-clockwise, it moves to Position C. At C, an observer on Earth would see only a small sliver of the right side of the Moon illuminated by the sun on the right (which appears as the left side being lit in the sky), which corresponds to a waning crescent. Position A is a waxing crescent, and Position B is a waxing gibbous.

Question 8

A lunar eclipse can only occur during a Full Moon, but one does not occur every month. What is the primary reason for this?

  1. The Moon is usually too far from the Earth for the Earth's shadow to cover it completely.
  2. The Moon's orbit is tilted about 5 degrees relative to the Earth's orbital plane (the ecliptic). (correct answer)
  3. The Earth's axial tilt causes its shadow to point in different directions throughout the year.
  4. The Moon's orbital speed varies, so it often passes through the shadow too quickly for an eclipse.
Explanation: For a lunar eclipse to occur, the Sun, Earth, and Moon must be perfectly aligned so the Moon passes through Earth's shadow. This alignment corresponds to the Full Moon phase. However, the Moon's orbit is tilted about 5° with respect to the ecliptic plane (Earth's orbital plane). Because of this tilt, the Moon usually passes above or below Earth's shadow during a Full Moon. Eclipses only happen when a Full Moon occurs as the Moon is crossing the ecliptic plane.

Question 9

At 6:00 PM local time, an observer sees the Moon 90 degrees east of the setting Sun. Six hours later, at midnight, where will this observer see the Moon?

  1. Setting on the western horizon. (correct answer)
  2. At its highest point in the sky (on the meridian).
  3. Rising on the eastern horizon.
  4. It will have already set and will be below the horizon.
Explanation: A Moon that is 90 degrees east of the Sun is a First Quarter Moon. At 6:00 PM (sunset), a First Quarter Moon is at its highest point in the sky (on the meridian). The Earth rotates 360 degrees in 24 hours, or 15 degrees per hour. In six hours, the Earth will have rotated 6 * 15 = 90 degrees. This rotation will carry the Moon from its highest point down to the western horizon, where it will be setting. Therefore, at midnight, the observer will see the First Quarter Moon setting.

Question 10

When observing a thin crescent Moon, it is possible to see the rest of the lunar disk glowing faintly. This phenomenon, known as Earthshine, is brightest when the Moon is a waxing or waning crescent because:

  1. from the Moon's surface, the Earth would appear nearly full and thus reflect the most sunlight. (correct answer)
  2. the Moon is at its closest point to Earth during the crescent phases.
  3. the Earth's atmosphere bends more sunlight towards the Moon during these phases.
  4. the Sun, Earth, and Moon are almost perfectly aligned, focusing light onto the lunar surface.
Explanation: When you encounter questions about lunar phases and lighting phenomena, think about the Earth-Moon system from both perspectives—what we see from Earth and what an observer on the Moon would see. Earthshine occurs because Earth acts like a giant mirror, reflecting sunlight back to the Moon's dark side. The key insight is understanding the phase relationship: when we see a thin crescent Moon, an observer on the Moon would see Earth in nearly the opposite phase. Since only a sliver of the Moon is illuminated (crescent phase), most of the Earth-facing side is in darkness—but this dark region receives reflected light from an almost fully illuminated Earth as seen from the Moon's surface. The "fuller" Earth appears from the Moon, the more sunlight it reflects back, making Earthshine brightest during crescent phases. Answer A correctly identifies this inverse phase relationship. When the Moon appears as a thin crescent to us, Earth appears nearly full from the Moon, reflecting maximum sunlight back to the lunar surface. Answer B incorrectly suggests orbital distance matters. The Moon's distance varies throughout its elliptical orbit, but this has no correlation with crescent phases, which depend on Sun-Earth-Moon alignment. Answer C misrepresents atmospheric effects. While Earth's atmosphere does scatter light, this doesn't preferentially direct more light toward the Moon during crescent phases. Answer D describes a solar eclipse scenario (perfect alignment), which actually occurs during new moon phases when Earthshine would be least visible from Earth. Remember: lunar phenomena often involve thinking about the complementary perspective. When the Moon shows one phase from Earth, Earth shows roughly the opposite phase from the Moon.

Question 11

The terminator on the Moon is the line separating the illuminated and dark hemispheres. An astronomer focusing a telescope on the terminator of a waxing gibbous Moon would observe features with long shadows because:

  1. the Sun is at a low angle in the sky as viewed from that region of the Moon. (correct answer)
  2. the observer on Earth is viewing these features at an oblique angle.
  3. the Earth is partially blocking the sunlight, a phenomenon known as a penumbral eclipse.
  4. the Moon's lack of atmosphere makes all shadows appear longer and darker than on Earth.
Explanation: When analyzing lunar surface features, the key is understanding how sunlight angle affects shadow length and visibility. The terminator—the boundary between light and dark on the Moon—is where dramatic topographical details become most apparent. The correct answer is A because at the terminator, the Sun appears very low on the horizon from the perspective of that lunar region. Just like sunrise or sunset on Earth, when the Sun is low in the sky, it casts long shadows behind any elevated features like crater rims, mountains, or rocks. This low-angle illumination creates the stark contrast that makes lunar topography so visible along the terminator line. Option B incorrectly suggests that Earth's viewing angle causes the long shadows. While our viewing angle affects how we see features, it doesn't change the actual length of shadows cast by sunlight on the lunar surface. Option C is wrong because penumbral eclipses are rare events where Earth partially blocks sunlight from reaching the Moon—this isn't what's happening during normal lunar phases when we observe the terminator. Option D misunderstands shadow physics; while the Moon's lack of atmosphere does make shadows appear very dark (no atmospheric scattering to soften them), it doesn't make shadows longer. Shadow length depends solely on the angle of the light source. Remember this principle: shadow length is determined by light source angle, not viewing angle or atmospheric conditions. When you see questions about lunar observations, focus on where the Sun is positioned relative to the features being illuminated.

Question 12

An observer notes a crater near the center of the Moon's visible disk. Over the course of a single night, from moonrise to moonset, what apparent change will the observer see in the illumination of this crater?

  1. The crater will go from being in darkness to being fully lit as the Sun rises on the Moon.
  2. The crater's illumination will change significantly, corresponding to about 12 hours of a lunar day.
  3. The crater's illumination will not change noticeably, but its orientation will appear to rotate. (correct answer)
  4. The crater will move into and out of the Earth's shadow as the Moon crosses the sky.
Explanation: The phase of the Moon, and thus the illumination of any feature on its surface, changes very slowly. A full lunar day is about 29.5 Earth days long. Over the course of a single Earth night (8-12 hours), the angle of solar illumination on the Moon changes by a negligible amount. Therefore, the crater's lighting will appear constant. However, as the Moon moves across the sky from east to west due to Earth's rotation, its orientation relative to the observer's local horizon changes, making it appear to rotate.

Question 13

An observer in Santiago, Chile (Southern Hemisphere) and an observer in Anchorage, Alaska (Northern Hemisphere) both view the Moon on the same night when it is in its First Quarter phase. Which of the following best describes their observations?

  1. Both observers see the right half of the Moon illuminated.
  2. The observer in Santiago sees the left half illuminated, while the observer in Anchorage sees the right half illuminated.
  3. Both observers see the left half of the Moon illuminated.
  4. Both observers see the same half illuminated, but the Moon appears 'upside down' in Santiago compared to Anchorage. (correct answer)
Explanation: The First Quarter Moon is defined by the Sun-Earth-Moon geometry, where the Moon has completed one quarter of its orbit from the New Moon position. This means the western hemisphere of the Moon is illuminated by the Sun. From any location on Earth, it is this same physical hemisphere that is seen as lit. However, an observer's orientation relative to the Moon depends on their hemisphere. An observer in the Southern Hemisphere is 'upside down' relative to an observer in the Northern Hemisphere. Thus, the Moon will appear inverted; the lit portion that is on the right for an Alaskan will appear on the left for a Chilean, but it's the same half of the Moon.

Question 14

An observer in the Northern Hemisphere sees the First Quarter Moon on the meridian. At that same instant, an observer at the South Pole would see the Moon:

  1. directly overhead.
  2. below the horizon and not visible.
  3. on the horizon. (correct answer)
  4. on the meridian but appearing as a Third Quarter.
Explanation: The First Quarter Moon is on the meridian (at its highest point) at approximately sunset. The Moon, like the Sun and planets, is always located near the ecliptic plane. For an observer at the South Pole, the ecliptic plane intersects the horizon at a shallow angle. The celestial equator lies exactly on the horizon. Since the Moon's orbit is only tilted 5 degrees from the ecliptic, and the ecliptic is never far from the celestial equator, the Moon will always be very near the horizon as viewed from the poles. Therefore, the observer would see it on or very close to the horizon.

Question 15

When observing a thin crescent Moon, it is possible to see the rest of the lunar disk glowing faintly. This phenomenon, known as Earthshine, is brightest when the Moon is a waxing or waning crescent because:

  1. from the Moon's surface, the Earth would appear nearly full and thus reflect the most sunlight. (correct answer)
  2. the Moon is at its closest point to Earth during the crescent phases.
  3. the Earth's atmosphere bends more sunlight towards the Moon during these phases.
  4. the Sun, Earth, and Moon are almost perfectly aligned, focusing light onto the lunar surface.
Explanation: When you encounter questions about lunar phases and lighting phenomena, think about the Earth-Moon system from both perspectives—what we see from Earth and what an observer on the Moon would see. Earthshine occurs because Earth acts like a giant mirror, reflecting sunlight back to the Moon's dark side. The key insight is understanding the phase relationship: when we see a thin crescent Moon, an observer on the Moon would see Earth in nearly the opposite phase. Since only a sliver of the Moon is illuminated (crescent phase), most of the Earth-facing side is in darkness—but this dark region receives reflected light from an almost fully illuminated Earth as seen from the Moon's surface. The "fuller" Earth appears from the Moon, the more sunlight it reflects back, making Earthshine brightest during crescent phases. Answer A correctly identifies this inverse phase relationship. When the Moon appears as a thin crescent to us, Earth appears nearly full from the Moon, reflecting maximum sunlight back to the lunar surface. Answer B incorrectly suggests orbital distance matters. The Moon's distance varies throughout its elliptical orbit, but this has no correlation with crescent phases, which depend on Sun-Earth-Moon alignment. Answer C misrepresents atmospheric effects. While Earth's atmosphere does scatter light, this doesn't preferentially direct more light toward the Moon during crescent phases. Answer D describes a solar eclipse scenario (perfect alignment), which actually occurs during new moon phases when Earthshine would be least visible from Earth. Remember: lunar phenomena often involve thinking about the complementary perspective. When the Moon shows one phase from Earth, Earth shows roughly the opposite phase from the Moon.

Question 16

An observer watches the Moon transition from a waxing crescent to a waxing gibbous phase over the course of a week. During this period, the percentage of the Moon's total surface area that is illuminated by the Sun:

  1. increases from approximately 25% to 75%.
  2. remains constant at approximately 50%. (correct answer)
  3. decreases as the Moon's far side rotates into darkness.
  4. remains constant at 100% as the Sun always shines on it.
Explanation: Except during a lunar eclipse, the Sun always illuminates one full hemisphere of the Moon. Therefore, 50% of the Moon's total surface area is always sunlit. The lunar phases we observe are a result of our changing viewing angle of this illuminated hemisphere from Earth. The amount of the visible illuminated portion changes, but the total illuminated portion of the entire sphere does not.

Question 17

The time between two consecutive New Moons (the synodic period) is 29.5 days. The time for the Moon to complete one orbit relative to the distant stars (the sidereal period) is 27.3 days. This 2.2-day difference exists because:

  1. the Moon's orbital plane is tilted 5 degrees to the ecliptic, requiring extra time to realign.
  2. the Earth's rotation on its axis slightly alters the Moon's orbital path from our perspective.
  3. the Earth-Moon system is also orbiting the Sun, so the Moon must travel further to realign with the Sun. (correct answer)
  4. the gravitational pull of the Sun slightly slows the Moon's orbit when it is moving away from the Sun.
Explanation: A phase cycle (like New Moon to New Moon) depends on the Sun-Earth-Moon alignment. While the Moon is orbiting the Earth, the Earth is also moving in its orbit around the Sun. After the Moon completes a 360° orbit relative to the stars in 27.3 days, the Earth has moved about 1/12 of its way around the Sun. The Moon must orbit for an additional 2.2 days to 'catch up' to the new position required for the Sun-Earth-Moon alignment of the New Moon phase.

Question 18

An observer watches the Moon transition from a waxing crescent to a waxing gibbous phase over the course of a week. During this period, the percentage of the Moon's total surface area that is illuminated by the Sun:

  1. increases from approximately 25% to 75%.
  2. remains constant at approximately 50%. (correct answer)
  3. decreases as the Moon's far side rotates into darkness.
  4. remains constant at 100% as the Sun always shines on it.
Explanation: Except during a lunar eclipse, the Sun always illuminates one full hemisphere of the Moon. Therefore, 50% of the Moon's total surface area is always sunlit. The lunar phases we observe are a result of our changing viewing angle of this illuminated hemisphere from Earth. The amount of the visible illuminated portion changes, but the total illuminated portion of the entire sphere does not.

Question 19

On March 20th, an observer sees a Full Moon rising at 6:00 PM. On which of the following dates would the observer most likely see a Third Quarter Moon?

  1. March 28th (correct answer)
  2. March 24th
  3. April 6th
  4. April 13th
Explanation: When you encounter questions about lunar phases and timing, think about the lunar cycle: the Moon completes its phases every 29.5 days, moving through New Moon, First Quarter, Full Moon, and Third Quarter in sequence. If you see a Full Moon on March 20th, you need to determine when the Third Quarter phase occurs. The lunar phases are evenly spaced in time - each phase transition takes about 7.4 days (29.5 ÷ 4 = 7.375 days). Since Third Quarter comes after Full Moon in the cycle, you add approximately 7-8 days to March 20th, giving you March 27th-28th. Looking at the answer choices: A) March 28th perfectly matches this timing - it's about 8 days after the Full Moon, exactly when Third Quarter should occur. B) March 24th is only 4 days later, which would still be in the waning gibbous phase, not yet Third Quarter. C) April 6th is 17 days after March 20th, which would put you past Third Quarter and approaching the next New Moon phase. D) April 13th is 24 days later, nearly a complete lunar cycle, so you'd be approaching the next Full Moon. The key insight is that lunar phases follow a predictable 7-8 day spacing. When solving lunar phase problems, always count forward or backward in roughly week-long intervals from your reference point. Remember that the sequence never changes: New → First Quarter → Full → Third Quarter → New Moon again.

Question 20

The time between two consecutive New Moons (the synodic period) is 29.5 days. The time for the Moon to complete one orbit relative to the distant stars (the sidereal period) is 27.3 days. This 2.2-day difference exists because:

  1. the Moon's orbital plane is tilted 5 degrees to the ecliptic, requiring extra time to realign.
  2. the Earth's rotation on its axis slightly alters the Moon's orbital path from our perspective.
  3. the Earth-Moon system is also orbiting the Sun, so the Moon must travel further to realign with the Sun. (correct answer)
  4. the gravitational pull of the Sun slightly slows the Moon's orbit when it is moving away from the Sun.
Explanation: A phase cycle (like New Moon to New Moon) depends on the Sun-Earth-Moon alignment. While the Moon is orbiting the Earth, the Earth is also moving in its orbit around the Sun. After the Moon completes a 360° orbit relative to the stars in 27.3 days, the Earth has moved about 1/12 of its way around the Sun. The Moon must orbit for an additional 2.2 days to 'catch up' to the new position required for the Sun-Earth-Moon alignment of the New Moon phase.