Earth, Moon & Sun: Moon Phases — Practice · Grade 9 Science

Chapter: Earth, Moon & Solar System

What this practice covers

By the end of Earth, Moon & Sun: Moon Phases you should be able to:

10 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: phase, waxing, waning, new moon, synodic month.

Problem 1

What actually causes the Moon's phases?
  1. Earth's shadow falls on the Moon
  2. Different amounts of the Moon's sunlit half face Earth as it orbits
  3. The Moon changes how much light it produces
  4. Clouds on Earth block our view of the Moon
Answer and explanation

Different amounts of the Moon's sunlit half face Earth as it orbits

Half the Moon is always lit by the Sun; phases come from how much of that lit half faces Earth as the Moon orbits us.

Problem 2

Which phase comes right after a new moon?
  1. Waxing crescent
  2. Full moon
  3. Waning gibbous
  4. Third quarter
Answer and explanation

Waxing crescent

Right after new moon, a thin sliver of light begins to grow — that's the waxing crescent.

Problem 3

About how many days does one full cycle of moon phases take, from new moon back to new moon?
Answer and explanation

29.5 days

One synodic month is about 29.5 days.

Problem 4

Tonight the Moon is waxing gibbous. What does that tell you?
  1. More than half lit, and growing each night
  2. Less than half lit, and shrinking each night
  3. Completely dark
  4. Fully lit and about to shrink
Answer and explanation

More than half lit, and growing each night

Gibbous means more than half lit but not full; waxing means it's still growing toward full moon.

Problem 5

A classmate says, 'There's a lunar eclipse every single month at full moon.' Explain why this isn't correct.
Answer and explanation

A lunar eclipse can only happen at full moon, but it also requires the Sun, Earth, and Moon to line up almost exactly. Since the Moon's orbit is tilted about 5° relative to Earth's orbit around the Sun, that precise lineup only happens a few times a year, not every month.

Problem 6

At full moon, Earth lies roughly between Sun and Moon. Why is the Moon usually not eclipsed? Explain the reasoning in your own words.
Answer and explanation

Orbital tilt usually prevents an exact shadow alignment.

A full moon places the bodies roughly along a line.

The tilted lunar orbit usually places the Moon above or below Earth's shadow.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 7

Why can a crescent Moon be visible even when no eclipse occurs? Show the reasoning, not just the conclusion.
Hints

Separate the visible disk from the whole sphere.

Answer and explanation

Only a small part of the sunlit hemisphere faces Earth at that viewing angle.

Problem 8

At full Moon, what approximate Sun–Earth–Moon arrangement applies?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

The Moon is roughly opposite the Sun in Earth's sky, with Earth between their directions; orbital tilt usually prevents an eclipse.

Problem 9

Phase geometry helps explain approximate viewing timesPhase geometry helps explain approximate viewing timesObserver’s line of sightNear full phase: Moon lies opposite the Sun’s sky directionAs the Sun sets, the full Moon is generally rising.Approximate timing depends on place, date and orbit geometry.
Original ideal alignment for interpreting full-phase viewing. It omits orbital tilt and real horizon/atmospheric effects and does not depict an eclipse. Labels state approximate timing, not an operational observation schedule or encouragement to view the Sun directly.
Interpret the supplied visual model. Why is a full Moon commonly visible at night even though sunlight supplies its illumination?
Answer and explanation

It lies approximately opposite the Sun from Earth, so its illuminated face can be above the night-side observer’s horizon while the Sun is below it. Exact times depend on the stated conditions.

Read the arrangement as an ideal full-phase sky-direction relation. Distances are exaggerated and the Moon’s illuminated side faces back toward the Sun.

It lies approximately opposite the Sun from Earth, so its illuminated face can be above the night-side observer’s horizon while the Sun is below it. Exact times depend on the stated conditions.

Response criteria

  • Read the labeled quantities, reference frame or objects correctly.
  • Explain the relation between the visual evidence and the mechanism.
  • Keep the drawing’s stated assumptions and limits in the conclusion.

Problem 10

A student argues that first quarter must show one quarter of the lunar disk lit and every new Moon causes a solar eclipse. Correct both statements with geometry.
Answer and explanation

First quarter describes about one-quarter of the phase cycle and the visible disk is roughly half lit. New Moon alone does not guarantee a solar eclipse because the tilted lunar orbit usually misses the shadow line; alignment near a node is also needed.

First quarter describes about one-quarter of the phase cycle and the visible disk is roughly half lit. New Moon alone does not guarantee a solar eclipse because the tilted lunar orbit usually misses the shadow line; alignment near a node is also needed.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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