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School-college Physics Notes: Astronomy 7. The phases of the Moon

GCSE level Physics exam revision notes on astronomy

ASTRONOMY: 7. A diagram naming and explaining the phases of the Earth's moon

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Astronomy 7.

Explaining the phases of the Earth's moon

Phases of the moon waxing waning as seen from Earth

In sequence as the Moon orbits the Earth in 28 days. The phases in sequence are:

1. new moon,  2. waxing crescent moon,  3.first quarter moon,  4. waxing gibbous moon

5. full moon,  6. waning gibbous moon,  7. third quarter moon,  8. waning crescent moon

A few details about our moon

The gravitational field strength on the moon is much smaller than on Earth because of its much smaller mass.

Gravitational field strengths: Earth 9.8 m/s2 or 9.81 N/kg;  Moon 1.62 m/s2 or 1.62 N/kg (~1/6th of Earth)

The moon does NOT have an atmosphere, its mass, and hence its gravity, is too low to hold on to an atmosphere.

There are no sedimentary rocks on the moon because there is no atmosphere, so there has been no weather to bring about weathering, erosion, transportation, deposition and compression to form sedimentary rocks.

There will be metamorphic rocks on the moon because there is evidence of volcanic activity and even igneous rocks when heated can re-crystallise to form a 'new' metamorphic rock.

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Key points for Physics - naming and explaining the phases of the Earth's moon

Phases of the Moon: Names and Explanations

The Moon’s phases are caused by its orbit around Earth, changing how much of its sunlit side we can see.

The full cycle takes about 29.5 days and includes eight main phases:

1. New Moon

  • Position: Moon is between Earth and Sun
  • Appearance: Invisible from Earth (dark side faces us)
  • Why: The side facing Earth is not illuminated

2. Waxing Crescent

  • Appearance: Thin crescent on the right side
  • "Waxing" means growing — more of the Moon becomes visible each night

3. First Quarter

  • Appearance: Right half of the Moon is lit
  • Why "Quarter": Moon is ¼ of the way through its orbit

4. Waxing Gibbous

  • Appearance: More than half lit, but not yet full
  • Continues to grow toward full illumination

5. Full Moon

  • Position: Earth is between the Sun and Moon
  • Appearance: Fully illuminated disk
  • Fun fact: Only time lunar eclipses can occur

6. Waning Gibbous

  • "Waning" means shrinking — light begins to decrease
  • Appearance: Left side still mostly lit

7. Last (Third) Quarter

  • Appearance: Left half of the Moon is lit
  • Moon is ¾ through its orbit

8. Waning Crescent

  • Appearance: Thin crescent on the left side
  • Leads back to New Moon and the cycle restarts

Keywords, phrases and learning objectives for astronomy

With the help of a diagram be able to explaining the phases of the moon.

Know the moon has a much weaker gravitational field (1/6th of Earth), geologically the rocks are similar to those found on Earth (but no sedimentary rocks) and the moon has no atmosphere.

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