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PHYSICS · Stage 7 · Chapter 5: The Earth in Space

Physics 5.5 The Moon — Lunar Phases, Solar Eclipses and Lunar Eclipses

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5.5 The Moon — Lunar Phases, Solar Eclipses and Lunar Eclipses — Alhussan International Academy — Physics (Stages 7–9) notes: Explain why we see phases of the Moon, solar and lunar eclipses, and tides.Seen day and night — you can see the Moon in the sky during the day and during …

  • Reflected light
  • Orbit time
  • Cycle length
  • Not the Earth's shadow
  • Expectation
  • Tilted orbit

Section 1 of 5Moonlight

  • Seen day and night
    • you can see the Moon in the sky during the day and during the night.
  • Reflected light
    • the Moon does not give out light itself, and moonlight is reflected sunlight.
  • Lit half
    • the light from the Sun lights up half of the Moon, just as it lights up half of every planet and moon in the Solar System.

Section 2 of 5Phases of the Moon

  • Orbit time
    • the Moon orbits the Earth every 27.3 days.
  • Phases = the regular changes in the Moon's shape seen from Earth.
    • The Moon appears to change shape because it moves around the Earth, so we see different amounts of its lit half.
  • Order
    • new Moon, waxing crescent, first quarter, waxing gibbous, full Moon, waning gibbous, last (third) quarter and waning crescent.
    • At new Moon we see none of the lit half, and at full Moon we see the whole lit half.
  • Cycle length
    • the cycle of phases takes about 29.5 days, because the Earth also moves round the Sun.
  • Same face
    • the Moon always shows the same face to the Earth because it spins once in each orbit.
  • Not the Earth's shadow
    • phases come from viewing the lit half from different angles.

Section 3 of 5Eclipses

  • Shadow
    • when an object moves in front of a source of light, a shadow is formed, which can happen when the Moon or the Earth moves in front of the Sun.
  • Solar eclipse = when the Moon passes between the Sun and the Earth and blocks the light from the Sun.
    • The region of deep shadow, or umbra, is where you see a total eclipse.
    • Outside that region, in the penumbra, you see a partial eclipse.
    • Total eclipses are seen because the Moon happens to be the right size and distance from the Earth to exactly cover the Sun, which is a coincidence.
  • Lunar eclipse = when the Earth comes between the Sun and the Moon, so the Moon is in the shadow of the Earth.

Section 4 of 5Why eclipses are rare

  • Expectation
    • you might expect an eclipse every 27.3 days, because the Moon orbits the Earth in that time.
  • Tilted orbit
    • the orbit of the Moon is tilted slightly, and the Earth, Sun and Moon need to be lined up for an eclipse to happen.
  • If the orbit was not tilted
    • the Earth would block out the light every month and we would never see a full moon.

Section 5 of 5Tides

  • Cause
    • water in the oceans is free to move, and the gravitational attraction of the Moon and Sun produces tides.
  • Bulges
    • there is a bulge of water on either side of the Earth due to the force of gravity from the Moon.
    • The Earth spins through the bulges, producing two high tides a day.
  • Spring tides
    • when the Sun and Moon are aligned, their pulls add up, giving a higher tide with very high high tides and very low low tides.
    • They happen at new Moon and full Moon.
  • Neap tides
    • when the Sun and Moon are not aligned, such as at right angles at the quarter Moons, the pulls partly cancel, giving a lower tidal range.
EventAlignment
Solar eclipseSun, Moon, Earth
Lunar eclipseSun, Earth, Moon
Spring tideSun and Moon in line with Earth
  • Connection
    • a new Moon, a solar eclipse and a spring tide all happen when the Sun, Moon and Earth are in line.

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