The Moon: Notes

The Moon in Motion

  • The Moon is the brightest object in the night sky.

  • It's the natural satellite of Earth.

  • Distance from Earth: 385000 km (three days by spacecraft).

  • Diameter at Equator: 3475 km (Earth’s diameter is 12750 km).

  • Period of orbit around Earth: about 29.5 days.

  • Period of rotation around its own axis: about 29.5 days.

  • Surface gravity: about one-sixth that of Earth.

  • Surface temperature: ranges from -175 ºC in darkness to 125 ºC in sunlight.

Phases of the Moon

  • The Moon is visible from Earth only because it reflects light from the Sun.

  • As the Moon orbits Earth, it rotates so that the same side of the Moon always faces Earth (it is tidally locked).

    • New Moon: The entire side of the Moon facing Earth is completely dark.

    • Quarter Moon and Gibbous Moon: When half of the moon face is lit.

    • Full Moon: When the side facing Earth is completely bathed in sunlight.

  • As the Moon moves on in its orbit, less of the lit face starts to become visible.

  • Eventually, the near side is completely dark again, and there is another new Moon, and so the sequence continues.

  • During the period between a new Moon and a full Moon, the Moon is said to be waxing.

  • As the phases move from full Moon to new Moon, it is said to be waning.

  • New Moon – The Moon is between the Earth and Sun; it appears completely dark.

  • Order:

  • Waxing Crescent – A small sliver of the Moon is visible on the right side.

  • First Quarter – Half of the Moon is visible (right side lit).

  • Waxing Gibbous – More than half is visible and growing.

  • Full Moon – The entire face of the Moon is lit and visible.

  • Waning Gibbous – More than half is visible but shrinking.

  • Last Quarter (or Third Quarter) – Half of the Moon is visible (left side lit).

  • Waning Crescent – A small sliver is visible on the left side before returning to new moon.

Lunar Surface Features

  • Large dark, flat areas are known as "seas" (or maria), even though they do not hold water.

    • They are the result of ancient lava flows.

    • These regions cover about 16\% of the moon's surface.

  • The lighter regions of the moon are called the "highlands".

    • Some of the mountains in these regions reach heights of 6 km.

  • The moon's surface is covered in craters of different sizes.

    • Astronomers believe these are the results of meteorites striking the surface.

Lunar Environment

  • The moon has:

    • No atmosphere.

    • No air.

    • No flowing water on the surface.

    • It does not experience wind or rain.

    • There is no erosion of its cratered surface.

Lunar Cycle

  • The lunar cycle, the period between one new Moon and the next, is approximately 29.5 days.

  • The phases of the moon during the lunar cycle include:

    • New Moon

    • Crescent Moon (Waxing)

    • Quarter Moon (Waxing)

    • Gibbous Moon (Waxing)

    • Full Moon

    • Gibbous Moon (Waning)

    • Quarter Moon (Waning)

    • Crescent Moon (Waning)

Ocean Tides

  • Each day, the waters of Earth’s oceans rise and fall against the coastlines of islands and continents.

  • These changes in sea level are called tides.

  • Tides are caused mainly by the gravitational pull of the Moon on Earth’s oceans.

  • Due to the Earth’s rotation under the tidal bulges, two high tides and two low tides occur each day.

  • The period between high tide and low tide when the sea level falls is called the ebb tide.

  • The flow tide occurs when the sea levels rise between low tide and high tide.

Spring and Neap Tides

  • The Sun also exerts a gravitational pull on Earth’s oceans, but its effect is much smaller than the Moon’s because the Sun is much further away from Earth than the Moon.

  • During the full Moon and new Moon phases, the Sun, Moon, and Earth line up.

    • At these times, the gravitational pulls of the Moon and the Sun combine to make the ocean bulges larger.

    • This results in high tides that are higher than usual and low tides that are lower than usual. These are called spring tides.

  • During the first quarter and third quarter phases of the Moon, the Moon and Sun are at right angles relative to Earth.

    • Their gravitational pulls on the oceans work against each other, so we see lower high tides and higher low tides. These are called neap tides.

  • Each month, there are two spring tides and two neap tides, occurring during a full Moon and a new Moon.

Review Questions

  • Q1: What term is used to describe the dark, flat areas on the moon that are visible from Earth? (Answer: Seas/Maria)

  • Q2: How were the craters on the moon formed?

    • A. Erosion by wind

    • B. Through the formation of the solar system

    • C. Erosion by water

    • D. Meteorite impact (Correct Answer)

  • Q3: The phase of the moon that we see when the Earth is between the sun and the moon is called a:

    • A. Full moon (Correct Answer)

    • B. New moon

    • C. Quarter moon

  • Q4: Why does the Moon have a stronger gravitational pull on Earth than the Sun?

    • A. It is closer than the sun (Correct Answer)

    • B. It spins faster than the sun

    • C. It is made of rocks while the sun is made of gases.

  • Q5: Identify the position of the moon during a full moon, new moon, and quarter moon in relation to the Sun and Earth:

  • 🌑 New Moon

    • Position: Moon is between the Earth and the Sun.

    • The side of the Moon facing Earth is in shadow, so we can’t see it.

    🌕 Full Moon

    • Position: Earth is between the Sun and the Moon.

    • The entire face of the Moon that faces Earth is lit up by the Sun.

    🌓 First Quarter Moon

    • Position: The Moon is at a right angle to the Earth and Sun (90° angle).

    • The right half of the Moon is lit from Earth’s view.

    🌗 Last (Third) Quarter Moon

    • Position: Again, the Moon is at a right angle to the Earth and Sun, but on the opposite side from First Quarter.

    • The left half of the Moon is lit from Earth’s view.