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Moon Phases: Crash Course Astronomy #4
CrashCourse
Overview
This video explains the phenomenon of moon phases, clarifying that they are not caused by the Earth's shadow, but by the changing angles at which we view the Moon's illuminated surface as it orbits Earth. It details the cycle from New Moon to Full Moon and back, explaining the terms waxing and waning, and introduces concepts like the terminator and Earthshine. The video emphasizes that the Moon is always half-lit by the Sun, and our perspective from Earth determines the phase we observe.
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Chapters
- The Moon appears to change shape due to its orbit around Earth and the Sun's illumination.
- The Moon, like Earth, is a sphere that is always half-lit by the Sun.
- The 'phase' we see is the portion of the Moon's illuminated half that is visible from Earth.
- The line dividing the lit and dark sides of the Moon is called the terminator.
Understanding that the Moon is always half-lit and that its phases are a result of geometry is fundamental to grasping the entire cycle and dispelling common misconceptions.
When the Sun is behind you, you see the fully illuminated half of the Moon (Full Moon); when the Moon is between you and the Sun, you see the dark half (New Moon).
- The lunar cycle, which gives us the word 'month,' takes about 29.5 days.
- New Moon occurs when the Moon is between Earth and the Sun, making its illuminated side invisible from Earth.
- As the Moon orbits, a sliver of light becomes visible, creating a waxing crescent.
- First Quarter occurs when the Moon is one-quarter of the way through its orbit, appearing half-lit from Earth.
This section introduces the beginning of the lunar cycle and the first key phases, explaining how the visible portion of the Moon grows after the New Moon.
During a waxing crescent, the Moon is visible shortly after sunset, appearing as a thin curve low in the western sky.
- Waxing gibbous describes the phase when more than half of the Moon is illuminated and the lit portion is still growing.
- Full Moon occurs when Earth is between the Sun and Moon, presenting the entire illuminated face to us.
- A Full Moon rises around sunset and sets around sunrise, being visible all night.
- The term 'gibbous' refers to the shape when more than half but not all of the Moon is lit.
This covers the progression towards the most visually striking phase, the Full Moon, and explains the terminology for the increasing illumination.
The Full Moon is opposite the Sun in the sky, meaning it appears when the Sun sets and disappears when the Sun rises.
- After the Full Moon, the illuminated portion begins to decrease, entering the waning phases.
- Waning gibbous is when the Moon is still more than half lit, but the illuminated part is shrinking.
- Third Quarter occurs three-quarters of the way through the orbit, appearing half-lit again, but the opposite half from the First Quarter.
- Waning crescent is the final phase before New Moon, where only a sliver of light is visible, shrinking each day.
- The cycle repeats every 29.5 days.
This section completes the lunar cycle, explaining how the illuminated portion shrinks and the Moon returns to its New Moon phase.
A Third Quarter Moon rises around midnight and is visible in the morning sky, setting around noon.
- From the Moon, Earth shows phases opposite to what we see; when we have a New Moon, the Moon-dweller sees a Full Earth.
- Earthshine is the reflection of sunlight off Earth onto the Moon's dark side, making the unlit portion of a crescent Moon faintly visible.
- Earth is significantly brighter than the Moon, making Earthshine much more intense than moonlight.
- We always see roughly the same side of the Moon because its rotation period matches its orbital period around Earth.
This introduces fascinating related phenomena like Earthshine and explains why we always see the same face of the Moon, adding depth to our understanding.
The 'ghostly face' sometimes seen on the dark part of a crescent Moon is due to Earthshine, the light reflected from Earth.
Key takeaways
- Moon phases are caused by the geometry of the Sun, Earth, and Moon, specifically how much of the Moon's sunlit side we can see from Earth.
- The Moon is always half-lit by the Sun; the phase we observe depends on our viewing angle from Earth.
- The lunar cycle, from New Moon to Full Moon and back, takes approximately 29.5 days and is the origin of the word 'month'.
- Waxing means the illuminated portion of the Moon is growing larger, while waning means it is shrinking.
- Key phases include New Moon (invisible), First Quarter (half-lit), Full Moon (fully-lit), and Third Quarter (half-lit on the opposite side).
- Earthshine is sunlight reflected from Earth onto the Moon, making the unlit portion of a crescent Moon visible.
- We always see the same side of the Moon because its rotation is tidally locked with its orbit around Earth.
Key terms
Moon PhasesOrbitIlluminationTerminatorNew MoonWaxing CrescentFirst QuarterWaxing GibbousFull MoonWaning GibbousThird QuarterWaning CrescentEarthshineTidal Locking
Test your understanding
- Why do we see different phases of the Moon, and what is the primary cause?
- What is the 'terminator' on the Moon, and how does it relate to the Moon's phases?
- How does the concept of 'waxing' and 'waning' apply to the Moon's appearance throughout its cycle?
- What is Earthshine, and how does it affect what we see during certain Moon phases?
- Explain the geometric relationship between the Sun, Earth, and Moon during a New Moon and a Full Moon.