Key Idea.
Section 1
The Moon Reflects, It Does Not Glow
In our solar system, the Sun is a light source because it generates its own visible energy. In contrast, the Moon is a non-luminous object made of gray rock.
The Moon is visible from Earth only because it acts like a mirror. Sunlight travels through space, strikes the surface of the Moon, and reflects toward our eyes. If the Sun were turned off, the Moon would instantly vanish from our view.
Section 2
The Half-Lit Principle (The Model)
Now we apply the "straight line" rule to the Moon. Since the Moon is a sphere and the Sun is a distant light source, the geometry forces a specific result: The Sun always illuminates exactly one half of the Moon.
There is always a "Day Side" facing the Sun and a "Night Side" facing away.
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Section 1
The Moon Reflects, It Does Not Glow
In our solar system, the Sun is a light source because it generates its own visible energy. In contrast, the Moon is a non-luminous object made of gray rock.
The Moon is visible from Earth only because it acts like a mirror. Sunlight travels through space, strikes the surface of the Moon, and reflects toward our eyes. If the Sun were turned off, the Moon would instantly vanish from our view.
Section 2
The Half-Lit Principle (The Model)
Now we apply the "straight line" rule to the Moon. Since the Moon is a sphere and the Sun is a distant light source, the geometry forces a specific result: The Sun always illuminates exactly one half of the Moon.
There is always a "Day Side" facing the Sun and a "Night Side" facing away.
Book overview
Jump across lessons in the current chapter without opening the full course modal.
Continue this chapter