Academic journal article The Science Teacher

Ask the Experts

Academic journal article The Science Teacher

Ask the Experts

Article excerpt

Q Why does the Moon show phases when viewed from Earth, but Earth always looks the same from the Moon, with only the top half illuminated?

Elaine Catalanotti, Teacher

International School 77

Ridgewood, New York

A This misconception may arise from the repeated portrayal of some popular photographs taken from the Moon's surface. From the surface of the Earth, we see the Moon go through phases that are due to the changing Sun-Moon-Earth angle as it orbits around us. Moreover, the Moon is in "synchronous rotation" with the Earth, which means that it spins around its own axis in exactly the same amount of time it takes to orbit (in other words, the Lunar day is exactly one Lunar month long). As a consequence, the Moon always points the same face toward the Earth.

In fact, the apparent phase of the Earth as seen from the Moon is exactly the opposite of the phase of the Moon as seen from the Earth. When the Moon is full the Earth is new, and vice versa. Also, when the Moon appears as a half-Moon, the Earth appears as a half-Earth from the Moon. The Apollo astronauts must have taken that famous photograph of the half-Earth at such a time.

Q Why are there two tides per day; i.e., why is there a high tide on the side of the Earth facing away from the Moon?

Amy Telford

Science Instructor

Sandoval Jr./Sr. High School

Sandoval, Illinois

A The "tidal force" created by the Moon is due to the change in the force of gravity as you get farther from the Moon. The water on the side of the Earth nearest to the Moon feels a stronger force than the water on the opposite side. Of course, the rocks that make up the land feel the same forces, but the water is easier to move and is pulled toward the Moon more so than the solid parts of the Earth. This creates a "tidal bulge" on the side of the Earth facing the Moon. On the opposite side of the planet, the solid Earth (which moves like a single, connected object) feels a greater force of gravity toward the Moon than the liquid water (which can freely flow). Relative to the water, the center of the Earth is pulled out from under it toward the Moon, creating a second tidal bulge on the side of the Earth opposite the Moon. …

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