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What does a Moon twice as big actually mean?

“A Moon twice as big” leaves an important question unanswered: twice the radius, twice the mass, or twice the size it appears in the sky? At the same distance, a wider Moon could have very different gravitational effects depending on what happened to its density.

5–7 min

Questions you could explore

  • What would stay the same if the radius doubled but the mass did not?
  • Why is a stronger tidal force not a prediction of a local tide height?

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Space

Why does the Moon change shape during a month?

A crescent Moon is not a partly switched-off Moon. Sunlight still illuminates half of it, while its changing position around Earth changes how much of that lit half we can see.

3–5 min

Why does the Moon change shape during a month?

Space3–5 minA crescent Moon is not a partly switched-off Moon. Sunlight still illuminates half of it, while its changing position around Earth changes how much of that lit half we can see.

Space

Two eclipses, two different shadows

A solar eclipse and a lunar eclipse both involve the Sun, Earth, and Moon, but the shadow falls in a different place. In one, the Moon’s shadow reaches Earth; in the other, the Moon moves into Earth’s shadow. The Moon’s tilted orbit helps explain why these alignments do not happen every month.

5–7 min

Two eclipses, two different shadows

Space5–7 minA solar eclipse and a lunar eclipse both involve the Sun, Earth, and Moon, but the shadow falls in a different place. In one, the Moon’s shadow reaches Earth; in the other, the Moon moves into Earth’s shadow. The Moon’s tilted orbit helps explain why these alignments do not happen every month.

Space

What can a century-old photograph tell us about a changing sky?

An old astronomical glass plate preserves an observation that cannot be made again today. Harvard’s plate archive brings such records into digital comparison, alongside dates, logbooks, and calibration work. The glass image alone is not the whole dataset; knowing how and when it was made changes what can be inferred from it.

8–10 min

What can a century-old photograph tell us about a changing sky?

Space8–10 minAn old astronomical glass plate preserves an observation that cannot be made again today. Harvard’s plate archive brings such records into digital comparison, alongside dates, logbooks, and calibration work. The glass image alone is not the whole dataset; knowing how and when it was made changes what can be inferred from it.

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