Pihu learns that a lunar eclipse happens when the Earth comes between the Sun and the Moon
Pihu learns that a lunar eclipse happens when the Earth comes between the Sun and the Moon. She argues that this happens at every full moon, so there should be a lunar eclipse every month. Arnav says the Moon's path is slightly tilted, so at most full moons it passes just above or just below the Earth's shadow. Whose reasoning holds up?
Who do you think is right? Pick one.
The answer: Arnav is right, the tilted path usually lets the Moon miss the shadow
Why
Pihu's reasoning is careful, and her starting point is correct: at full moon the Earth is roughly between the Sun and the Moon, and a lunar eclipse needs exactly that. The catch is the word 'roughly'. The Moon's path around the Earth is tilted by about 5 degrees compared with the Earth's path around the Sun. So at most full moons the three are not in a perfectly straight line — the Moon passes a little above or a little below the Earth's shadow and stays fully lit. Only when a full moon happens close to the points where the two paths cross do the Sun, the Earth and the Moon line up closely enough for the Moon to move into the shadow. That is why there are only a few lunar eclipses a year, not one at every one of the twelve or so full moons. Test: hold a ring tilted a little around a ball lit by a lamp, and move a small bead along the ring — the bead passes through the ball's shadow only where the ring crosses the level of the lamp. Arnav is right: because the Moon's path is tilted, most full moons pass above or below the Earth's shadow, so there is no eclipse every month.
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