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THE SKY GUIDE CHALLENGE

BBC Sky at Night Magazine

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October 2025

Make a composite that reveals how the Moon's diameter changes over a lunar cycle

THE SKY GUIDE CHALLENGE

Supermoons, micromoons and all the in-between Moons occur because the Moon's orbit around Earth (or, more accurately, around the Earth–Moon barycentre) isn't circular but elliptical. This means the Moon's physical distance from Earth varies over the course of its orbit. As a result, its apparent diameter changes too.

Recording these changes is surprisingly easy. All you need is a camera, preferably attached to a telescope or long-focal-length lens, that's able to capture the entire disc of the Moon in one go. The larger the image, the easier it will be to see the effects, but the critical point is you must use the same equipment setup for every image.

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THE SKY GUIDE CHALLENGE

Make a composite that reveals how the Moon's diameter changes over a lunar cycle

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2 mins

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Create a striking moonrise composite

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