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Poster of types of optical telescope: Newtonian, Refracting, and Casserian.
Telescopes Poster

MOON: simplified animation of the Moon orbiting the Earth showing the phases of the moon.

THE MOON varies in appearance throughout the lunar month as it revolves around the Earth. This animation shows in a very simplified form how the sunlight (coming from lower right) illuminates the Earth (bluish-white) and the Moon (grey). The viewpoint is fixed on the Earth and shows the moon rotating around the Earth. This rotation takes just over 27 days (a lunar month). The animation at the upper left shows approximately what the moon would look like viewed from Earth. The animations are synchronised and the two views should help make the phases of the moon understandable. The phases are full moon, gibbous moon, half moon & crescent moon. Moonlight is simply sunlight reflected from the surface of the moon.

NOTES: The scales are approximately correct for the relative sizes of the Earth and moon but the distance between the two has been greatly reduced to fit within a reasonable frame. The plane of the moon orbit is, in reality, slightly tilted with respect to the plane of the Earth's orbit around the sun. The Earth and moon are locked in an orbit around each other and actually rotate about the pair's centre of gravity which is towards the surface of the Earth on the side facing the moon. This point is called the barycentre (Greek heavy centre). The moon continues to face the Earth as it orbits around, locked into synchrony by tidal (gravitational) forces. This is equivalent to the moon performing one revolution around its own axis for every complete revolution it makes around the Earth. The gravitational pull of the moon on the Earth distorts the oceans creating the tides.

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