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[music]

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On October 16th 2017 the Lunar
Reconnaissance Orbiter mission

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celebrates one hundred lunar
days of being at the Moon. It’s

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a big accomplishment. Now you
may be wondering – if the

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spacecraft launched way back in
2009, how are we only at one

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hundred days? What is a lunar
day, and how does it differ from

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a day on Earth? On Earth, a day
is about the time it takes for

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the Earth to turn once on its
axis with respect to the Sun. A

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lunar day is also about the time
it takes the Moon to turn once

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on its axis with respect to the
Sun, and that’s about

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twenty-nine and a half Earth
days. So, it’s about a month.

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Now you may be confused about
how the Moon rotates, if we

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always see the same face from
Earth. The answer lies in a

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process called synchronous
rotation, which is caused by

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tidal forces between the Earth
and the Moon. We always see the

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near side of the Moon, because
as the Moon orbits around the

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Earth, it is also continuously
turning. In fact, it rotates at

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the same rate it orbits the
Earth, on average. So, a lunar

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day takes about the same amount
of time as one complete lunar

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orbit. One hundred lunar days
means one hundred chances to

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observe a complete day/night
cycle on the Moon -

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photographing the surface with
different Sun angles, measuring

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the rising and falling
temperatures, and studying the

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way certain chemicals react to
those daily changes. If we want

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a better idea about long-term
trends and processes on the

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Moon, being able to study it for
one hundred days and counting is

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a good place to start. And
thanks to all the data gathered

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from LRO during this time, our
understanding of the Moon has

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increased exponentially. Stay
tuned for our next video that

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explores the significance of
being at the Moon for one

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hundred days, and what we’ve
been able to accomplish

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in that time.

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[beeping]