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On October 16th 2017 the Lunar Reconnaissance Orbiter mission
celebrates one hundred lunar days of being at the Moon. It’s
a big accomplishment. Now you may be wondering – if the
spacecraft launched way back in 2009, how are we only at one
hundred days? What is a lunar day, and how does it differ from
a day on Earth? On Earth, a day is about the time it takes for
the Earth to turn once on its axis with respect to the Sun. A
lunar day is also about the time it takes the Moon to turn once
on its axis with respect to the Sun, and that’s about
twenty-nine and a half Earth days. So, it’s about a month.
Now you may be confused about how the Moon rotates, if we
always see the same face from Earth. The answer lies in a
process called synchronous rotation, which is caused by
tidal forces between the Earth and the Moon. We always see the
near side of the Moon, because as the Moon orbits around the
Earth, it is also continuously turning. In fact, it rotates at
the same rate it orbits the Earth, on average. So, a lunar
day takes about the same amount of time as one complete lunar
orbit. One hundred lunar days means one hundred chances to
observe a complete day/night cycle on the Moon -
photographing the surface with different Sun angles, measuring
the rising and falling temperatures, and studying the
way certain chemicals react to those daily changes. If we want
a better idea about long-term trends and processes on the
Moon, being able to study it for one hundred days and counting is
a good place to start. And thanks to all the data gathered
from LRO during this time, our understanding of the Moon has
increased exponentially. Stay tuned for our next video that
explores the significance of being at the Moon for one
hundred days, and what we’ve been able to accomplish
in that time.
[beeping]