﻿1 00:00:00,500 --> 00:00:08,609 [music] 2 00:00:10,611 --> 00:00:14,982 On October 16th 2017 the Lunar Reconnaissance Orbiter mission 3 00:00:14,982 --> 00:00:18,585 celebrates one hundred lunar days of being at the Moon. It’s 4 00:00:18,585 --> 00:00:22,222 a big accomplishment. Now you may be wondering – if the 5 00:00:22,222 --> 00:00:25,659 spacecraft launched way back in 2009, how are we only at one 6 00:00:25,659 --> 00:00:29,329 hundred days? What is a lunar day, and how does it differ from 7 00:00:29,329 --> 00:00:33,066 a day on Earth? On Earth, a day is about the time it takes for 8 00:00:33,066 --> 00:00:36,837 the Earth to turn once on its axis with respect to the Sun. A 9 00:00:36,837 --> 00:00:39,640 lunar day is also about the time it takes the Moon to turn once 10 00:00:39,640 --> 00:00:42,609 on its axis with respect to the Sun, and that’s about 11 00:00:42,609 --> 00:00:47,214 twenty-nine and a half Earth days. So, it’s about a month. 12 00:00:47,214 --> 00:00:50,250 Now you may be confused about how the Moon rotates, if we 13 00:00:50,250 --> 00:00:53,954 always see the same face from Earth. The answer lies in a 14 00:00:53,954 --> 00:00:57,124 process called synchronous rotation, which is caused by 15 00:00:57,124 --> 00:01:00,961 tidal forces between the Earth and the Moon. We always see the 16 00:01:00,961 --> 00:01:03,563 near side of the Moon, because as the Moon orbits around the 17 00:01:03,563 --> 00:01:08,068 Earth, it is also continuously turning. In fact, it rotates at 18 00:01:08,068 --> 00:01:12,139 the same rate it orbits the Earth, on average. So, a lunar 19 00:01:12,139 --> 00:01:15,409 day takes about the same amount of time as one complete lunar 20 00:01:15,409 --> 00:01:19,212 orbit. One hundred lunar days means one hundred chances to 21 00:01:19,212 --> 00:01:21,448 observe a complete day/night cycle on the Moon - 22 00:01:21,448 --> 00:01:25,319 photographing the surface with different Sun angles, measuring 23 00:01:25,319 --> 00:01:28,221 the rising and falling temperatures, and studying the 24 00:01:28,221 --> 00:01:32,326 way certain chemicals react to those daily changes. If we want 25 00:01:32,326 --> 00:01:34,928 a better idea about long-term trends and processes on the 26 00:01:34,928 --> 00:01:38,632 Moon, being able to study it for one hundred days and counting is 27 00:01:38,632 --> 00:01:42,336 a good place to start. And thanks to all the data gathered 28 00:01:42,336 --> 00:01:45,639 from LRO during this time, our understanding of the Moon has 29 00:01:45,639 --> 00:01:49,843 increased exponentially. Stay tuned for our next video that 30 00:01:49,843 --> 00:01:52,245 explores the significance of being at the Moon for one 31 00:01:52,245 --> 00:01:54,815 hundred days, and what we’ve been able to accomplish 32 00:01:54,815 --> 00:01:57,918 in that time. 33 00:01:58,385 --> 00:02:03,991 [beeping]