It was on Dec. 24, 1968 that astronauts aboard the Apollo 8 mission orbiting the Moon took one of the most famous images in the history of photography. It's become known as "Earthrise on the Moon," and we will soon be celebrating its 45th anniversary.
Moon chronicler Andrew Chaikin has worked with NASA to make a short video of the complete story of how this image came to be taken, and wrote me about recently. I thought I would share it with all of you -- whether you are an old geezer like me (and were alive in 1968) or whether you are newer to life on Earth and saw the picture later. You can find the video at:http://www.youtube.com/watch?v=dE-vOscpiNc
I love the image and its portrayal of a small, fragile planet Earth isolated in space above the airless surface of the Moon. But since this is the AstroProf's page, I have to add, like a good teacher should, that the name creates a bit of a misimpression.
When we talk about sunrise on Earth, we mean that as the Earth turns, we can see the Sun rise above the eastern horizon. As the day goes on, we see the Sun move through our skies, until, in the evening, it sets below the western horizon. What we see as the rising and setting of the Sun is just the effect of the 24-hour rotation of our planet. The spinning of the Earth causes the whole sky to go around us in a day.
The Moon's spin is much slower than Earth's! It takes the Moon 27.3 Earth days to spin once. And in exactly the same 27.3 days, the Moon also orbits once around the Earth. This "tidal locking" of the two motions, the Moon's spin and its revolution around us, means that the Moon always keeps the same face to the Earth.
If you don't believe or see this, try the experiment with your little brother. Put him in the middle of an empty space, face him, and then go around him in the exact same time as you turn your shoulders in a circle. If you take the same time to go around him as to go around yourself, your little brother will always see the front side of your body and never the back side.
For the same reason, if you stand on the Moon, you will either see the Earth in your sky (if you are on the near side of the Moon) or you will not (if you are on the far side). But the Earth will not rise or set in the Moon's sky the way the Sun or Moon rise and set in our sky.
In the famous picture, the Earth only seemed to be "rising" to the astronauts because the Apollo spacecraft was orbiting the Moon.
But if future visitors to the Moon's surface really want to see an Earthrise, it is possible. The Moon wobbles back and forth just a bit (we call it "lunar libration") and so, if you stand in just the right place on the Moon, you could see the Earth dip very slowly below the horizon and then come up again. (Google lunar libration if you want the technical details.)
For everyone else, just enjoy the image and the video. In these times of strife and mistrust everywhere, it's good to remember that -- in the larger picture -- we are all passengers aboard spaceship Earth, sharing its limited area and limited resources.
