Date: September 13, 2009
Title: Why We See The Same Side of the Moon
Podcaster: Gordon Houston
Organization: JPL/NASA Solar System Ambassadors Program
Description: Have you ever wondered why we always see the same side of the Moon? Did you know that the Moon only rotates once each time it orbits the Earth? Today’s Podcast will explore the relationships between the Earth and the Moon and the causes of synchronous rotation, which places the same face of the Moon as the only side observable from Earth. This in turn causes what we call the “Dark side of the Moon.” These mechanisms are at work throughout the solar system causing similar characteristics of many of the Solar System’s larger moons. Learn about the tidal dynamics that creates this phenomenon.
Bio: A lifelong observer of the sky and space enthusiast, Gordon L. Houston is enrolled in a Masters of Astronomy at Swinburne University and has earned the designation of “Master Observer” through the Astronomical League. Gordon is a member of the Houston Astronomical Society, The Association for Lunar and Planetary Observers, Astronomical Society of the Pacific, and a George Observatory volunteer, where he is a certified operator of the 36” Gueymard Research Telescope. Gordon owns an astronomy shop, where he teaches a basic observational astronomy course and was recently hired by Blinn College to teach astronomy.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by the American Astronomical Society, the major organization for professional astronomers in North America, whose members remind everyone that One Sky Connects Us All. Find out more or join the AAS at aas.org.
WHY WE SEE THE SAME SIDE OF THE MOON
Podcast by Gordon L. Houston, September 13, 2009
Hello and welcome to another addition of the 365 Days of Astronomy podcasts. I am Gordon Houston, Solar System Ambassador for NASA’s Jet Propulsion Laboratory.
Have you ever wondered why we always see the same side of the Moon? Did you know that the Moon only rotates once each time it orbits the Earth? Today’s Podcast will explore the relationships between the Earth and the Moon and the causes of synchronous rotation. This places the same face of the Moon as the only side observable from Earth. This in turn causes what we call the “Dark side of the Moon.” These mechanisms are at work throughout the solar system causing similar characteristics of many of the solar system’s larger moons.
If the moon did not rotate, the side facing the sun at new Moon would be the side we would see facing the Earth at the full moon position. In this situation, the Moon would remain still as it orbits the Earth and would always have the same position orientation. So, in one complete orbit of the Earth, we would in fact observe the complete surface of the Moon. Synchronous rotation, on the other hand, causes us to always see the same side of the Moon, whether it is illuminated or not.
The Moon rotates once on its axis for each time it orbits the Earth. The Earth rotates once every 24 hours, which makes up a solar day, yet it takes the Moon almost 30 days to rotate once. The rotation on its axis of a celestial object in the same time as it takes to orbit another object is called synchronous rotation. The Moon’s rotation and revolution periods are identical. Synchronous rotation results from tidal forces between the two bodies, where the rotation on its axis of one celestial body becomes tidally locked to the other body.
Briefly, the tidal mechanism works like this. A gravitational gradient forms between the two bodies of different size mass; this causes the smaller body to become tidally locked to the larger body, creating the synchronous rotation. To be more specific, the torque applied to the smaller body creates tidal bulges in the object, this in turn slows the object’s rotation down, until the rotation becomes tidally locked. Conservation of angular momentum operates in the system, so as the Moon’s rotation slows down, its orbital speed picks up, and there may be some effect on the Earth’s motion as well.
Most major moons of the solar system are in synchronous rotation with their host planets. However, a very unique situation exists between Pluto and its major satellite Charon. Pluto and Charon are in dual synchronous rotation about each other. That means that the same face of both always faces the other. If we lived on Pluto, Charon would always be up, day and night, and always in the same position in the sky. Charon would not move and just appear to be suspended there, but the same would be true if we lived on Charon, Pluto would be suspended in the same position in the sky, never seeming to move as well.
The final effect of synchronous rotation, allowing us to only see the same side of the Moon, creates what has become known as “The Dark Side of the Moon,” or the side we never can observe from Earth. In reality, we see just a little more than 50% of the lunar surface due to libration effects. Libration is the wobble of the Moon on it axis, which allows us to see about 59% of the total lunar surface, not all at once, but at different times. The wobble can be polar north and south movements, enabling us to see further over one pole or another or the movements can be equatorial east and west. Typically the libration is a combination of the two.
Another interesting phenomenon of the moon is our ability to still see what appears to be the dark side of the moon during the thin waxing crescent phase, just after new Moon. This is not the “dark side of the moon,” but is the same side of the Moon we always see, and it is caused by reflected light from Earth illuminating that portion hidden from the Sun. This is called earthshine and was explained in the 1500’s by Leonardo Da Vinci, when he realized that both the Earth and Moon reflect sunlight. This effect has now taken on a broader definition called planetshine.
This year, we celebrate the 400th Anniversary of Galileo pointing a telescope to the celestial sphere. One of those discoveries included the four main moons of Jupiter, which are also known as the Galilean moons. All of these moons are in synchronous rotation with Jupiter, just like our Moon is with Earth. He also discovered that our Moon was not smooth, but covered with mountains and craters and that Venus went through phases just like our Moon. This latter discovery was one of the most important discoveries of all time. It was the first observational evidence that the Sun and not the Earth was the center of the solar system.
Finally, I don’t want to forget to mention several lunar missions currently active involving our Moon, the LRO and the LCROSS missions. They both were launched in the same payload on June 18, 2009. The LRO or Lunar Reconnaissance Orbiter is mapping the moon with the greatest detail ever to date. This data will be used to determine landing sites and potential locations for future lunar bases. The LCROSS or Lunar CRater Observation and Sensing Satellite, mission is searching for water on the Moon and will be creating an exciting event on October 9, 2009, when they crash the Centaur probe into a permanently shadowed lunar crater in the southern highlands. The main spacecraft will then impact four minutes later. These impacts will are predicted to through up large plumes of material, which will be analyzed by spectrographs, attempting to identify the signatures of water. The plumes of these impacts are predicted to be visible from Earth, even with modest sized telescopes. So, mark your calendars!!!
Well, I hope you have learned something about our Earth-Moon system today. I will finish with one of my favorite quotes from the ancient astronomer Ptolemy.
“I know that I am mortal and the creature of a day; but when I search out the massed wheeling circles of the stars, my feet no longer touch the earth, but, side by side with Zeus himself, I take my fill of ambrosia, the food of the gods.” — Ptolemy
This is Gordon Houston, signing off, Ad Astra, “To the Stars.”
TIME: 7 minutes, 34 seconds
End of podcast:
365 Days of Astronomy
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