According to mythology, Mercury was the messenger of the Gods. This had to do in part with its rapid motion around the Sun. It is the closest planet to the Sun, therefore it has the shortest orbital period. Also due to its proximity to the Sun, it is very difficult to see in the morning or evening twilight. It rarely gets more than 25 degrees away from the Sun.
Mercury is so close to the Sun that astronomers thought that there would probably be orbital coupling, like the Earth-Moon system (a tidally locked system). They expected the rotation period to be the same as the orbital period, so that one side of Mercury would always face the Sun, just like one side of the Moon always faces the Earth. Does the orbital period equal the rotation period for Mercury? It is a small planet and difficult to observe, so astronomers were not sure of its rotation period. They had to wait for the invention of radar to check it out. By bouncing a radar signal off the planet, they could measure the speed of its rotation and therefore determine the period of rotation (sort of the same way a highway patrol officer can get you clocked on the radar for being slightly over the speed limit on the highway). Are the orbital period and rotation period the same? No. The period for one orbit is about 88 days, while the period for one rotation is 59 days (pretty slow spinner, eh?). At first glance this doesn't seem to be very significant, but if you were to take the ratio of the periods, 59/88, you get a number close to 2/3. What's so great about that?
The basic upshot is that there is a kind of coupling between the orbit and rotation, but not the one that was expected. For every two orbits around the Sun, Mercury rotates three times on its axis. Figure 1 shows how the orientation of an astronaut would change over time as Mercury went about the Sun. By the time the planet had made one orbit, Mercury would have made 1.5 (or 3/2) rotations; one more orbit, and there would be a total of three rotations - sort of unusual.
Figure 1. An astronaut's view on Mercury would change slowly. An astronaut is located on the side of the planet away from the Sun to begin with (position 1). As the planet moves around the Sun and rotates on its axis (both counterclockwise, as seen here), then by the time the planet has moved 1/3 of the way in its orbit, it would have rotated around 1/2 way (position 3). One complete rotation is reached by position 5, where the astronaut is again upright, and the planet has gone 2/3 of its way in its orbit. By the time one orbit is completed (position 7), the planet has completed 3/2 or 1.5 rotations. Notice how the numbers "two" and "three" keep popping up here.
The Sun's mass influences the space Mercury occupies and adds some extra distortion to Mercury's orbit in a manner known as orbital precession. Observations of its orbit over many years showed that the locations of perihelion shifts (or you could say its orbit wobbles). Early astronomers didn't know what caused it, and even Newton's law of gravity wasn't able to adequately explain it. The distortion of space around the Sun, as described by Einstein's theory of General Relativity, helps to explain the cause of the orbital precession of Mercury, so that is one mystery that was solved by a better theory (not to say that Newton's law of gravity is lousy, but sometimes it needs help).
We don't know a lot about Mercury, in part because we haven't really spent a lot of time exploring it. Also since it is so close to the Sun and relatively small, it is very difficult to view from the Earth, even with the largest telescopes. The earliest close exploration was by the Mariner 10 spacecraft which flew by in 1974-75 and was able to photograph only 1/2 of the surface. For nearly 34 that was the best information available. In 2008, a spacecraft named MESSENGER flew past the planet for the first time. After that it flew by 2 more times before changing its orbit enough to enter into a long-term orbit about the planet. From 2011 to 2015, MESSENGER was a "satellite" of Mercury and has been able to provide us with a significant amount of new information about the planet. Before 2008, we only had images of 1/2 the surface and now we have pretty much the entire surface imaged. Messenger has provided a great deal of information about the mineral characteristics of the surface as well as information about the magnetic field and likely evolutionary history of this small world. The mission ended on April 30, 2015 when the spacecraft crashed into the surface at 8,750 mph (this was an intentional impact). It probably left a good sized crater due to this, but because we don't currently have any spacecraft around Mercury, we can't see that new crater yet. There are various videos of the things Messengerobserved at Mercury available on YouTube, so you can check those out for more information.
Mercury is so close to the Sun that astronomers thought that there would probably be orbital coupling, like the Earth-Moon system (a tidally locked system). They expected the rotation period to be the same as the orbital period, so that one side of Mercury would always face the Sun, just like one side of the Moon always faces the Earth. Does the orbital period equal the rotation period for Mercury? It is a small planet and difficult to observe, so astronomers were not sure of its rotation period. They had to wait for the invention of radar to check it out. By bouncing a radar signal off the planet, they could measure the speed of its rotation and therefore determine the period of rotation (sort of the same way a highway patrol officer can get you clocked on the radar for being slightly over the speed limit on the highway). Are the orbital period and rotation period the same? No. The period for one orbit is about 88 days, while the period for one rotation is 59 days (pretty slow spinner, eh?). At first glance this doesn't seem to be very significant, but if you were to take the ratio of the periods, 59/88, you get a number close to 2/3. What's so great about that?
The basic upshot is that there is a kind of coupling between the orbit and rotation, but not the one that was expected. For every two orbits around the Sun, Mercury rotates three times on its axis. Figure 1 shows how the orientation of an astronaut would change over time as Mercury went about the Sun. By the time the planet had made one orbit, Mercury would have made 1.5 (or 3/2) rotations; one more orbit, and there would be a total of three rotations - sort of unusual.
Figure 1. An astronaut's view on Mercury would change slowly. An astronaut is located on the side of the planet away from the Sun to begin with (position 1). As the planet moves around the Sun and rotates on its axis (both counterclockwise, as seen here), then by the time the planet has moved 1/3 of the way in its orbit, it would have rotated around 1/2 way (position 3). One complete rotation is reached by position 5, where the astronaut is again upright, and the planet has gone 2/3 of its way in its orbit. By the time one orbit is completed (position 7), the planet has completed 3/2 or 1.5 rotations. Notice how the numbers "two" and "three" keep popping up here.The Sun's mass influences the space Mercury occupies and adds some extra distortion to Mercury's orbit in a manner known as orbital precession. Observations of its orbit over many years showed that the locations of perihelion shifts (or you could say its orbit wobbles). Early astronomers didn't know what caused it, and even Newton's law of gravity wasn't able to adequately explain it. The distortion of space around the Sun, as described by Einstein's theory of General Relativity, helps to explain the cause of the orbital precession of Mercury, so that is one mystery that was solved by a better theory (not to say that Newton's law of gravity is lousy, but sometimes it needs help).
We don't know a lot about Mercury, in part because we haven't really spent a lot of time exploring it. Also since it is so close to the Sun and relatively small, it is very difficult to view from the Earth, even with the largest telescopes. The earliest close exploration was by the Mariner 10 spacecraft which flew by in 1974-75 and was able to photograph only 1/2 of the surface. For nearly 34 that was the best information available. In 2008, a spacecraft named MESSENGER flew past the planet for the first time. After that it flew by 2 more times before changing its orbit enough to enter into a long-term orbit about the planet. From 2011 to 2015, MESSENGER was a "satellite" of Mercury and has been able to provide us with a significant amount of new information about the planet. Before 2008, we only had images of 1/2 the surface and now we have pretty much the entire surface imaged. Messenger has provided a great deal of information about the mineral characteristics of the surface as well as information about the magnetic field and likely evolutionary history of this small world. The mission ended on April 30, 2015 when the spacecraft crashed into the surface at 8,750 mph (this was an intentional impact). It probably left a good sized crater due to this, but because we don't currently have any spacecraft around Mercury, we can't see that new crater yet. There are various videos of the things Messengerobserved at Mercury available on YouTube, so you can check those out for more information.
I'm a young researcher, i like documentary movies that taken by national geographic channel or bbc documentary channel, so collect them in our faeries, as i think it will be cool if i do so.
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