Saturn
Saturn is the farthest planet known to the ancients. Although it is far from us, it is quite bright. At its brightest, it can reach a magnitude of -0.75, which is brighter than any star except Sirius. Brighter than mercury, it is easier to observe than mercury anyway, because Saturn is farther from the sun than us, unlike mercury, which can only stay around the sun, so it can't appear in the midnight sky.
The average distance between Saturn and the sun is14.3x108km, which is 1.833 times that of Jupiter. It takes about 29.458 years to make a revolution around the sun. Compared with Jupiter's period of revolution 1 1.862, Saturn's year is about 2.5 times that of Jupiter.
In many ways, Saturn is inferior to Jupiter. In terms of size, it is the second largest planet in the solar system, second only to Jupiter. The equatorial diameter is 120000 km, which is only 5/6 of Jupiter's. Because Saturn is relatively small and far away from the sun, the intensity of sunlight shining on Saturn is only half that of Jupiter, which makes Saturn much darker than Jupiter. On the other hand, Saturn is still big enough to make it have considerable brightness.
Saturn's mass is 95. 1 times that of the earth, and it is the second largest planet after Jupiter. Its mass is only 3/ 10 of Jupiter, but its volume is 6/ 10 of Jupiter. With such a large volume and such a small mass, the density of Saturn must be very low. Indeed, Saturn's density is only 0.7 times that of water, making it the least dense planet in our solar system. If we imagine that Saturn can be wrapped in plastic sheets to prevent it from melting or scattering, and then put it into an ocean that can accommodate it, it will float on the water. Therefore, it can be inferred that Saturn contains more hydrogen than Jupiter and less other contents. At the same time, because Saturn's gravity is very weak, it can't compress its own matter as tightly as Jupiter compresses its own.
Although Saturn is small and rotates fast, it is still slower than Jupiter. Saturn's rotation period is 10.67 days, so Saturn's day is 8% longer than Jupiter's day.
Although Saturn's rotation is slower than Jupiter's, the density of Saturn's outer layer is low, and the attraction to the outer layer is also small; Therefore, Saturn bulges greatly near the equator and becomes the flattest planet in the solar system. Its flatness is 0. 102, which is 6 times that of Jupiter and 30 times that of the earth. Although Saturn's equatorial diameter is 120000 km, its polar diameter is only 108000 km, with a difference of 12000 km, which is almost the whole length of the earth's diameter!
Rings of Saturn
On the other hand, Saturn is unique. When Galileo first observed Saturn through his original telescope, he found its shape a bit strange, as if there were two small balls on both sides of its sphere. He continued to observe and found that the two balls gradually became difficult to see. By the end of 16 12, they finally disappeared at the same time.
Other astronomers have also reported this strange phenomenon of Saturn; But it was not until 1656 that Huygens put forward the correct explanation. He claimed that Saturn was surrounded by a bright and thin ring; The rings are not in contact with Saturn.
Saturn's axis of rotation, like the earth, is also inclined. Saturn's axis inclination is 26.73, while the earth's axis inclination is 23.45. Since Saturn's ring is on the same plane as the equator, it leans towards the sun (and towards us). When Saturn runs to one end of its orbit, we can see the near side of the ring from top to bottom, while the far side is still covered. When Saturn is at the other end of the orbit, we can see the near side of the ring from the bottom to the top, while the far side is still covered. It takes 14 years for Saturn to move from one side of its orbit to the other. During this time, the halo gradually moved from the bottom to the top. Halfway through the line, the halo just moved to the middle position. At this time, we observed that the edges on both sides of the halo were connected together, like a "line". Subsequently; Saturn continues to run, circling back to its original starting point along the other half orbit, and then the halo gradually moves from the top to the bottom; When we moved to the middle, we saw the edges connected again. Because Saturn's aura is very thin, it seems to disappear when it looks like a "line". Galileo saw this at the end of 16 12; It is said that he never observed Saturn again because of his chagrin.
Saturn's rings lie on Saturn's equatorial plane. Before space exploration, Saturn was observed to have five rings from the ground, including three main rings (A ring, B ring and C ring) and two dark rings (D ring and E ring). B ring is wide and bright. It has a C ring on the inside and an A ring on the outside. Between ring A and ring B is a Cassini seam about 5000 kilometers wide, which was discovered by astronomer Cassini in 1675.
1826, Struve, a Russian astronomer of German descent, named the outer ring as the A ring and the inner ring as the B ring. 1850, American astronomer W.C. Bond claimed that there was a faint halo closer to Saturn than the B ring. This dim halo is the C ring, and there is no obvious boundary between the C ring and the B ring.
There is nothing like Saturn's rings anywhere in the solar system, or we can't see them anywhere with any instruments. Indeed, we now know that there is a thin material halo around Jupiter, and any gas giant planet like Jupiter and Saturn may have a debris halo near them. However, if Jupiter's rings are taken as the standard, these rings are pitiful and insignificant, while Saturn's ring system is magnificent and moving. Seen from the earth, from one end of Saturn's ring system to the other, it extends 269,700 kilometers (65,438+067,600 miles), which is equivalent to 265,438+0 times the width of the earth, and actually almost twice the width of Jupiter.
What exactly is Saturn's ring? J.D. Cassini thinks they are solid rings as smooth as iron rings. However, in 1785, Laplace (who later put forward the nebula hypothesis) pointed out that due to the different distances between the rings and Saturn's center, the degree of attraction to Saturn's gravitational field will be different. This difference in gravity (that is, the tidal effect I mentioned earlier) will pull the ring apart. Laplace thinks that the halo is composed of a series of thin rings arranged together, and these circle permutation are so close that they look like a solid from the distance of the earth.
But in 1855, Maxwell (who later predicted the existence of broadband electromagnetic radiation) suggested that even this statement was not perfect. The only reason why the halo is not broken by tidal effect is that the halo is composed of countless relatively small meteorite particles, and the distribution of these particles around Saturn makes people feel like a solid ring from the distance of the earth. Maxwell's hypothesis is correct, and no one has questioned it now.
French astronomer Roach used another method to study the tidal effect. He proved that any solid celestial body, when approaching another celestial body much larger than it, will be subjected to a powerful tidal force action and will eventually be torn to pieces. This smaller celestial body is called roche limit, which is usually 2.44 times the equatorial radius of a large celestial body.
In this way, Saturn's roche limit is 2.44 times of its equatorial radius of 60,000 kilometers, that is,146,400 kilometers, and the distance from the outermost edge of ring A to Saturn's center is136,500 kilometers (84,800 miles), so the whole ring system is within roche limit. (Jupiter's rings are also within the roche limit. )
Obviously, Saturn's rings are debris that can never be gathered on satellites (more debris than roche limit has gathered on satellites-obviously), or debris left by satellites that are too close to Saturn for some reason. In either case, there are some small celestial bodies left behind. (The smaller the affected celestial body, the smaller the tidal effect will be. After the debris is small enough, it will not continue to break apart unless two small celestial bodies collide occasionally. It is estimated that if all the substances in Saturn's rings are aggregated into a celestial body, the result will be a sphere slightly larger than our moon.