The reasons why Pluto was excluded from the major planets: first, because the process of its discovery was based on a wrong theory; second, because its mass was originally estimated incorrectly and it was mistakenly included in the list. When it reaches the rank of a large planet, it is classified as a dwarf planet. The definition of a planet is that it must be a celestial body orbiting a star;
It must be massive enough to rely on its own gravity to make the celestial body appear spherical;
There must be no other objects near its orbit object.
Pluto does not meet the last condition. Pluto's orbit intersects with Neptune. [Edit this paragraph] Mercury English name: Mercury
Mercury is the closest to the sun and is the smallest and lightest planet in the solar system. Mercury is smaller in diameter than Ganymede and Titan.
Basic parameters of Mercury:
Orbital semi-major diameter: 57.91 million kilometers (0.38 astronomical units) Ganymede
Revolution period: 87.70 days
Rotation direction: from west to east
Average orbital speed: 47.89 kilometers per second
Orbital eccentricity: 0.206
Orbital inclination: 7.0 degrees
Planet’s equatorial radius: 2440 km
Mass (Earth mass = 1): 0.0553 Density: 5.43 g/cm3
Rotation period: 58.65 days< /p>
Number of satellites: None
Orbit: 57,910,000 kilometers (0.38 astronomical units) from the sun
In ancient Roman mythology, Mercury was the planet of commerce, travel and theft. The god, Hermes in ancient Greek mythology, was the messenger of the gods. Perhaps Mercury got its name because of its rapid movement in the sky.
As early as 3000 BC in the Sumerian era, people discovered Mercury. The ancient Greeks gave it two names: when it first appeared in the morning, it was called Apollo; when it twinkled, it was called Apollo. In the night sky it is called Hermes. However, ancient Greek astronomers knew that the two names actually referred to the same star. Heraclitus (a Greek philosopher in the 5th century BC) even believed that Mercury and Venus did not orbit the earth, but orbited the sun. run.
Only the Mariner 10 probe visited Mercury three times, in 1973 and 1974. It surveyed only 45% of Mercury's surface (and unfortunately, Mercury was too close to the sun for Hubble to safely image it).
Mercury’s orbit deviates greatly from a perfect circle. Its perihelion is only 46 million kilometers away from the sun, but its aphelion is 70 million kilometers away. It has a large distance around the sun at perihelion. The slow precession phenomenon is called "Mercury's perihelion orbital precession". (Precession: The precession of the earth's axis causes the vernal equinox to move slowly westward at a speed of 0.2" per year and one week in about 25,800 years, making the tropical year shorter than the sidereal year. There are two types of equinox precession and planetary precession, the latter is caused by the gravity of the planet (Caused by changes in the ecliptic plane.) In the 19th century, astronomers made very careful observations of Mercury's orbital radius, but were unable to adequately explain it using Newtonian mechanics between the actual observed and predicted values. The subtle difference is a minor one (one-seventh of a degree per thousand years) but has puzzled astronomers for decades. Some believe there is another planet in orbit close to Mercury, as it is sometimes called. Vulcan, "Zhu Rongxing"), to explain this difference, and the final answer is quite dramatic: Einstein's general theory of relativity. In the early days when people accepted this theory, the correct prediction of Mercury's movement was very important. Factor. (Mercury revolves around it due to the gravitational field of the sun, and the gravitational field of the sun is extremely huge. According to the general theory of relativity, mass generates a gravitational field, and the gravitational field can be regarded as mass, so the huge gravitational field can be regarded as mass, producing small The gravitational field causes its orbit to deviate. Similar to the divergence of electromagnetic waves, the changing magnetic field generates an electric field, and the changing electric field generates a magnetic field that travels far away.
Before 1962, people always thought. Mercury takes the same amount of time to rotate once and revolve around it, so that the side facing the sun remains constant. This is very similar to the fact that the same half of the moon always faces the Earth. But in 1965, it was observed by Doppler radar. This theory was found to be wrong. Now we know that Mercury rotates three times while orbiting twice. Mercury is the only known object in the solar system whose revolution period and rotation period are not 1:1.
Due to the above situation and the extreme deviation of Mercury’s orbit from a perfect circle, observers on Mercury will see a very strange scene. Observers at certain longitudes will see that after the sun rises, As it slowly moves toward the zenith, it will gradually and significantly increase in size. The Sun will pause at the zenith, undergo a brief retrograde process, pause again, and then continue its journey toward the horizon, while shrinking significantly in the meantime. , stars will move across the sky three times faster. Observers at other locations on Mercury's surface will see different but equally unusual celestial motions.
The temperature difference on Mercury is the largest in the entire solar system, with the temperature range ranging from 90 to 700 K. In comparison, the temperature of Venus is slightly higher but more stable.
Mercury is similar to the Moon in many ways. Its surface is cratered and very old; it also has no plate tectonics. Mercury, on the other hand, is much denser than the Moon, (Mercury 5.43 g/cm3 Moon 3.34 g/cm3). Mercury is the second densest celestial body in the solar system after the Earth. In fact, the high density of the Earth is partly due to the compression of gravity; or otherwise, the density of Mercury would be greater than that of the Earth, which indicates that the iron core of Mercury is relatively larger than that of the Earth, and is likely to constitute most of the planet. Therefore, Mercury has only a thin silicate mantle and crust, relatively speaking.
The huge iron core has a radius of 1800 to 1900 kilometers and dominates the interior of Mercury. The silicate shell is only 500 to 600 kilometers thick, and at least part of the core is probably molten.
In fact, Mercury's atmosphere is very thin and is composed of atoms destroyed by the solar wind. Mercury's temperature is so high that these atoms are quickly dispersed into space, so that Mercury's atmosphere is replenished more frequently than the stable atmospheres of Earth and Venus.
Mercury's surface shows huge sharp slopes, some of which are hundreds of kilometers long and three thousand meters high. Some lie across the outer ring of the crater, while others have sharply sloping features that suggest they were formed by compression. It is estimated that Mercury's surface has shrunk by approximately 0.1% (or approximately 1 km in planet radius).
One of the largest landform features on Mercury is the Caloris Basin (pictured on the right), which is approximately 1,300 kilometers in diameter and is thought to be similar to Maria, the largest basin on the Moon. Like the moon's basin, the Caloris basin was likely formed during a large collision in the early solar system. That collision probably also created the strange topography on the other side of the planet facing the basin (pictured left).
In addition to the cratered terrain, Mercury also has relatively flat plains, some of which may be the result of ancient volcanism, but others are probably the result of the deposition of ejecta formed by meteorites.
Data from the Mariner spacecraft provide some early indications of recent volcanic activity on Mercury, but we need more data to confirm.
Surprisingly, radar scans of Mercury's North Pole, an area not surveyed by Mariner 10, showed signs of ice in well-protected recesses in some craters.
Mercury has a small magnetic field with a magnetic field strength of about 1% that of Earth.
No satellites of Mercury have been discovered so far.
Mercury can usually be observed through binoculars or even directly with the naked eye, but it is always very close to the sun and difficult to see in the twilight. Mike Harvey's planet-finding chart points out Mercury's position in the sky (and the positions of other planets) at that time, which can then be customized in more detail with the "Starlight" astronomical program. [Edit this paragraph] Venus English name of Venus: Venus
One of the eight planets, it was called Taibai or Taibai Venus in ancient China. Sometimes it is a morning star, appearing in the eastern sky before dawn, and is called "Qiming"; sometimes it is an evening star, appearing in the western sky after dusk, and is called "Chang Geng". Venus is the brightest star in the sky besides the sun and the moon. It is like a dazzling diamond, so the ancient Greeks called it Aphrodite, the goddess of love and beauty, while the Romans called it Venus. --God of beauty.
Basic parameters of Venus
Rotation direction: from east to west
Revolution period: 224.701 days
Average orbital speed: 35.03 kilometers /per second
Orbital eccentricity: 0.007
Orbital inclination: 3.4 degrees
Equatorial diameter: 12,103.6 kilometers
Diameter: 12105 Kilometers
Mass (Earth mass = 1): 0.8150
Density: 5.24 g/cubic centimeter
Number of satellites: 0
Radius of revolution: 108,208,930 km (0.72 astronomical units)
Surface area: 460 million square kilometers
Rotation time: 243.02 days
Escape velocity: 10.4 kilometers /sec
Venus has been known since prehistory. After the sun and moon, it is the brightest one.
Venus is an inner planet. If you observe it with a telescope from the Earth, you will find that it has phase changes. Galileo's observation of this phenomenon was important evidence in favor of Copernicus' heliocentric theory of the solar system.
The first spacecraft to visit Venus was Mariner 2 in 1962. Subsequently, it was successively visited by other spacecraft: Venus Pioneer, Soviet Dignity 7 and Dignity 9.
The rotation of Venus is very unusual. On the one hand it is very slow (a Venusian day is equivalent to 243 Earth days, slightly longer than a Venusian year), and on the other hand it is inverted. In addition, the rotation period of Venus is synchronized with its orbital period, so when it reaches its closest point to the Earth, the side of Venus facing the Earth is always fixed.
Whether this is a ringing effect or just a coincidence is unknown.
Venus is sometimes called Earth's sister planet, and in some ways they are very similar:
--Venus is slightly smaller than Earth (95% of Earth's diameter, 80% of Earth's diameter) quality).
--There are some craters on the relatively young surface.
--Their density and chemical composition are very similar.
Because of these similarities, it is sometimes thought that Venus may be very similar to Earth beneath its thick clouds, and may contain life. But unfortunately, many in-depth studies of Venus have shown that Venus is fundamentally different from Earth in many ways.
The atmospheric pressure of Venus is 90 standard atmospheres (equivalent to the pressure at a depth of 1 kilometer in the Earth's ocean). The atmosphere is mostly composed of carbon dioxide, and there are also several layers of clouds composed of sulfuric acid several kilometers thick. These clouds block our view of Venus' surface, making it appear very blurry. This dense atmosphere also produces a greenhouse effect, causing the surface temperature of Venus to rise by 400 degrees to over 740 degrees Kelvin (enough to melt lead bars). The surface of Venus is naturally hotter than that of Mercury, even though Venus is several times farther from the sun than Mercury. There are strong winds at the top of the clouds, about 350 kilometers per hour, but the surface wind speed is very slow, less than a few kilometers per hour.