Las dificultades o los logros y sus relaciones con los estilos de enseñanza universitaria

The Moon is the only natural satellite of the Earth. It was first visited by the Soviet spacecraft Luna 2 in 1959. It is the only extraterrestrial body which humans have visited. The first manned landing on the Moon was on July 20, 1969; the last was in December, 1972. The Moon is also the only body from which astronauts have brought back samples to Earth. In the summer of 1994, the Moon was mapped in detail by the little spacecraft Clementine, and again in 1999 by Lunar Prospector.

The gravitational forces between the Earth and the Moon have some interesting effects. The most obvious is the tides. The Moon's gravitational attraction is stronger on the side of the Earth nearest to it and weaker on the opposite side. Since the Earth is not perfectly rigid, i.e. fixed and inflexible, it is stretched out toward the Moon. This results in two small bulges on the Earth's surface, one in the direction of the Moon and one directly opposite. The effect is much stronger in the ocean water than in the solid crust, so the water bulges are higher. And because the Earth rotates much faster than the Moon moves in its orbit, the bulges move around the Earth about once a day creating two high tides in oceans and lakes.

The Moon appears to turn slightly (due to its slightly non-circular orbit) so we can see a few degrees of the far side from time to time. The majority of the far side was completely unknown until the Soviet spacecraft Luna 3 photographed it in 1959. Actually, there is no "dark side" of the Moon; other than a few areas with deep craters near the poles, all parts of the Moon get sunlight half the time.

There are basically two types of terrain, or land, on the Moon: the heavily cratered and very old highlands, and, opposite them, the relatively smooth and younger maria. Most of the surface of the cratered highlands is covered with regolith, a mixture of fine dust and small rocks produced by meteor crashes. For some unknown reason, the maria are concentrated on the near side. The maria (which cover about 16% of the Moon's surface) are huge craters that were later filled with lava.

A total of 382 kg of rock samples were returned to the Earth by the Apollo and Luna programs. These provide most of our knowledge of the Moon. They are particularly valuable because they can be dated. Even today, 30 years after the last Moon landing, scientists still study these precious samples. Most rocks on the surface of the Moon seem to be between 4.6 and 3 billion years old. Thus, the Moon provides certain evidence about the early history of the solar system which is not fully available on the Earth.

Before the study of the Apollo, Luna 2 and Luna 3 samples, there was no agreement among scientists about the origin of the Moon. There were three principal theories: co-accretion, which said

that the Moon and the Earth formed at the same time from the Solar Nebula; fission, which claimed that the Moon was formerly a part of the Earth; and capture, which held that the Moon formed somewhere else and was later captured by the Earth's gravitational field. None of these is very reasonable. The new and detailed information from the Moon rocks led to the impact theory: that the Earth crashed into a very large object (as big as Mars or more) and that the Moon formed from the discharged material. There are still details that scientists are working on, but the impact theory is now widely accepted.

46 Venus

Venus is one of the most unusual planets in our solar system. It is the second planet from the Sun, located in between Mercury and Earth. The Orbit, or path, Venus follows around the Sun is circular, and the planet's distance from the Sun is about 65 million miles. As Venus is closer to the Sun than Earth, a Venusian year is just 225 Earth-days. Venus rotates around itself much more slowly than Earth, so a day on Venus is 243 Earth-days. This means that a day on Venus is longer than its year! Even more strange is the fact that Venus rotates from East to West. This is just the opposite of Earth and most of the other planets.

Venus is the brightest object in the sky after the Sun and the moon. It is visible to the naked eye for several months. As Venus appears both in the morning sky and the evening sky, ancient astronomers thought it was two different objects and called it Eosphorus and Hesperus.

Venus is similar to Earth in some ways. Venus is almost the same size as Earth, with a diameter of a little over 7,200 miles. Both have craters on their surfaces. Their densities and chemical compositions are similar. Venus probably once had large amounts of water like Earth, but it all boiled away so Venus is quite dry now. Because of these similarities, scientists thought that below its dense clouds Venus might be like the Earth and there might even be life on it. However, more detailed study of Venus showed that in many ways it is different from the Earth.

The atmosphere of Venus is about 90 times heavier than the Earth's atmosphere. It consists mainly of carbon dioxide (96%). 3.5% of the atmosphere is nitrogen and less than 1% is made up of carbon monoxide, argon, sulfur dioxide and water vapor. There are also several layers of clouds that cover the planet. These clouds are many kilometers thick and they are made of sulfuric acid, which is an extremely dangerous substance. These clouds obscure our view of the surface of Venus. In other words, we can't see the surface of Venus from the Earth. There are strong winds at the cloud tops, averaging around 350 kilometers per hour, but the winds on the surface are very light, no more than a few kilometers per hour.

A lot of information about Venus comes from spacecraft that study the planet from a safe distance. They cannot get very close to the planet because of the high temperature. The density of the atmosphere of Venus leads to an increase in the surface temperature and makes the planet one of the hottest places in the solar system. The temperature on Venus reaches 450°C, which is very hot and can melt most metals; that is, the high temperature can turn most metals into liquid. For this reason, scientists are looking for a different way of collecting information about Venus from Earth. A few years

ago, they made a map of the planet's surface using radar telescopes here on Earth. They found that Venus' surface has many mountains (some higher than Mount Everest) and volcanoes. There are also large flat lands called plains on its surface.