Tuesday, April 15, 2008

Astronomy (Sun,Moon,and Earth)

Sun, Moon, and Earth
For us, Earth is the most important celestial body. Earth is a spherical object 7,926 miles (12,756 km) in diameter and enveloped by a layer of air (the atmosphere) that, by protecting us from dangerous cosmic radiation, makes life possible for us. Once a day, every 24 hours, Earth rotates around its own axis, which is an imaginary straight line that runs from the north to the south pole. Because of Earth's rotation we see the stars rise in the east and set in the west. In the course of the night the starry sky changes its appearance completely. New stars appear, and others vanish from sight.

The sun is crucial for Earth and its inhabitants. It is a huge, blazing ball of gas, in whose core atomic nuclei are fused in a process that releases energy. Thanks to the incredibly vast amounts of energy produced, the sun radiates great quanti­ties of light and heat toward Earth and provides the energy that is necessary for life to emerge. The rising and setting of the sun create our days and nights.

When the sun is in the sky during the day, it shines so brightly that all other celestial bodies which are overhead not just at night but during the day as well, fade from sight. It is only in the evening, when the sun sinks below the horizon and darkness falls, that we can see the other, much less luminous celestial bodies.

The sun's disk frequently displays dark spots. The number of these so-called sunspots fluctuates in an eleven-year cycle. The "solar activity" associated with sunspots will increase during most of the period from 2007-2010. Astronomers expect the next sunspot flare-up in about 2011.
Only one other celestial body is sometimes visible during the day, the moon, which measures 2,159 miles (3,476 km) in diameter and is considerably smaller than Earth. The moon revolves around Earth approximately once a month. It is Earth's companion or satellite, a cold celestial body that is inhospitable to life. Humans first set foot on the moon on July 20, 1969.

Like Earth, the moon derives its light from the sun, and it reflects this light back to Earth. The moon appears to us in continually changing form as it circles around Earth, and we speak of its "waxing" and "waning" as, in the course of 29 1/2 days, it passes through the different phases of the lunar cycle. Depending on the moon's position in relation to the sun, we see sometimes the entire lit-up side of the moon (full moon), sometimes only half of the lit-up side (half-moon) and when the moon is closest to the sun, we don't see anything at all because the side of the moon facing us is unlit (new moon).

As it revolves around Earth, the moon occasionally produces the most dramatic celestial events we can observe, lunar and solar eclipses. Earth and the moon throw shadows, like any other body that is lit up. When Earth's shadow falls on the moon, we see a lunar eclipse, when the moon's shadow hits Earth, it causes a solar eclipse. Solar eclipses are always visible from only a small part of Earth's surface lunar eclipses on the other hand can be seen on about half of Earth's night side.

Although the moon goes completely into the Earth's shadow during a totol lunar eclipse, it does not completely disappear. Earth's atmosphere deflects the sun's rays into the shadows and colors them red. This gives the moon an impressive copper tone during an eclipse. Both lunar and solar eclipses can be total or partial depending on whether the solar or lunar disk is entirely covered up or only partially. In a total solar eclipse we can see the sun's corona for a few minutes. This ring of radiating light consists of highly diffused gases heated to a temperature of several million degrees Celsius, but its luminosily is too small to be seen except during total eclipses.
Solar and lunar eclipses are quite rare, and they are not visible from all parts of Earth.