The Geomagnetic Field And Magnetosphere

Helical fluid motions in Earth's electrically conducting liquid outer core have an electromagnetic dynamo effect, giving rise to the geomagnetic field. The planet's sizable, hot core, along with its rapid spin, probably accounts for the exceptional strength of the magnetic field of Earth compared with those of the other terrestrial planets. Earth's main magnetic field permeates the planet and an enormous volume of space surrounding it. A great teardrop-shaped region of space called the...

Energy Generation and Transport

The energy radiated by the Sun is produced during the conversion of hydrogen (H) atoms to helium (He). The Sun is at least 90 percent hydrogen by number of atoms, so the fuel is readily available. Since one hydrogen atom weighs 1.0078 atomic mass units, and a single helium atom weighs 4.0026, the conversion of four hydrogen atoms to one helium atom yields 0.0294 mass unit, which are all converted to energy, 6.8 million electron volts (MeV), in the form of gamma (y) rays or the kinetic energy of...

Literary and Historical References

The following subsections address some aspects of how eclipses were regarded by the great ancient civilizations. When reading these accounts of long-gone people who grappled with the implications of startling and portentous events in the sky, one cannot help but be struck by the immediacy of their emotions, and one must also admire the beginnings of astronomy, of humanity's understanding, however dimly, of its place in the universe. According to long-established tradition, the history of...

Solution to the Angular Momentum Puzzle

The angular momentum problem that defeated Kant and Laplace why the planets have most of the solar system's angular momentum while the Sun has most of the mass can now be approached in a cosmic context. All stars having masses that range from slightly above the mass of the Sun to the smallest known masses rotate more slowly than an extrapolation based on the rotation rate of stars of higher mass would predict. Accordingly, these sunlike stars show the same deficit in angular momentum as the Sun...

Composition and Surface Pressure

Carbon dioxide constitutes 95.3 percent of the atmosphere by weight, nine times the quantity now in Earth's much more massive atmosphere. Much of Earth's carbon dioxide, however, is chemically locked in sedimentary rocks the amount in the Martian atmosphere is less than a thousandth of the terrestrial total. The balance of the Martian atmosphere consists of molecular nitrogen, water vapour, and noble gases (argon, neon, krypton, and xenon). There are also trace amounts COMPOSITION OF THE...

Eclipses In History

Eclipses of the Sun and Moon are often quite spectacular, and in ancient and medieval times they were frequently recorded as portents usually of disaster. Hence, it is not surprising that many of these events are mentioned in history and literature as well as in astronomical writings. Well over 1,000 individual eclipse records are extant from various parts of the ancient and medieval world. Most known ancient observations of these phenomena originate from only three countries China, Babylonia,...

Notable Surface Features

Prominent and noteworthy features of the lunar surface range from prominent craters such as Copernicus and Tycho to sites important in humanity's brave history of lunar exploration such as Taurus-Littrow and Hadley Rille. Copernicus is one of the most prominent craters on the Moon. It constitutes a classic example of a relatively young, well-preserved lunar impact crater. Located at 10 N, 20 W, near the southern rim of the Imbrium Basin (Mare Imbrium) impact structure, Copernicus measures 93 km...

Basic Astronomical Data

Mars moves around the Sun at a mean distance of 228 million km (140 million miles), or about 1.5 times the distance of Earth from the Sun. Because of Mars's relatively elongated orbit, the distance between Mars and the Sun varies from 206.6 million to 249.2 million km (128.4 million to 154.8 million miles). Mars orbits the Sun once in 687 Earth days, which means that its year is nearly twice as long as Earth's. At its closest approach, Mars is less than 56 million km (35 million miles) from...

The Astronomical Unit

Astronomers studying the solar system use a unit of length, the astronomical unit (AU), that provides a convenient way to express and relate distances of objects in the solar system and to carry out various astronomical calculations. The astronomical unit is effectively equal to the average, or mean, distance between Earth and the Sun. Alternately, it can be considered the length of the semimajor axis i.e., the length of half of the maximum diameter of Earth's elliptical orbit around the Sun....