Introduction

In additon to the planets that are hundreds of other bodies in our solar system.

Asteroids

Asteroids are rocky fragments left over from the formation of the solar system about 4.6 billion years ago. Most of these fragments of ancient space rubble - sometimes referred to by scientists as minor planets - can be found orbiting the Sun in a belt between Mars and Jupiter.

This region in our solar system, called the Asteroid Belt or Main Belt, probably contains millions of asteroids ranging widely in size from Ceres, which at 940 km in diameter is about one-quarter the diameter of our Moon, to bodies that are less than 1 km across. There are more than 20,000 numbered asteroids.

As asteroids revolve around the Sun in elliptical orbits, giant Jupiter’s gravity and occasional close encounters with Mars or with another asteroid change the asteroids’ orbits, knocking them out of the Main Belt and hurling them into space across the orbits of the planets. For example, Mars’ moons Phobos and Deimos may be captured asteroids.

Scientists believe that stray asteroids or fragments of asteroids have slammed into Earth in the past, playing a major role both in altering the geological history of our planet and in the evolution of life on it. The extinction of the dinosaurs 65 million years ago has been linked to a devastating impact near the Yucatan peninsula in Mexico.

Comets

Throughout history, people have been both awed and alarmed by comets, stars with "long hair" that appeared in the sky unannounced and unpredictably. We now know that comets are dirty-ice leftovers from the formation of our solar system around 4.6 billion years ago. They are among the least-changed objects in our solar system and, as such, may yield important clues about the formation of our solar system. We can predict the orbits of many of them, but not all.

Around a dozen "new" comets are discovered each year. Short-period comets are more predictable because they take less than 200 years to orbit the Sun. Most come from a region of icy bodies beyond the orbit of Neptune. These icy bodies are variously called Kuiper Belt Objects, Edgeworth-Kuiper Belt Objects, or trans-Neptunian objects. Less predictable are long-period comets, many of which arrive from a distant region called the Oort cloud about 100,000 astronomical units (that is, 100,000 times the mean distance between Earth and the Sun) from the Sun. These comets can take as long as 30 million years to complete one trip around the Sun. (It takes Earth only 1 year to orbit the Sun.) As many as a trillion comets may reside in the Oort cloud, orbiting the Sun near the edge of the Sun’s gravitational influence.

Meteoroids

Shooting stars" or meteors are bits of material falling through Earth's atmosphere; they are heated to incandescence by the friction of the air. The bright trails as they are coming through the Earth's atmosphere are termed meteors, and these chunks as they are hurtling through space are called meteoroids. Large pieces that do not vaporize completely and reach the surface of the Earth are termed meteorites.

Scientists estimate that 1,000 tons to more than 10,000 tons of meteoritic material falls on the Earth each day. However, most of this material is very tiny - in the form of micrometeoroids or dust-like grains a few micrometers in size. (These particles are so tiny that the air resistance is enough to slow them sufficiently that they do not burn up, but rather fall gently to Earth.)

Where do they come from? They probably come from within our own solar system, rather than interstellar space. Their composition provides clues to their origins. They may share a common origin with the asteroids. Some meteoritic material is similar to the Earth and Moon and some is quite different. Some evidence indicates an origin from comets.

Classification of the Planets

Composition:

- Terrestrial or rocky planets: Mercury, Venus, Earth, and Mars:
The terrestrial planets are composed primarily of rock and metal and have relatively high densities, slow rotation, solid surfaces, no rings and few satellites.

- Jovian or gas planets: Jupiter, Saturn, Uranus, and Neptune.The gas planets are composed primarily of hydrogen and helium and generally have low densities, rapid rotation, deep atmospheres, rings and lots of satellites.

- Pluto.

Size:

Small planets: Mercury, Venus, Earth, Mars and Pluto.
The small planets have diameters less than 13000 km.

Giant planets: Jupiter, Saturn, Uranus and Neptune.
The giant planets have diameters greater than 48000 km

Mercury and Pluto are sometimes referred to as lesser planets (not to be confused with minor planets which is the official term for asteroids).
The giant planets are sometimes also referred to as gas giants.

Relative Position to the Sun:

Inner planets: Mercury, Venus, Earth and Mars.
Outer planets: Jupiter, Saturn, Uranus, Neptune and Pluto.

The asteroid belt between Mars and Jupiter forms the boundary between the inner solar system and the outer solar system.

Relative Position to Earth:

Inferior planets: Mercury and Venus.
- Closer to the Sun than Earth.
- The inferior planets show phases like the Moon's when viewed from Earth.
- Earth.

Superior planets: Mars through Pluto.
- Farther from the Sun than Earth.
- The superior planets always appear full or nearly so.

History:

Classical planets: Mercury, Venus, Mars, Jupiter, and Saturn.
- known since prehistorical times
- visible to the unaided eye

Modern planets: Uranus, Neptune, Pluto.
- discovered in modern times
- visible only with telescopes
- Earth.

Galaxies: The Milky Way

Our own galaxy consists of about 200 billion stars, with our own Sun being a fairly typical specimen. It is a fairly large spiral galaxy and it has three main components: a disk, in which the solar system resides, a central bulge at the core, and an all encompassing halo.

Disk: The disk of the Milky Way has four spiral arms and it is approximately 300pc thick and 30kpc in diameter. It is made up predominantly of Population I stars which tend to be blue and are reasonably young, spanning an age range between a million and ten billion years.

Bulge: The bulge, at the centre of the galaxy, is a flattened spheroid of dimension 1kpc by 6kpc. This is a high density region where Population II stars predominate---stars which tend toward red and are very old, about 10 billion years. There is growing evidence for a very massive black hole at its centre.

Halo: The halo, which is a diffuse spherical region, surrounds the disk. It has a low density of old stars mainly in globular clusters (these consist of between 10,000 - 1,000,000 stars).The halo is believed to be composed mainly of dark matter which may extend well beyond the edge of the disk.

Black Holes

A black hole is a massive object (or region) in space that is so dense that within a certain radius (the Schwarzschild radius, which determines the event horizon), its gravitational field does not let anything escape from it, not even light.

The point at the center of a black hole is called a singularity. Within a certain distance of the singularity, the gravitational pull is so strong that nothing--not even light--can escape. That distance is called the event horizon. The event horizon is not a physical boundary but the point-of-no-return for anything that crosses it. When people talk about the size of a black hole, they are referring to the size of the event horizon. The more mass the singularity has, the larger the event horizon. The structure of a black hole is something like this:

The Formation of Black Holes:

It is thought that giant stars (those with a mass over 3 times the mass of the Sun) will evolve into a red supergiant, a supernova, and then into a black hole. The typical black holes is believed to have a mass of roughly 10 times that of the Sun, but the range must be huge. For a typical black hole with a mass 10 times that of the Sun, the Schwarzschild radius would be roughly 18.6 miles (30 km).

The Phrase Black Hole:

The phrase 'black hole' was coined by the physicist John Archibald Wheeler; before Wheeler, black holes were called 'frozen stars.' Astronomers think that there may be a black hole at the center of each galaxy.

Eclipses

Eclipses have long been a source of mystery and spectacle. These events were viewed with fear and dread in the past and, even today, still thrill.

There is a lot of special vocabulary involved in eclipses but there is a way to keep from being confused. The eclipse is named for the object that is being eclipsed, or obscured. In a solar eclipse you observe the Sun (using only safe methods, of course).In a lunar eclipse you observe the Moon. A portion of its surface will be obscured.

Another way to avoid confusion is to consider the time at which you will be viewing the eclipse. Because of the geometry described below, you can only view a solar eclipse when the Sun is up. You view lunar eclipses when the Moon is up.

Eclipses occur when the Sun, Earth and Moon line up. They are rare because the Moon usually passes above or below the imaginary line connecting Earth and the Sun. In a solar eclipse the Moon passes directly in front of the Sun. This can only happen when the phase of the Moon is "new." That occurs because, for Earth-based observers, the far side of the Moon is illuminated while the side facing Earth is in darkness. The Moon, like any sphere, casts a shadow. A solar eclipse occurs when that shadow sweeps across Earth. The black cone is called the umbra, as in umbrella. An observer anywhere in that region is completely in shade. None of the Sun is visible from there.

Surrounding the umbra is the penumbra. An observer there will see some, but not all, of the Sun. Outside of these regions, all of the Sun is visible. Note that the tip of the umbra barely touches Earth. At the current time the position of the Moon relative to the Sun is such that the Moon, which is 400 times smaller that the Sun, is 400 times closer! This means that the two objects appear to be the same size in the sky. Only observers at the tip of the umbral cone will see a total solar eclipse. A large number of observers across the globe will see a partial solar eclipse if they are in the penumbra.

An annular eclipse is a special partial solar eclipse. Because the Moon's orbit around Earth is an ellipse, not a circle, the Moon's distance from Earth varies. When the Moon is far from Earth it appears slightly smaller in the sky. (Earth's orbit around the Sun is also an ellipse, and during January, Earth is at its closest point to the Sun. The Sun's size is slightly larger than during the rest of the year.) With a "small" Moon and a "large" Sun the Moon will not completely block out the Sun. The umbra does not touch Earth. An observer would have to be above the surface of Earth to see a total eclipse. For individuals in just the right location, the Sun appears as a ring (annulus) around the silhouetted Moon.

In a lunar eclipse the Moon moves into Earth's shadow. They can only occur when the moon is "full." Observers on the night side of Earth see the Moon take on a reddish hue as it moves into Earth's umbra. If the entire disk of the Moon falls into the umbra it is total lunar eclipse. If only a portion does, then it is a partial lunar eclipse. Penumbral lunar eclipses are very difficult to detect because the Moon dims only slightly while moving through that region. Lunar eclipses are more common than solar eclipses. Total eclipses of the Sun and Moon are partial before and after totality.