The Solar System A Brief Introduction

This is primarily a book about imaging the planets with webcams and is aimed at amateur astronomers who already have some basic knowledge of the solar system. However, we all have to start somewhere and it is quite possible that a few complete beginners will be attracted to this book, with virtually no prior knowledge of astronomy at all. This chapter is for the total novice. If you are familiar with the structure of our solar system, feel free to skip this introduction. If not, then start your adventure here. I have deliberately written this introduction to be as simple as possible, so that almost anyone will (hopefully) understand it.

We live in a solar system (Figure 1.1) comprising one Sun, nine planets, hundreds of thousands of asteroids, and well over one thousand comets. If I sound a bit vague about exactly how many asteroids and comets there are, well, that is deliberate. Firstly, no one knows. Secondly, if you count objects no bigger than large boulders or snowballs as asteroids and comets, there must be millions. Figures 1.2 through 1.7 show some of the solar system's major bodies.

Billions of years ago, material in our solar system started to drift toward a common center of gravity. The vast bulk of the material, comprising mainly hydrogen, formed the Sun. As soon as enough mass was present, nuclear fusion started and the Sun began to shine, illuminating the early solar system and announcing its presence as a star in this part of our galaxy. All of the stars in the night sky are Suns; some are bigger, some are smaller, but all of them are a huge distance away.

In passing, I would like to stress early on that you must never, ever stare at the Sun, even with the naked eye, and certainly never with a telescope (unless the telescope has special full-aperture solar filters and you are an expert; see Chapter 16 for more details). The Sun is dangerously bright and permanent eye damage can easily occur. You have been warned!

The distances to other stars (and, therefore, other solar systems) are measured in light years. The closest star to us, Proxima Centauri, is 4.2 light years away.

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Figure 1.1. The orbits of the inner planets (top) and outer planets (bottom) are shown. Diagram: courtesy of Gerald North. Average solar distances are represented and Pluto is excluded due to space limitations.
Figure 1.2. Mars, imaged at its closest to the Earth. Image taken by the Hubble space telescope on August 26, and 27, 2003. South is up. The Solis Lacus is shown on the left of the left-hand image; the Syrtis Major is shown on the left of the right-hand image. Image: NASA/STScI.
Figure 1.3. The 33 x 13 km asteroid Eros, imaged by the NEAR/Shoemaker spaceprobe. Image: NASA.
Figure 1.4. Comet Hyakutake from Tenerife on the night of March 24/25, 1996. Image: M. Mobberley.
Figure 1.5. Jupiter, Io, and Io's shadow, taken by the Cassini space probe. Image: NASA.
Figure 1.6. Saturn imaged by Cassini. Image: NASA.

In other words, it would take a beam of light, traveling at 300,000 kilometers per second, 4.2 years to get from here to that star. By comparison, light takes just over a second to get from the Moon to the Earth and just over eight minutes to get from the Sun to the Earth. As nobody knows (yet) how to travel faster than light, it can be seen that traveling to other stars in a reasonable timescale is pretty well impossible with current technology. One only has to look at the stars in the sky and see

Figure 1.7. Uranus imaged by Voyager 2: Image: NASA.

how faint they are to appreciate how far away they are. Each one would be a blind-ingly brilliant Sun when seen close up.

In addition to the material that formed our Sun almost five billion years ago, the leftover chunks of heavy elements clustered together and became the planets and asteroids that we see today.

All of the planets in the solar system orbit the Sun in the same direction as the Sun rotates. But, whereas the Sun rotates in 25 days, the planets orbit the Sun in much longer periods. Looking from above the solar system almost everything orbits the Sun counterclockwise. The only real exceptions to this are the long-period comets, which can orbit the Sun in any direction and at any angle.

In order outward from the Sun, the major planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Most (but not all) of the asteroids or minor planets live in the asteroid belt between Mars and Jupiter. These are worlds smaller than our Moon, but there are hundreds of thousands of the smaller (kilometer-sized) ones. As one moves inward from Mars to the Earth and closer to the Sun, we encounter asteroids closer to home, too. These are classified as Amor, Apollo, and Aten asteroids, and some NEOs (Near Earth Objects) are considered a threat to the Earth. The most dangerous ones are called PHAs, or Potentially Hazardous Asteroids.

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