How to read the solar system : a guide to the stars and planets

269 Pages·2015·3.62 MB·English

by  Paul Abel

HOW TO READ THE SOLAR
SYSTEM
A GUIDE TO THE STARS AND PLANETS
CHRIS NORTH PAUL ABEL
AND
for Patrick
Contents
Foreword
1 What is the Solar System?
2 Ancient Stargazers
3 Celestial Mechanics
4 The Sun
5 Mercury
6 Venus
7 Earth and Moon
8 Mars
9 Jupiter
10 Saturn
11 Uranus and Neptune
12 Asteroids and Dwarf Planets
13 Comets
14 One Among Many
Acknowledgements
Glossary of Useful Terms
Index
Picture Credits
Foreword
How To Read the Solar System is a book that would have warmed Sir Patrick
Moore’s heart – for many reasons. It has a simple aim – to provide, for those just
beginning in astronomy, a guide to the awesome motley collection of objects
that make up the family of our own star – the yellow ball of incandescence
which we call the Sun.
To call this assortment of rocks a ‘family’ is not quite as far-fetched as it may
seem, for although they come in an array of vastly differing shapes and sizes,
these lumps of matter all share a common history – the history of the solar
system, and many of their births are related. The more we discover about these
planets, moons, comets and meteors, the more surprises they reveal. Yet, on a
cosmic scale, we are looking here at the tiniest portion of the known Universe –
an almost infinitesimally small fraction of the vast expanse of space that we see
looking up on a clear night, if we are lucky enough to be able to escape the all-
pervasive light pollution of the modern world.
What we see in the heavens, even with the naked eye, is a myriad of stars –
each one a Sun in its own right, and recent research indicates that most of them
probably have families of planets of their own. This appears to be the norm,
rather than the exception, so there are probably even more planets than there are
stars out there.
With binoculars or a small telescope, more wonders are revealed. We are able
to glimpse, from within, the structure of our own Milky Way galaxy, a kind of
greater family into which all the stars in our skies fit. We can look at the
brightest parts of the Milky Way and see the massive dense centre of our galaxy,
and look in the opposite direction out into the outer reaches of its spiral arms.
And looking further out from our galaxy we are even able to see other galaxies
of similar size, each containing billions of stars of their own. The most powerful
telescopes now available have peered into unimaginably distant space, and,
because of the finite speed of their light coming towards us, seen billions of
years back in time. What they have shown us is billions upon billions of galaxies
stretching out to the limits of the observable Universe.
Sir Patrick Alfred Caldwell Moore (1923-2012).
This is a very big picture, and the study of it, on this enormous scale, is called
cosmology. Yet, this book has no truck with cosmology. It’s impossible to see
any of this vastness at close quarters, but many of us, including Sir Patrick
Moore, have found great thrills in examining what is on our own doorstep,
relatively speaking. Looking up at the skies of our own Solar System, we soon
become aware of those ‘wanderers’ which move their position on the starry vault
every night – the planets which have been known to our ancestors since before
recorded history: Mercury, Venus, Mars, Jupiter, Saturn. And we may be lucky
enough to see a visiting comet or a minute piece of rock flash its burning path
through our atmosphere as it burns up – the ‘shooting stars’ which also
fascinated our forbears.
It is this close-up drama that is the subject of this book. It is a modest aim, to
survey only material within a couple of light years of our Sun, but in this study,
we will see in miniature the whole of the Universe, since it seems that no matter
where we go in the Universe things look very much the same. So a careful study
of our immediate surroundings can tell us so much about the wider cosmos.
Indeed, Sir Patrick Moore, though well versed in every aspect of current
knowledge in astronomy and astrophysics, devoted a large fraction of his entire
life to detailed observations of the planets, and in particular, our own Moon, a
life to detailed observations of the planets, and in particular, our own Moon, a
mere quarter of a million miles from Earth. His meticulous lunar mapping was
so extraordinarily accurate that is was used by the first US astronauts, to plan
their conquest of the Moon.
Sir Patrick was not the only astronomer to become absorbed in the
uncovering the secrets of our Solar System, but our knowledge of this subject
has exploded in the last 20 years, as a succession of unmanned probes have been
launched on missions to rendezvous with the planets, their moons, and even with
cometary nuclei and asteroids. The photographs and data these probes have sent
back to Earth have brought us shockingly clear information about all these
objects, and it is fair to say that the truth about the composition of all these
objects was a surprise in every case.
Does that ‘uniformity’ we see in the Universe include life? Are those other
planets inhabited too, with the wonderful array of creatures which Man has done
such a good job of bringing to the brink of extinction? We may never know. But
looking at those rocks and stones, and the very dust that surrounds us on our tiny
blue planet is a very good place to start. Certainly for this Sir Patrick, to whom
this admirable book is dedicated, and who is the reason so many of us have
discovered the thrills of astronomy, would be smiling.
Dr Brian May
1
What is the Solar System?
We should probably start this book by stating what is meant by the Solar
System, and what better place to look than the dictionary. The word ‘solar’
comes from the Latin solaris, which means ‘of the Sun’, and so we define the
Solar System to be the collection of objects that ‘belong to’ the Sun – that is to
say, everything that orbits it.
The Sun is the powerhouse of the whole Solar System, and presents a
perfectly good place to start our introduction to the Solar System. Although it
may be incredibly important to our brief lives here on Earth – and was portrayed
by our ancestors as an all-seeing deity – the Sun is in fact more or less an
ordinary star. But in astronomy, the word ‘ordinary’ is often overused. The Sun
is over a million kilometres across, and weighs in at a whopping two thousand
million million million million tonnes. Its surface is pretty hot by human
standards, at around 5,500°C, but its centre is at a temperature of tens of millions
of degrees – so hot and dense that nuclear fusion is converting four million
tonnes of matter into pure energy every single second.
This incredible star, while not significantly different from the others in our
Galaxy, is host to a magnificent array of planets, moons, asteroids and comets –
including one very special planet that we call home. There are, of course, the
eight major planets, which in order of increasing distance from the Sun are
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. We often
groups these into two main sections, the inner planets (Mercury through to Mars)
and outer planets (Jupiter to Neptune), and there are useful distinctions to be
made at this level.
The orbits of the planets in our Solar System, with distance scales shown in light travel time.