The Observer's Guide to Planetary Motion: Explaining the Cycles of the Night Sky

246 Pages·2014·5.98 MB·English

by  Dominic Ford (auth.)

Dominic Ford
The
Observer’s
Guide to
Planetary
Motion
Explaining the Cycles
of the Night Sky
The Patrick Moore
The Patrick Moore Practical Astronomy Series
For further volumes:
http://www.springer.com/series/3192

The
Observer’s
Guide to
Planetary
Motion
Explaining the Cycles of the Night Sky
Dominic Ford
Dominic Ford
The Naked Scientists
Cambridge , UK
ISSN 1431-9756 ISSN 2197-6562 (electronic)
ISBN 978-1-4939-0628-4 ISBN 978-1-4939-0629-1 (eBook)
DOI 10.1007/978-1-4939-0629-1
Springer New York Heidelberg Dordrecht London
Library of Congress Control Number: 2014937791
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About the Author
Dominic Ford is a science radio presenter and producer at the N aked Scientists ,
where he presents the monthly podcast N aked Astronomy. He is also a regular guest
on BBC and ABC radio stations, talking about the latest astronomical news. In his
spare time, he runs I n-The-Sky.org , a website which lists forthcoming astronomical
events, and manages the website of the British Astronomical Association (BAA).
D ominic previously worked as a professional astronomer at the University of
Cambridge, where he completed a doctorate on the processes by which stars form.
He went on to design components of the computer software needed to process
observations made by the Square Kilometre Array (SKA). When it commences
operations in around 2019 this will be the world’s largest radio telescope, split over
two sites in Southern Africa and Australasia.
Dominic also works on open-source software projects for the amateur community.
In 2005, he wrote G repNova , an automated image-comparison tool for supernova
hunters which has assisted in the discovery of over 50 events to date. Among its
users is the British supernova hunter Tom Boles, who presently holds the world
record for the largest number of supernova discoveries made by any single indi-
vidual. More recently, Dominic has worked as the lead author of the vector graphics
package P yxplot , which was used to produce most of the line diagrams in this book.
v

Pref ace
A stronomy is among the oldest recorded human activities, and the planets have
always held a special fascination. As seen by the unaided eye, their motion across
the night sky is the only way in which the heavens appear to change noticeably
from one century to the next. While the patterns of the constellations have remained
largely unchanged for thousands of years, the Sun, Moon and five naked-eye plan-
ets have always been the seven exceptions to the rule. Efforts to understand their
motion date back to at least 1500 B C , when Babylonian court astronomers were
already compiling systematic tables of observed planetary positions and trying to
formulate mathematical procedures to predict their future paths. More recently, as
we shall see, it was this same problem that motivated Isaac Newton to formulate his
laws of motion and gravity.
This book is about the ways in which the orbits of the planets produce complex
patterns in their brightnesses, sizes and positions from one week to the next. It tells
the story of how mankind has struggled to understand those orbits over the past
centuries, and what they can tell us today about the Universe we live in. For active
observers, it provides tabulated almanacs of significant events and alignments in the
paths of all the planets over the period 2010–2050. In the cases of rarer events, I
have been able to cover much longer time periods still, sometimes extending to
2200. These tables serve not only as catalogs of examples of each of the types of
events that will be described, but also as a long-term handbook for active observers,
which can be used to plan suitable dates for future star parties, for preparing ‘sky
diary’ columns in advance, or simply for personal use.
Since the computer revolution, the role of printed celestial almanacs has changed
considerably. Nowadays, almost every amateur astronomer has access to a com-
puter, and there is no need to look far to find out where the planets are on any given
vii
viii Preface
night. There is a wide range of planetarium programs to choose from, ranging from
freely available packages such as S tellarium and G oogle Sky to deluxe paid-for
packages such as T he Sky and S tarry Night, to countless astronomy apps that are
available for each and every type of smartphone or tablet computer.
Few astronomers would now choose to pore over printed tables of planetary
positions to find out where they might find Mars on any particular night. The only
reason why they might is that such planetarium software remains, for the time being
at least, accessible only on devices with backlit screens. Holding a planetarium
program up to the night sky is a fun way to navigate when learning the constella-
tions and bright stars, but at a dark site it can take several minutes to reacquire night
vision after using one. Nonetheless, just as printed encyclopedias have had to
evolve in response to the wealth of information that Wikipedia places at the finger-
tips of anyone with access to a web browser, so too printed ephemerides have had
to evolve.
The drawback of planetarium software is that having so much information avail-
able can make it difficult to see the wood for the trees. A recurring theme in this
book is that there are long-term cycles in the positions and visibilities of the planets.
Some, such as eclipses and the phases of the Moon, are very evident. Others, such
as ocean tides, are no less evident, but it is not immediately obvious that they are
triggered by astronomical mechanisms. The vast majority, however, are rather more
subtle and can only be spotted by compiling detailed observations over long time
periods. In light of this, it is rather remarkable that there is extensive evidence that
ancient civilizations were very familiar with many of them. The Babylonians, for
example, were fully aware by 1500 B C that days can vary in length by up to 30 s,
depending on the time of year (see Chap. 4 ).
I t remains interesting to study such patterns, not just to better understand when
and where the planets will appear in the sky, but also to understand the gravitational
and geometric mechanisms which lie behind them. Why does Saturn sometimes
remain unfavorably placed for northern-hemisphere observers for several years at a
time? Why does Mars appear much brighter at some oppositions than others? Why
does Venus move from the evening sky to the morning sky in only 4–5 months, but
take over a year to make the return journey? Such are the questions with which
astronomers have had to grapple over the centuries in working out the three-
dimensional structure of the solar system and the gravitational forces that act
between the planets.
As we shall see, in modern times, new questions have been added to the list.
Why did the solar system form with all of its gas giants further from the Sun than
its terrestrial planets? Are Saturn’s rings a permanent structure, or a temporary
result of some recent event? Why do we sometimes have to insert leap seconds into
our system of timekeeping?
This book is free from equations, relying instead on diagrams of the physical
geometry of the solar system, so as to be accessible to readers with little or no
mathematical background. It is intended both for amateur astronomers who are
Preface ix
interested in the mechanics which underlie the motion of the solar system and also
for science students who are interested in the historical and observational origin of
the physical models of gravity and planetary motion that remain in use by
astronomers.
Cambridge, UK Dominic Ford
March 2013