How We See the Sky: A Naked-eye Tour of Day & Night

December 22, 2011

In a book that considers the appearance of the heavens from the point of view of naked-eye observations on the Earth's surface, Thomas Hockey writes about the practical astronomy that anyone can do without using a telescope - and, of course, that is how astronomy was done throughout most of human history. His goal is to explain phenomena from a modern point of view, and to provide a cultural and historical setting.

The main themes holding the narrative together are clear explanations of a multitude of phenomena in the astronomical sky, and the way we describe and explain those phenomena. The first chapter sets the scene by describing the "bowl" of the night sky, which is how the ancients tried to make sense of the heavens and the motions of the planets. This leads into a description of coordinate systems and the celestial sphere.

With this introductory material the reader is well placed to understand phenomena associated with the apparent motion of the stars: the Earth is a rotating world spinning on an inclined wobbly axis, the consequences of which are the precession of the equinoxes, seasons, solstices and more. Three chapters artfully explain the complicated apparent motions of the Sun and the Moon, which have all kinds of practical implications for calendars and time-keeping.

Cultural aspects of sky-watching are well covered, too, with concise accounts of megalithic astronomy and its sites, notably Stonehenge, and the passage grave at Newgrange in the Republic of Ireland, which is aligned to sunset at the winter solstice. The alignments at neolithic sites appear to mark the annual cycle of the Sun in a symbolic way, capturing the moment of death and resurrection of the Sun. That's how Hockey colourfully imagines the purpose of these sites, and it's the story rolled out to tourists. Scholars may be sceptical.

In order to review this book I conducted the following thought experiment. I give popular lectures on astronomy on cruise liners, so I recalled some of the questions I have been asked in the queue for the buffet or the cocktail bar. For example: what's a blue Moon? Why does the Moon look big when it is close to the horizon? Why are there leap years? Those questions are answered here, and thus I would happily put this book on a reading list for newcomers to elementary astronomy who are curious to know more about the night sky.

In his preface, the author explains that he restricts the account to purely astronomical phenomena, and he therefore rejects those sky phenomena that arise because we view the heavens through an atmosphere. This is a mistake. When we look towards the Sun we may be able to see a parhelion (sun dog), or a circumzenithal arc (an upside-down rainbow), as well as rainbows and halos, and the green flash at sunset. The Moon in winter may have a halo.

I would also have welcomed more extensive coverage of meteors and comets, as well as solar eclipses. Despite these limitations, this is an easy introduction to astronomy from your back yard.

How We See the Sky: A Naked-eye Tour of Day & Night

By Thomas Hockey. University of Chicago Press. 224pp, £39.00 and £13.99. ISBN 9780226345765 and 5772. Published 7 October 2011

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