Saturday, October 27, 2007

That was the day that was - 6 megahits

kw: book reviews, nonfiction, sun, moon, earth

I just Googled "that was the day that was" and got 5.9 million hits (Yes, I used quotes. Without them, it's 1.1 Billion(!) hits). Raconteur Michael Sims spends a day with us in Apollo's Fire: A Day on Earth in Nature and Imagination, and like all semi-fictional days, it is one packed day. Authors aplenty have written "day in the life of" novels and memoirs. This book is a day in the life of...a Day.

It is only recently, historically speaking, that people determined that the Sun is large and comparatively far away. How large? Its diameter is more than 100 times the Earth's, so its volume is greater than one million Earth volumes. How far? Whether you think in miles (93 million) or km (149 million), it is FAR: nearly 12,000 times Earth's diameter, or 3,700 times around, and around, and around...

But for thousands, perhaps hundreds of thousands, of years, since we had a thinker to think with, the Sun was thought of as a powerful but familiar Presence, perhaps the size of the next big town down the road, hung on a big bowl of a sky held up by the distant hills or mountains.

By turns didactic, lyrical, bare-bones factual, yet always engaging and witty, Sims takes us, not hour by hour, but event by event, through one day: pre-dawn darkness, dawn, morning, midday, afternoon and "brillig", sunset, twilight, and night. Here both science and mythology have a place.

Every culture bases its religion or mythology on daily events and the dominant players, the Sun and Moon. It could hardly be otherwise. Suppose that one side of Earth faced the sun perpetually, as the Moon's visible hemisphere always faces Earth. There would be no notion of "reliable as the sunrise". The Moon, which circles Earth monthly, would be the primary mobile item in a fixed Heaven, and would likely be the dominant deity; the Sun is "just there all the time, and does nothing". Subtler events like the slow orbits of Mars and Saturn take a people with plenty of leisure to puzzle out.

Many in the West know the Biblical formulation, "The Sun to rule the day, and the Moon to rule the night." But have you noticed the Moon in the daytime? This image from a spacecraft on its way to the outer planets captures an amazing view: Not only is the Moon at First Quarter, so is the Earth.

Those in Earth's nighttime see the half-moon high in the sky at sunset, rapidly descending to set about midnight. But consider those on the late afternoon portion of Earth. The Moon is (barring overcast skies) visible as a white half-cookie pasted on a blue sky, about halfway around the sky from the Sun's position. About half the time, the Moon is a daylight object, and near New Moon, it hardly appears after dark.

And that blue color? It is the blue sky seen from above. The air scatters light; even very pure air containing no dust will scatter light. Blue scatters more than red, because the short wavelength of blue light is more closely matched to the size of the molecules that make up the air. When you look at a mountain range such as the Rockies, seen from forty or fifty miles away, the blue, hazy effect is the blueness of the air between. Air is visible. Even after dark, if the moon is up, the sky appears grayish, not black. If our color vision were more sensitive, we would see that moonlit skies are also blue. Maybe lemurs, with their huge eyes see blue nighttime skies. But they are dark to us.

Darkness. What is the reason? The existence of a dark sky is visible proof that the Universe is finite, either in time, in extent, or probably both. Sims takes us rapidly through the wranglings of Galileo, Kepler, Olbers, and finally Slipher, as the puzzle of the dark sky was gradually solved...well, as solved as it can be at present. We still don't know for sure whether the expansion following the Big Bang is slowing down or, as many now contend, speeding up (My take: slowing down. The apparent anomaly in supernova brightness at very great distances is related to elemental evolution. The stars billions of years ago, having very low "metals" abundance, got bigger, burned brighter, and blew up more energetically than is the rule now. Type Ia supernovae in the early universe were genuinely brighter.).

By the end of the book, I felt like I'd been sitting with a comfortable, knowledgeable companion through a long, pleasant day.

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