Classic Asimov. Lucid and occasionally wryly humorous.Edmund Halley gets numerous mentions, although William Herschel gets even more. I think Asimov sometimes didn't bother to verify some non-scientific bits of history, and got those wrong in their particulars. But his science history is always good, because it is necessarily more coherent than the rest of history.

This was the only Asimov non-fiction that I could get on audio. How I wish all his history were on audio. In the end, after listening to it forward on audio, obviously, I decided to read the book in reverse order, starting with the very last questions.

Detailed Review:

The discussion is really mostly chronological, starting with the basic questions asked and answered by the ancients and gradually moving toward more modern times. Parts are my own:

Part I: The Earth, its Shape, Dimensions, Mass, Age, Position and Internal Temperature.

1. What is the Shape of the Earth?
Intelligent observation does convince one that the earth is a sphere. I was interested to find out from Asimov's own "How Did We Find out the Earth is Round?", that some Greeks philosophers postulated that the Earth was a cylinder.

2. What is the Size of the Earth?
Eratosthenes' idea is built up nicely.

3. If the Earth is a Sphere, Why Don't We Slide Off?
A very good question, this troubled me when I was a child.

4. Does the Earth Move?
How we got from "No" to "Yes", ending with Foucault's pendulum. There's a fine example in the Boston Science Museum.

4. When You Jump Up, Why Don't You Come Down in a Different Place?
Shared momentum. It's still pretty shocking to think that, at the equator, you're moving at 1000 miles an hour.

6. What Makes the Wind Blow?
Not so certain that this is quite correct. Coriolis was more a mechanical engineer than a meteorologist, and I don't know that much about the weather.

7. Why is Summer Warmer than Winter?
A well-known explanation, but no history about the discovery of this explanation.

8. How Do We Measure Time?
This is a discussion of calendars.

9. How Do We Measure Time Intervals Less Than A Day Long?
A discussion of clocks and quite advanced clepsydras. The most interesting bit to me is the bit about Christian Huygens, the pendulum clock, and the cycloid. I must know more about the cycloid trick! Promising detailed discussion here: http://www.antique-horology.org/Piggott/RH/Images/81V_Cycloid.pdf.

10. How Old is the Earth?
Various scientific efforts at calculation lead to lots of different answers. But these answers tend toward the way longer than the biblical chronology by several orders of magnitudes, causing all sorts of cultural stress. Asimov is probably incorrect about Bernard Pallissy's demise; he was not burned alive, he seems to have died in the Bastille of malnourishment, etc.

11. How Was the Age of the Earth Finally Determined?
A lower limit is set using radioactive decay of uranium, which has a conveniently long half-life. That lower limit is 3.8 billion, i.e., 3.8e9.

12. What is Mass?
This is more like a definition, but it is pointed out that gravitational and inertial mass are the same, which is interesting.

13. What is the Mass of the Earth?
Clever measurement allowed Henry Cavendish to arrive at a reasonable value in the 1700s. He determines the force exerted by some heavy balls of known mass, and then the force exerted by the earth on the same apparatus. After that it's just ratios.

14. What is Density?
Definitional. Also, with the answer to (2) and (13) we can now calculate the density of the Earth, about 5.5 times that of water.

15. Is the Earth Hollow?
Lots of fun novels written about an imaginary hollow earth. Who could resist? Asimov argues that the answer to (14) must demonstrate that the earth is not hollow, but it doesn't immediately. The earth could have a highly variable density in its interior, at least allowing for some empty spaces somewhere deep inside.

16. What is the Earth's Core Really Like?
The study of earthquake waves reveals pressure discontinuities. Contemporary geologists think the earth has three main regions, the crust, the mantle, and the core, and that the core is almost certainly molten metal. Analogy w/ meteorites suggests a composition of 90% iron/10% nickel.

17. Do the Continents Move?
The emergence of plate tectonics as the base theory of geology. "Telegraph Plateau" is discovered and named by Matthew Fontaine Maury rather earlier. "PlateTectonics" wins out in the 60s.

18. What Causes Earthquakes and Volcanoes?
Plate tectonics, by various mechanisms.

19. What is Heat?
Eventually gets to the kinetic theory of heat by way of Count Rumford's cannon boring experiments. Unfortunately, Rumford's cannon boring experiments don't mean anything to me, perhaps because I've absorbed the kinetic theory of heat so thoroughly.

20. What is Temperature?
No actual answer to this question, but the relation to specific heat is discussed.

22. How do we Measure Temperature?
Discussion of the classic thermometer, the mercury bulb one and its evolution from the Galileo's first attempt a glass tube w/ air in it, with its bottom end in a pool of water.

23. What is Energy?
The math definition: force over distance. And then a discussion of conservation of energy.

24. What is the Internal Temperature of the Earth?
Well, the temperature increases as humans dig our deep mines within the Earth's crust. If it increases at the same rate throughout, then the temperature at the core is 9000F. Jordan Ellenberg would point that this is an unwarranted assumption of linearity, and he would be right. Contemporary estimates vary quite a bit, but 7000F is one approximation: https://www.scientificamerican.com/article/why-is-the-earths-core-so/.

25. Why Doesn't the Earth Cool Off?
Well, the crust insulates it, and it generates more heat by various means, including nuclear fission in the mantle and crust. Asimov gives more weight to nuclear fission than do some more contemporary scientists, the amount of fissionable material is very hard to estimate.

Part II: The Solar System

26. Does the Sky Turn in one Piece?
Mostly, except for the planets: Mercury, Venus, Sun, Moon, Mars, Jupiter, Saturn.

27. Is the Earth the Center of the Universe?
Surely it must be.

28. Once Again, Is the Earth the Center of the Universe?
Galileo observes the phases of Venus, which are consistent with a heliocentric model but not with a geocentric model. He also observes some moons of Jupiter, which are definitely not orbiting the earth.

29. Can Copernicus' View be Improved?
Kepler derives his laws of planetary motion, and Newton invents calculus and postulate the force of gravity. This force can allow elliptical orbits such as Kepler observed and calculated. It doesn't really explain them exactly, because circular orbits would also work, but all these things are consistent with each other.

30. How was the Earth Formed?
Various ideas about the formation of the planets from the sun, or of the planets and sun together. They had their proponents and detractors, based on the physics and known forces at the time. Asimov asserts that all the planets rotate in the same direction, a fact which supported the theory that the planets and sun had formed together (the "nebular" hypothesis). The nebular hypothesis is still the winner, but it is now known that several of the planets have odd rotations. Venus rotates backward, and one of the outer gas giants is flipped almost 90 degrees. The earth is still so hot now, because of all those collisions in the early days, where much kinetic energy was turned to heat.

31. Is the Earth a Magnet?
Yes, it is. What establishes the earth's magnetic field? This is harder to explain. There is no large solid magnetic at the earth's core, since all the evident points to a molten core. All that molten iron might be sloshing about conducting electricity and thereby creating a magnetic field, which is why the magnetic field migrates and occasionally flips, rather than remaining stable.

32. Is the Earth a _Perfect_ Sphere?
This question, of course, means that you must start by ignoring any surface irregularities. The force of gravity causes it to tend toward sphericity, but at the equator the surface is moving at 1000 mph, which is kind of fast, and hence tends to want to fly out. At the poles it's moving slowly, and doesn't. So, the earth is an oblate spheroid, about 50 miles shorter pole to pole. This works out differently for other bodies. Jupiter and Saturn are spinning really fast at the surface and gravity is not so high, so they are more distorted, while the Sun is less distorted.

33. Why Does the Moon Change its Shape?
Explanation of the phases of the moon. Pretty good.

34. Does the Earth Shine?
Yes! When the moon is new, you can see the dark part of the moon rather dimly due to the reflected light from the earth. At one point this phenomenon was explained by the moon being partly transparent, or glowing with its own light, but Galileo apparently thought up the correct answer, and it's the right one.

35. Why are there Eclipse of the Sun and Moon?
The moon gets between the sun and the earth or the earth gets between the sun and the moon. All else is explained by the geometry of shadows.

36. Does the Moon Turn?
Well it has an orbit around the sun, and it absolutely turns with respect to that. It's day and our month are the same. In its orbit around the earth, it is tidally locked, so it does not turn.

37. How Far Away is the Moon?
The Greek astronomer Hipparchus used parallax and geometry to come up with a figure of about 239,000 miles which is a fine estimate. Measurements of the apparent size of the moon, plus this figure, then indicated that the moon was something like a quarter or a third the width of the earth, which must have seemed surprising.

38. What is the Mass of the Moon?
If you already know the mass of the earth, and you can identify the barycenter of this two object system, and you know the distance to the moon, the calculation is simple. The moon weighs about 1/80 the mass of the earth.

39. What are the Tides?
They are caused by gravitational pull of the moon and the sun, which distort the shape of the earth, but most especially the shape of the water on the earth, which wobbles about. When the sun and the moon align, the bulge is simple and tides are especially extreme.

40. How do the Tides Affect the Earth?
They slow its spin, a very small amount, and as a consequence the earth drifts a bit away.

41. Is there Life on the Moon?
A good look says no, because there are no indications of an atmosphere and nothing much changes.

42. What Caused the Craters on the Moon?
Meteorite impacts. At first it was thought that they might be volcanoes, because geologists knew volcanoes, and they weren't really too familiar with meteorite craters on Earth, since they are obfuscated by weathering. Against the meteorite theory was the consideration that the meteorites would have come in from all angles and would have made all different shaped craters, not just mostly round ones. Eventually, a better understanding of meteorite impacts was formed, and it was understood that the craters wasn't caused by the impact directly but by the enormous explosion resulting from the impact, which would have roughly the same force in every direction, leading to circular craters. Nowadays, everybody on agrees on meteorites.

43. How was the Moon Formed?
The moon is oddly large in relation to the earth. It weighs about 100th what the earth weighs, but that's till a much larger ratio than the moons of Mars, which are relatively minuscule, or the moons of Jupiter; these are big, but Jupiter is much bigger. The moon is significantly less dense than the Earth, and we are quite certain now that it has no molten core. How did it get there? The current winning theory is that early in the Earth's history there was a collision with a relatively large body, maybe 10% the mass of the earth. These two stuck together, but a bunch of the surface matter of both was ejected, and clumped together to form the moon. This seems to answer the physical objections to the other theories, but constitutes a remarkable chance, nonetheless.

44. Can we Reach the Moon?
Done! But long before it happened people wrote novels about travels to the moon. Cyrano de Bergerac postulated something like a rocket trip to the moon. For a very long time, though, it seemed like traveling to the moon would be like flying, and it wasn't until the discovery and characterization of the atmosphere and space that it was realized that there wasn't a lot of air for most of the distance between the moon and the earth. If all of space was filled w/ air, then the orbits of the planets would degrade with great rapidity. It is like of friction that keeps the orbits relatively stable and allows other effects to dominate.

45. What are Meteorites?
Meteors are the "shooting stars" that are sometimes seen. From the Greek, meteor, meaning roughly, "object in the air", consider meteorology. It's not a bad name, although Asimov points out that it was the Greek way of saying UFO, and it really refers to the light that we see, not the object. A meteoroid is a meteor large enough to maintain its existence as an object for some time after it reaches the atmosphere, a meteorite is a meteoroid large enough to not be burned away before it hits. For a while, nobody believed it was possible for rocks from space to hit the earth. There is a quotation of Thomas Jefferson's where he says it's easier to believe that two Yankee professors would lie than that meteorites could exist. Now of course, people actively search for meteorites, mostly in Antarctica, and study them. They are reasonably certain that they have found meteorites from Mars and that they can reconstruct their history.

46. Might Meteorites be Dangerous to Life and Property?
You bet! Reference to the Tunguska event in Siberia shortly before WWI. No meteorite or actual crater was found, the hypothesis is that the meteroid exploded before it hit the ground.

47. What are Asteroids?
A formula, kind of like Kepler's, frequently named for someone called Bode, but really devised by Titus is used to relate the sizes of the orbits of the planets. It is noticed that there is a missing spot where the asteroid belt is eventually found to lie. "asteroid" means star-like and was proposed by William Herschel since, although these bodies were known to be planets, they were all so small that they appeared like stars even through the most powerful telescopes of the time. Not a good name, really, but it has stuck. Why isn't there one big planet there. Chaos and maybe Jupiter, which is very big, accreted so much material that there really wasn't enough left over to form a planet.

48. Are there Asteroids only in the Asteroid Belt?
No, some of them are roaming around quite close to the earth. One should hit the earth about once every 100 million years. The physicist, Alvarez, advanced the theory that the dinosaurs had died out due to an asteroid impact, based on the global layer of iridium deposits that could be dated to about that time. Asimov does not mention the hypothesis that the resultant crater has been spotted in the Yucatan peninsula, but that was probably after his book.

49. What are Comets?
Comets are irregular. They appear and then disappear and can take up a good portion of the sky. Ancients generally assumed they meant disaster, although William of Normandy spun the comet of 1066 effectively. Aristotle thought they must be close to the earth, because they were changeable. Tycho Brahe determined that the comet he saw was further from the earth than the moon was (for it had no observable parallax). Edmund Halley used calculus and prior observations to successfully predict the comet that is now named for him. From then on, comets seemed less extraordinary, as at least one had a predictable orbit.

50. Why do Comets Look Fuzzy?
They are dirty snowballs, and when they get close to the sun the ice melts and the little particles of dust are freed up and blown away from the sun by the solar wind. They are also reflective.

51. What Happens to Comets?
They disintegrate a bit more every time they pass by the Sun. Halley's comet is a virtual goner. Sometimes they come apart into a dust cloud and there was a lovely meteor shower over New England in the 1830s due to the earth passing through a cometary orbit.

52. Where do Comets Come From?
The hypothetical Oort cloud. Nobody has seen it, but it still seems the most likely explanation.

53. How Far is the Sun?
Aristarchus attempted a calculation which would theoretically have worked. He couldn't make his measurements accurately, but still he got a calculation that indicated that the sun was about 25 times as far from the earth as the moon was and therefore at least 7 times as wide. This was a huge underestimate, but he was on the right track. Later, using Kepler's rules about the orbits of the planet, Cassini and Richier used parallax to determine the distance to Mars. This, combined w/ a lot of math, gave an estimate that differed from the modern estimate by only about 7%. Not bad. About 100 years later the transit of Venus measurement yielded a distance only differing about 1% from the modern value. This modern value puts the sun at about 400 times the distance from the earth as the moon, and _really_ big, about 400 times the diameter of the moon.

54. Is the Earth Large?
Not compared to Jupiter, which has about 300 times the mass. It is bigger than all the other inner planets and then any satellite in the solar system, though. ( )

Considering this book was first published twenty years ago, this is still a solid guide to many basic concepts. There are 111 simple questions, such as "How large is the Earth" and "How fast does light travel" that a clearly explained. Some answers are extremely basic, and there are a (very) few condescending comments, but overall, this book is exactly what it claims to be. I would recommend this as a primer for anyone from high-school on, though most of the writing and a majority of the answers are simple enough for middle-schoolers as well. ( )