The Universe: From Flat Earth to Black Holes and Beyond

Asimov does his usual science as a gradually-unfolding-series-of-more-reliable-answers-to-more-and-more-well-formulated-questions story, and as usual, he does it well.

Since this is astronomy not a whole lot of the early stuff is new to me. However I did learn a new phrase "equatorial horizontal

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parallax" and then I looked it up and didn't get a whole lot of good information online. Here's my try:

Inscribe an equilateral triangle within the earth. One point is at the very center of the earth and the other two are touching the surface of the (perfectly spherical) earth. There is an observer at each point. There is a point in the celestial sphere directly above each observer, and an arc that connects these two points. The thing to be observed, call it o, is observed by both observers to be situated on that arc. However, there is another thing, also observed on that arc, call it p, wrt. the angular distance between o and p differs, depending on which observer is doing the observing. When observing the moon in this way, the difference of arc is almost 1 degree. That's big! Since this is an equilateral triangle we're observing from, we know that the base of the triangle is one sixth the circumference of the earth, i.e., exactly it's radius and trigonometry automatically yields us the distance to the moon.

The ancient Greek astronomer Hipparchus calculated the distance to the moon using a variation based on a convenient solar eclipse. Knowing that at the Hellespont it was total and that in Alexandria the sun was only 1/5th obscured gave him a good measurement, and the rest followed.

Although attempted by an ancient Greek astronomer, using sound geometrical methods, the determination of the distance the sun could not be made with any accuracy until Kepler discovered his law relating the relative distances of the planets from the sun to their periods. Then, the telescope, and parallax of the nearest planets could be used to determine the AU. Cassini did it first, then the transit of Venus measurements resulted in a more accurate estimate, and then Eros, an asteroid with an orbit that brings it closer to the earth than either Venus or Mars ever gets was discovered was discovered in the 1930s, allowing even more accurate estimates. In the 1960s, bouncing microwaves off Venus made the estimates even more precise and accurate.

As one would expect for a 1960s book, Pluto is a planet, but even here Asimov doesn't give it too much importance. It is the comets, believed to have eccentric elliptic orbits by Edmond Halley, that while still captive to the sun's gravitational force, travel the farthest distance from it. It's calculated orbit far exceeded that of all the known planets at the time it was calculated.

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