Endnotes
Note 1: Examples of potential “dark matter” that isn’t matter
- Quote: It’s not even clear that “dark matter,” a term used widely by astronomers and particle physicists alike, is actually matter.
- Endnote: Among possible dark matter particles are axions and dark photons, neither of which would obviously qualify as “matter.”
- Discussion
Note 6: True vs. apparent weightlessness
- Quote: On the Moon, where the Earth’s pull would be tiny, the Moon would pull you toward its center with gravity that’s about one-sixth of what you’re accustomed to. And if you traveled out into deep space, far from any large object, you’d weigh virtually nothing. Yet all the while, your body’s mass—the difficulty I would face if I tried to speed you up or slow you down—would never change.
- Endnote: Confusingly, astronauts orbiting Earth inside nearby space stations appear to float as though weightless. From Newton’s perspective, they are not truly weightless; if they were, they’d coast, leaving the Earth’s vicinity and moving rapidly into deep space. Instead, they and their spaceship are pulled by gravity into a common orbit around the Earth. Since they travel on the same path as their container and as the camera which films them, they seem and feel weightless. (This subtle issue is turned on its head in Einstein’s view of gravity.)
- Discussion
Note 9: Measuring the size of the Moon.
- Quote: The Moon’s distance from Earth (about 240,000 miles, or 400,000 kilometers) had been measured two thousand years earlier, using tricks of perspective.
- Endnote: The simplest trick: if the Moon appears to pass in front of a planet such as Jupiter, this eclipse can be seen from only a part of the Earth’s surface. The north-south width of that region is approximately the diameter of the Moon. Combining this information with the Moon’s apparent diameter on the sky, one can determine its distance from Earth.
- Discussion: In my series on Do-It-Yourself Astronomy, I wrote about how you yourself can estimate the Moon’s size using occultations of planets and also solar eclipses. From there you can estimate its distance.
Note 10: Moon’s orbital speed
- Quote: Newton knew right away that if the force of gravity were as powerful out by the Moon as it is at Earth’s surface—if the Moon accelerated toward the Earth at the same rate that your dropped keys do—then motion and gravity would be wildly out of balance [and so the Moon would have fallen and crashed into the Earth.]
- Endnote: To avoid disaster, the Moon’s orbital speed would need to be 40 miles per second, leading it to circle Earth twice a day!
- Discussion
Note 12: Why tides are twice daily
- Quote: Furthermore, because gravity dwindles at greater distances, the Moon’s pull is stronger on the near side of the Earth and weaker on the far side than it is on the Earth’s center. This uneven pull stretches our planet’s oceans slightly, resulting in a small bulge of water, not much taller than a human, both on the Earth’s side facing the Moon and on the opposite side, too.
- Endnote: To explain why gravity leads to a water bulge on both sides of the Earth is too complex for a footnote, and I’d rather not repeat the most commonly heard explanations, which are phibs. One can see a hint of the cause as follows: if one drops a water balloon in constant gravity, it will fall as a sphere, whereas if it is pulled more strongly at the bottom than at the top, it will stretch into an oval as it falls.
- Discussion
