* Strictly speaking, the Earth’s core consists of a small solid inner core and a larger molten outer core.

† The ancients believed that 36,000 years was the period of precession of the Earth’s axis, which they had observed from the movement of the positions of the equinoxes along the ecliptic relative to the fixed stars. The actual number is approximately 25,800 years. Periods of geological time, by contrast, are measured in millions of years.