Four hundred years ago this month, in December 1609, Galileo began his telescopic study of the moon and almost immediately found evidence that challenged the Aristotelian view that was fundamental to much of the scholarly and ecclesiastical thinking of the day.

The ancient Greek philosopher Aristotle set forth a world view that still influenced Galileo’s 17th-century world nearly 2,000 years later. According to Aristotle, earthly things, composed of earth, water, air, and fire, were imperfect, changing and subject to death and decay, whereas heavenly bodies, composed of aether (quintessence), were of a completely different nature. They were perfect, eternal and unchanging; thus, objects beyond Earth were perfectly round with perfectly smooth surfaces.

Viewed with the naked eye, the moon appears perfectly round, and even with its obvious darker and lighter areas, it could well have been perfectly smooth, like a multishaded marble.

But that’s not what Galileo’s telescope revealed. He saw what appeared to be mountains and other seemingly uneven terrain. After observing the same areas several nights and giving attention to the changing shadows they cast, it became clear to him that the moon was not perfectly smooth. Indeed, in many ways, its surface resembled that of Earth’s — the moon appeared more Earthlike than heavenly.

Being an astute mathematician, Galileo even used shadows and basic geometry to estimate the highest moon mountains to be some four miles high.

He also noted that the moon’s surface was covered with countless roundish areas of widely varying sizes. By studying their shadows, he determined that they were depressions in the moon, although he never knew what caused them. We now know them to be craters formed from meteor impacts.

When the moon is in its thin crescent phase, one can easily see its faintly lit night side, even with the naked eye. In Galileo’s day, many assumed the glow came from the moon itself, but from his observations, Galileo correctly deduced that earthshine — sunlight reflecting off Earth — produced the faint glow, showing that imperfect Earth could have an effect on a heavenly body. 

Galileo’s lunar discoveries put the first cracks in Aristotle’s world view, and there were more to come, which we will look at in future columns.

* Next two weeks: Avg. sunrise: 7:28 a.m.; avg. sunset: 5:37 p.m. Thursday’s full moon, the second of the month, is popularly referred to as a blue moon, although it has nothing to do with its color. Jan. 2, the Earth is at perihelion, its nearest point to the sun in its annual orbit. The moon is at last quarter Jan. 7.

* Naked-eye planets: (The sun, moon and planets rise in the east and set in the west due to Earth’s west-to-east rotation on its axis.) Evening: Jupiter is the brightest object in the Southwest; Mercury is just above the West-Southwestern horizon at dusk; Mars now rises late in the evening. Morning: Saturn is high in the South, with Mars high in the West-Southwest.

* Astro milestones: Dec. 25 is the 367th birthday of Isaac Newton (1642-1727), father of modern physics. Dec. 27 is the 438th birthday of Johannes Kepler (1571-1630), discoverer of elliptical orbits.

Stargazer appears every other Saturday. Paul Derrick is an amateur astronomer who lives in Waco. Contact him at 918 N. 30th, Waco 76707, (254) 753-6920 or paulderrickwaco@aol.com. See the Stargazer Web site at stargazerpaul.com.