Everyone knows the Sun is a brilliant round ball that travels around Earth each day. Perfect and unchanging, it is made of shiny quintessence, a heavenly substance not found on Earth.

At least that’s what sophisticated Europeans thought in the early 17th century when Galileo and others began studying the heavens with the newly invented telescope 400 years ago.

That view of the Sun began to crumble when Galileo and other early astronomers discovered sunspots. They seemed to be clear evidence that the Sun wasn’t perfect after all.

Further, the sunspot blemishes came and went and changed sizes, demonstrating that the Sun isn’t unchanging. And seeing the sunspots move across the Sun’s surface indicated that the Sun was rotating on its axis.

However, neither Galileo nor his contemporaries had any idea what sunspots were, or what the Sun was made of.

Galileo’s observations, especially of Jupiter and Venus, also led him to accept Copernicus’ theory that the Sun doesn’t go around the Earth, but rather Earth and the other planets go around the Sun. And as mentioned in previous columns, his promotion of these heretical ideas got him in serious trouble with the ecclesiasticalgovernmental authorities.

Today, of course, the Sun-centered view of our solar system is universally accepted, and we have a much better understanding of the nature of our Sun.

An ordinary star, the Sun is a huge gaseous ball composed not of any exotic heavenly substance but primarily of hydrogen, the most common element in the known universe. It’s heat, light, and other forms of energy come from nuclear reactions deep within its core.

At its center, the temperature is 27 million degrees whereas the temperature at the visible surface (called the photosphere) is a mere 10,000 degrees.

The sunspots which so intrigued and baffled Galileo are now known to be areas of magnetic disturbance; they are darker in appearance because they are cooler.

And finally, our Sun, like all stars, is not eternal. It was born 5 billion years ago and will die in another 5 billion years.

[Much of this information is from Stephen P. Maran and Laurence A. Marschall’s book, Galileo’s New Universe, reviewed in this column; my Web site contains an archive of previous columns.]

Sky Calendar

• Monday, March 15: The Moon is new.

• Tuesday, March 16: evening: A very thin crescent Moon is to the lower right of Venus low in the west at dusk, and above the planet the next evening.

• Saturday, March 20: Spring (vernal) equinox, the northern hemisphere’s first day of spring when day and night are (about) 12 hours long.

• Saturday, March 20: evening: The crescent Moon grazes the Pleiades star cluster, a sight best seen in binoculars; the star Aldebaran, the “red eye of the bull,” is to their upper right.

• Sunday, March 21: Saturn is at opposition (on the opposite side of Earth from the Sun) when it rises at sunset, is up all night, and sets at sunrise.

• Tuesday, March 23: The Moon is at 1st quarter.

• Wednesday, March 24: evening: The gibbous Moon is to the upper right of Mars, and below the planet the next night.

• 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: Mars is high in the east as Saturn rises an hour after sunset; Venus is visible very low in the west after sunset. Morning: Saturn, low in the west, is currently the only morning planet. Mercury and Jupiter are now in the Sun.

• Astro Milestones. March 13 is the birthday of William Herschel (1738-1822) who discovered the planet Uranus in 1781 from Bath, England. March 14 is the birthday of Albert Einstein (1879-1955), who set forth the theories of relativity in the early 1900s.

• Time Change. Set clocks forward (“spring forward”) to Daylight Saving Time Sunday, March 14, at 2 a.m.

(Stargazer appears every other week. 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.)