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 it was rotating on an 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.

Today, the sun-centered view of our solar system is universally accepted, and we have a better understanding of the nature of the 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. Its 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 10,000 degrees.

The sunspots that 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 is not eternal. It was born five billion years ago and will die in another five billion years.

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

* Next two weeks: Average sunrise: 6:41 a.m.; average sunset: 6:35 p.m.

The moon is at third quarter tomorrow and new March 15.

The evening of March 16, a very thin crescent moon will be to the lower right of Venus low in the west at dusk and above the planet the next evening.

* Naked-eye planets: (The sun, moon and planets rise in the east and set in the west because of 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 the only morning planet. Mercury and Jupiter are now in the sun.

* Star party: The Central Texas Astronomical Society’s free monthly star party begins at 7 p.m. today at the Lake Waco Wetlands. For directions, see my Web site.

* 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 at 2 a.m. March 14.

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