April 25 marks the 20th anniversary of the much-anticipated deployment of the Hubble Space Telescope by Discovery space shuttle astronauts in 1990.

Then, to the dismay of scientists and the public, it was quickly found that the HST had an optical defect that seriously degraded its views.

However, once corrective optics were installed in 1993, the magnificent telescope has been revealing a universe never before known in such depth and grandeur.

In 1609-10, Galileo and his new telescope revolutionized astronomy by revealing a cosmos humans had scarcely imagined, much less seen, and altered our understanding of our place in the universe.

Then in 1924, a young American astronomer, Edwin Hubble (1889-1953), again revolutionized astronomy. The then-prevailing theory was that our Milky Way galaxy constituted the entire universe.

But using the then-largest telescope in the world, the 100-inch Hooker Telescope on Mount Wilson near Los Angeles, he discovered the universe to be vastly larger than had been imagined and that our galaxy is but one of billions of galaxies.

Many argue that the HST, named for Edwin Hubble, has been no less revolutionary. It has enabled astronomers to determine the age of the universe (13.7 billion years) and confirm that supermassive black holes reside at the center of most galaxies.

It also has enabled scientists to better understand how stars and planets are formed and has detected organic molecules beyond our solar system, increasing the possibility for the existence of other organic life in the cosmos.

In its 20-year history, data from the HST have generated more than 7,500 scientific papers, making it one of the most productive scientific instruments in history.

Although not the largest telescope in the world, HST’s 94-inch (diameter) mirror is larger than McDonald Observatory’s original 82-inch telescope, which is still in use. At 43 feet long and 14 feet in diameter, our Toyota 4Runner and 23-foot travel trailer could park inside the body of the HST. It also has two rectangular solar panels, each 8.5 feet by 23 feet.

If you want to see the HST in the night sky, the Web site www.heavens-above.com provides exact viewing information on many Earth-orbiting satellites, including the HST. You’ll need to register (free) and enter your viewing location the first time you use the site, but then you won’t need to do it again.

*  Next two weeks. Average sunrise: 6:50 a.m.; average sunset: 8:03 p.m. Tonight and for the next few nights, Mars passes very near the Beehive star cluster high in the west — a sight best seen in binoculars. Wednesday night the first quarter moon is below Mars. The Lyrid meteor shower, which peaks Thursday morning, is best seen after the moon sets at 3 a.m. The evening of April 25, Venus passes near the Pleiades (Seven Sisters) star cluster low in the west at dusk. The April 28 full moon is called Egg Moon, Grass Moon and Easter Moon.

*  Naked-eye planets. Evening: Brilliant Venus is low in the west, with much fainter Mercury to its lower right, Mars is high overhead, and Saturn is high in the southeast. Morning: Before dawn, Saturn is setting in the west as Jupiter is rising in the east.

*  Star party. The Central Texas Astronomical Society’s free monthly star party begins at 8 tonight at the Lake Waco Wetlands, weather permitting. For directions, see my Web site.

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.