Cumberland Times-News

Bob Doyle - Astronomy

April 14, 2012

Special numbers key to running universe

— In science, there are a number of special constants that play key roles in making our universe the way that it is.

One of the most interesting sagas is the centuries old effort to determine the speed of light. In the early 17th century, Galileo became the first scientist to utilize the scientific method. Galileo insisted that all hypotheses or scientific explanations be tested, either by experiment or observations.

Galileo’s scheme to measure the speed of light was to have two observers, each with a covered lantern climb to the top of a different hill.

The first observer was to uncover his lantern and then wait for the second lantern to appear in the distance. The time interval between the first lantern being uncovered and the sighting of the second lantern would be the time for light to travel to the other hill and return to the first observer.

Unfortunately, the human reaction time is far larger than the time for light to travel this round trip between hills so light’s speed could not be determined.

In the middle of the 19th century, French physicists (Fizeau and Foucalt) used rapidly spinning mirrors, driven by steam engines to obtain the speed of light to within 5 per cent of the known value.

It fell to an instructor at the U.S. Naval Academy, Albert Michelson, to get the first accurate value for light speed.

Michelson was born in a small Polish town of Strezlno. His family immigrated to the United States, settling in Virginia City, Nev. In applying for the U.S. Naval Academy, Michelson was interviewed by U.S. President Ulysses S. Grant!

There were 10 at large positions and Michelson didn’t make the cut. Three days after the interview, as Michelson was about to board a train to start his long trek back to Nevada, a courier called out Michelson’s name.

President Grant was so impressed with Michelson that he created an 11th at large position so Michelson could enroll at the Naval Academy.

In 1907, Michelson became the first American scientist to win a Nobel Prize. Michelson’s value for light speed is 186,355 miles per second.

Another key constant is the Gravitational Constant G, used to compute the gravitational pull between any two masses. (Your weight is the gravitational pull between your body and the Earth’s mass, considered to lie at the Earth’s center.)

Isaac Newton in the late 17th century had shown that his law of gravitation could explain both the fall of an apple and the orbiting of our moon around the Earth.

But in Newton’s time, only the ratio of the pulls on each body were known. It wasn’t until the middle of the 18th century that the gravitational constant G could be determined.

This was done in an experiment by Henry Cavendish. French born, Cavendish was from a well-to-do family. After three years at Cambridge, Cavendish left the University without his degree.

He was painfully shy, possibly having Asperger’s syndrome, making it difficult for Cavendish to interact with others. Cavendish discovered that air was about 80 per cent nitrogen and about 20 per cent flammable gas (oxygen). Cavendish also favored the formula for water (H2O), instead of the HO of John Dalton.

Cavendish published little; his recognition came from the publication of his notes after his death by James Clerk Maxwell.

The Cavendish Experiment was designed to weigh the Earth by using two large spheres and two smaller spheres close by. The spheres were mounted as dumbbells and the smaller spheres turned a slight amount due to the gravitational attraction for the larger spheres.

Since the masses of the spheres were known, it allowed a measurement of the relative attraction of the large spheres compared to the attraction of Earth itself. The mass of the Earth was determined then; it was only after Cavendish’s death that the G value was determined. G’s value is 0.000000000067 Newton*meter squared/(kilograms squared).

The small value of G explains why there is little gravitational pull between people or between people and large buildings. But due to the Earth’s mass (about 6 followed by 24 zeroes in kilograms), our weight (gravity’s pull) is considerable, especially after a long day of standing on your feet.

This week’s evening sights: Tonight the planet Saturn is closest and brightest, rising in the southeast as the sun sets and hanging in the sky all through the night.

Saturn’s distance is now 811 million miles, so far that it takes 72.5 minutes for light reflected off Saturn’s clouds or rings to reach the Earth. Saturn is in Virgo, to the left of Virgo’s brightest star Spica. Saturn is more than twice as bright as Spica, shining with a steady light.

To find Saturn, find the Big Dipper upside down and high in the north. Follow the curve of the Big Dipper’s handle outward. You will first encounter the bright orange star Arcturus. Further along this same arc is the Saturn Spica pair.

Predator program at Frostburg State: Our April program on African Predators will be held this afternoon at 4 p.m. in Compton 224. Featured will be Jackals, Hyenas, Caracals, Leopards and Lions.

After our brief half hour interactive presentation, you are invited to meet these creatures (preserved) face to face in the Science Discovery Center. Cameras are welcome. This program will be repeated the next two Sunday afternoons.

Bob Doyle invites any readers comments and questions. E-mail him at rdoyle@frostburg.edu . He is available as a speaker on his column topics.

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Bob Doyle - Astronomy
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