Lost pegs are the pits for pendulum

Frequent Griffith Observatory visitor Pamela Garner thought she had the place pegged.

Her first stop was always the Los Angeles landmark’s soaring central rotunda. There, she paused to watch the giant Foucault pendulum swing gracefully beneath the famous Hugo Ballin murals depicting classical figures and the advancement of science.

The 240-pound brass ball that moves back and forth on a 40-foot steel cable suspended from Ballin’s painted dome is a powerful illustration of Earth’s rotation and its relationship with the heavens above.

At Los Angeles’ latitude, it takes 42 hours for the pendulum to seemingly complete a circle as the earth beneath it rotates. For decades, that movement has been visually represented by small wooden dowels that are knocked over one at a time by a pointer on the bottom of the shiny gold ball.

“You’ve got to see the pegs being knocked over,” Garner, of Santa Clarita, told Anne Fleming of Valencia as the pair peered over the waist-high polished granite wall that encircles the pendulum pit. “Wait! Where are my pegs?”

They were missing. It turns out the 20 dowels disappeared after they were removed from the pit at the start of the $93-million observatory makeover.

“People ask about them all the time,” observatory guide Jesus Gaspar said. “Everybody wants to know what’s going on with the pegs.”

Without the dowels, it’s almost impossible to visualize Earth’s rotation beneath the pendulum.

French physicist Jean Foucault exhibited the first such device in 1851 in Paris, where the pendulum ball’s pointer traced its route in sand spread on the ground beneath it.

In the Griffith Observatory’s setup, a three-inch dowel was knocked over every eight minutes or so. They were placed on a base that used a spring-like tension wire to return them to an upright position when the base was moved by a long stick pushed by observatory guides.

The semi-automated peg system was used for about 10 years, according to Tony Cook, the observatory’s astronomical observer -- himself a former guide. It replaced a ladder system used for more than half a century by workers who climbed in and out of the 7-foot-deep pit several times a day to return knocked-over dowels to an upright position and move them ahead of the pendulum.

Workers also manually started the pendulum swinging each morning when they switched on a circular electromagnet used to keep the pendulum’s swing from diminishing due to air friction and gravity. Since the renovation, the magnet has been left activated around the clock. It can be heard clicking each time the ball starts to swing.

Pit duty required a steady hand. It also required nerves of steel, thanks to the swinging brass ball.

“On my very first day here as a guide I was so nervous that I’d knock them down,” Cook said.

The pegs were packed away about five years ago when workers began constructing a 40,000-square-foot underground education center and upgrading main-floor displays in alcoves on both sides of the pendulum.

But nobody could find them when the observatory reopened in November.

“They were put in the city’s central storage, and they lost them,” Gaspar said.

Observatory director Edwin Krupp said a new peg system is being created. It will be radio-controlled, using the same type of technology found in remote control toy cars. An arrow will also be used to show where over the pit floor the pendulum was previously swinging.

“The things you did in 1935 aren’t necessarily what you do today,” Krupp said.

But Krupp denied rumors that health or safety inspectors were responsible for initially banning observatory workers from the pendulum pit. Instead, observatory officials themselves decided to change the procedure after repeated pit entry began damaging the enclosure’s decorative stone and glasswork, he said.

The worst damage occurred in 1985, when a drunken visitor who was turned away from a planetarium laser show programmed to the heavy metal rock sounds of Led Zeppelin grabbed the pendulum wire and held on to it until the ball broke loose and crashed onto the pit floor. A granite slab was chipped and one of the glass light panels was broken.

The visitor was charged with vandalism and was lucky to have escaped unhurt.

“The wire could have cut him in two,” Krupp said.

Observatory guides say visitors sometimes try to touch the wire. That causes it to vibrate, knocking the pendulum off its course. Workers then have to enter the pit, bring the ball to a complete halt and carefully restart its swing.

With the observatory’s new exhibits in place and its longtime favorites such as the huge Tesla coil, camera obscura and solar telescope renovated, the return of the wooden pendulum pegs will be the final touch.

For its fans, that means the place will have come full circle.

bob.pool@latimes.com