AidSpace Blog

Adjusting Our Atomic Clock for the Leap Second

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Our exhibition Time and Navigation features an atomic clock that will keep an accurate time within a tiny fraction of a second for the foreseeable future (see my earlier post to learn how atomic clocks work and how we installed ours into the exhibition). Except, of course, when we need to account for a leap second.

 What’s a Leap Second?
In the past, time was measured using the rotation of the Earth. With atomic clocks, we learned that the length of a day changes by a second here and there. To take this into account, in 1972 the International Telecommunications Union adopted “leap seconds.” A leap second is added whenever the Earth’s rotation gets out of sync compared to the international time reference measured with atomic clocks. There is a discussion about leap seconds and if they will continue to be added, but that is another story. For our purposes, we just needed to keep our atomic clock in sync with the world’s time when the last leap second was added in June 2015.

Adding the Leap Second
Because our atomic clock is not connected to outside data sources, we had to add the leap second manually. This required opening the front of the exhibit case and typing a series of commands on the keypad a few days before the leap second would occur. We told both the frequency standard and the time code generator to add the leap second on the last day of June. We also successfully tested the cable connection in the back of the atomic clock. From now on, we’ll use that to update the clock for future leap seconds.

Leap seconds are always added at midnight Coordinated Universal Time (UTC). For us, that was 8:00 pm in Washington, DC. With our commands entered earlier, I waited around in the Time and Navigation gallery to see what would happen as the clock struck 8:00. The leap second went off without a hitch.

It was interesting to watch the displays.

UTC on the cesium clock displayed these seconds:

…58…59…60…00…01…

Above the clock, local time from the time code generator displayed:

…58…59…00…00…01…

This video shows the time displays 5 seconds before and after 8:00 pm. The elapsed time was 11 seconds instead of 10 because of the leap second.

Please come visit the Time and Navigation exhibition to see the working atomic clock. You can set your watch to the correct local time displayed in the upper part of the case.

Andrew Johnston was a research associate in our Center for Earth and Planetary Studies. He is now the Vice President of Astronomy & Collections at the Adler Planetarium in Chicago.

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A “Box of Time” in the Time and Navigation Exhibition

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In our exhibition Time and Navigation visitors can set their watches by a working cesium frequency standard, commonly known as an “atomic clock,” on loan from the National Museum of American History. The exhibit allows visitors to see different methods of measuring time, including mechanical and electrical clocks. A digital display on the atomic clock shows the global reference known as the Coordinated Universal Time or UTC. A separate display connected to the clock shows local time, which visitors can use to set their watches. While the device is not connected to outside time sources, it will keep accurate time within a tiny fraction of a second over the foreseeable future. We jokingly called it our “Box of Time.”

What is an Atomic Clock?
Atomic clocks maintain very stable time references at specialized laboratories such as the U.S. Naval Observatory and the National Institute of Standards and Technology. The time is distributed all over the world by satellites, radio signals, fiber optic connections, and computer networks. These time standards are essential for synchronizing data connections, communications, transportation, and countless other aspects of modern society.

Atomic “clocks” can be more precisely called frequency standards. They maintain stable frequencies by measuring changes in the energy state of heavy elements such as cesium. These devices know exactly the length of each second with a precision of a billionth of a second. The unit sends out pulses exactly one second apart. By itself, the frequency standard doesn’t actually know the time of day. Keeping track of that requires a second piece of equipment: A time code generator. This device takes the pulses from the frequency standard to keep track of hours, minutes, and seconds. (Jump to minute 17:30 of this STEM in 30 episode to see the U.S. Naval Observatory and learn more about how an atomic clock works.)

Setting Up the Clock in the Museum
Our atomic clock was the last thing to be installed in our exhibition in 2014. Before it could be installed, we needed the frequency standard to be calibrated because each atomic clock can run at a slightly different rate. To determine how ours was working, we wanted to compare its operation to the national reference time. Fortunately for us, this originates right in Washington, DC at the U.S. Naval Observatory. The staff there agreed to let us bring our clock in for calibration. It was a tricky procedure. We had to calibrate both the frequency standard and the time code generator and then bring them back to the Museum. During all this, electrical power to the clock had to be maintained. We brought along a battery power unit used for computers. Along with the internal battery backup in the frequency standard, we hoped this would give us about 90 minutes of power. To be safe, we planned to plug in the whole system to the vehicle’s power supply.

With a plan in place, we picked up the clock from the National Museum of American History, along with its curator Roger Sherman. The unit had a helpful note on top that said, “Roger’s Atomic Clock.” The battery backup worked flawlessly as we made our way up Massachusetts Avenue to the Naval Observatory.

Once at the Observatory’s time service building, we plugged in the necessary cables to compare our clock with the U.S. master clock. The initial comparison showed that our clock was running about 24 nanoseconds (billionths of a second) slow. After a couple hours, this offset had changed to less than a nanosecond. This told us the frequency standard was running well. Over the next 10 years it will drift out of sync with the national time reference by only less than 1/10,000th of a second. That sounded good enough for museum visitors to set their watches. While there we also set the time of day on the time code generator.

We packed up the frequency standard, the time code generator, the battery backup, and began the drive back to the Museum. I was behind the wheel with the power supply plugged into the dashboard. Roger was in the back seat with the equipment. At one point, driving down Independence Avenue, something began to emit ear-splitting cries. Roger and I tried to determine which piece of equipment was complaining. It turned out to be the overloaded power supply. I pulled the plug out of the dashboard port, which was so hot it almost burned my fingers. Then the UPS on the floor started beeping loudly because it wasn’t getting power. Everything was confusion in the vehicle as we shouted above the noisy equipment while checking all the units and cables. But after that brief moment of excitement, we had enough juice in the battery backup to make it the rest of the way to the Museum. After some careful coordination with all the cables, we got it mounted in its display case where it continues to display the time.

installing an atomic clock

Andrew Johnston (left) and Roger Sherman (right) deliver the atomic clock to the Time and Navigation exhibition, which is under construction in the background. The atomic clock, sitting on the cart along with the time code generator and the battery power supply, is reading 16:35:13 Universal time (2:35 pm local time). Image Courtesy: Harold Dorwin

This wouldn’t be the last time we needed to adjust our atomic clock. In June 2015 we had to account for a leap second. In my next post I’ll explain what a leap second is and how we updated our atomic clock.

 A special thanks to everyone at the U.S. Naval Observatory, the people at Symetricomm (now Microsemi) who manufactured the clock, and Roger Sherman at the National Museum of American History.

Andrew Johnston was a research associate in our Center for Earth and Planetary Studies. He is now the Vice President of Astronomy & Collections at the Adler Planetarium in Chicago.

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Inside the Sally K. Ride Papers – Now Open for Research

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color portrait of Sally Ride with signature

A signed photograph of Sally Ride. Image: National Air and Space Museum, NASM 9A12598

Last October, we announced that we had acquired the collection of Sally K. Ride, the first American woman in space. Now, we can share that the archival portion of the collection has been processed and is available for research! See our finding aid for more detailed information.

The Sally K. Ride collection consists of more than 23 cubic feet of papers, photographs, certificates, and film created or collected by Ride chronicling her career from the 1970s through the 2010s. The papers document Ride’s lifetime of professional achievements and include material relating to her astronaut training and duties; her contributions to space policy; her work as a physicist; and her work as an educator.

A significant portion of the collection highlights her iconic role as a NASA astronaut from 1978 to 1987. Ride spent 343 hours in space, as a mission specialist on space shuttle missions STS-7 and STS-41G, where she operated a variety of orbiter systems and experiment payloads. She also operated the Remote Manipulator System (RMS) arm to maneuver, release, and retrieve a free-flying satellite.

color portrait of Sally Ride with signature

Here is the cover of Ride’s STS-7 ascent checklist. Image: National Air and Space Museum, NASM 2016-00309

color portrait of Sally Ride with signature

An inside view of another of Ride’s manuals, the PDRS Operations Checklist for space shuttle mission STS-7, including a drawing which highlights the RMS arm and features annotations by Ride. Image: National Air and Space Museum, NASM 9A12468

But Ride’s NASA’s career and legacy extend well beyond her missions in space. Ride was training for her third flight when the Space Shuttle Challenger disaster occurred and she was named to the Rogers Commission, the presidential commission investigating the accident. Ride later served on the Columbia Accident Board as well. She was the only person assigned to both shuttle disaster committees that investigated the causes and recommended remedies after the tragic losses.

color portrait of Sally Ride with signature

Two pages from one of the notebooks Ride kept containing her notes from the Challenger accident meetings. Image: National Air and Space Museum NASM-9A12458

In 1987, Ride left NASA to become a full-time educator. The collection mirrors those professional changes with material relating to her work as a physics professor at University of California at San Diego (UCSD) and later endeavors to improve science education for elementary and middle school students, with a special focus on science education for girls.

The Museum is proud to play a role in securing Ride’s legacy by making this collection available to researchers for years to come. And, on a personal note, it was a wonderful honor to process the papers. I leave you with my favorite image from the collection. It shows a very young Sally Ride looking at a book. A “thought bubble” caption has been adhered to the photo as though Ride is reading a technical manual. I found this image attached on the inside cover of one of her STS-7 manuals.

color portrait of Sally Ride with signature

Image: National Air and Space Museum, NASM-9A1246

Patti Williams is the acquisition archivist for the National Air and Space Museum

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Crossing the Atlantic in a Wicker Chair

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If you were going to fly non-stop for 33½ hours, what kind of chair would you want to sit in?

For Charles Lindbergh, it was this simple wicker chair. The Ryan NYP Spirit of St. Louis was a modified version of the Ryan M-2 aircraft created specifically for the long flight from New York to Paris. In an effort to save weight, Lindbergh opted for this wicker seat for the historic flight. Discover more about the Ryan NYP Sprit of St. Louis.

Inside the Spirit of St. Louis

Tom Paone is a Museum Technician in the Aeronautics Department at the National Air and Space Museum.

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A New Home for an Old Glove

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Who would think that a damaged, old leather glove, with the thumb badly torn, could be a valuable item? But if that damaged glove belonged to Luftwaffe pilot Günther Rall, with 275 aerial victories and the third highest scoring ace in aviation history, then it becomes an item of unique historic value. And now that item has found a home at the National Air and Space Museum. In addition to the glove, the Museum also received Rall’s diary from 1942, documenting his actions at the Eastern Front, and a portrait of the pilot in summer 1945, created by another prisoner of war, Wolfgang Willrich, during their time in captivity in Fouquainville, France.

Painted portrait of Gunther Rall.

This portrait of Günther Rall as prisoner of war was painted by fellow prisoner, Wolfgang Willrich, in early summer 1945 in France.

Günther Rall was born in 1918 at the end of World War I and became a pilot with the Luftwaffe in 1938. During World War II, he fought in the skies over France, Great Britain, Yugoslavia, Greece, Russia, and later in the air defense over Germany against the American and British strategic bombardment campaign—always flying the Messerschmitt Bf 109. In November 1941, after 37 air victories, Rall was shot down for the first time and rescued by a German tank crew, his back broken in three places. Told that he would never be able to walk (let alone fly) again, Rall returned to combat just one year later.

In April 1944, Major Günther Rall was made Group Commander of the 2nd group of Fighter Wing 11, defending the skies over Germany against the overwhelming powers of the Allied Air Forces. At that time, the Allies had seven to 10 times more aircraft in the air over Germany than Germany did. Even worse, U.S. pilots had about 400 flight hours of training when they were sent into battle, while German pilots, due to lack of instructors and fuel, had almost none. Many of these young, inexperienced German pilots were shot down before their 10th sortie.

Messerschmitt Me 109 airplane

The Museum’s Messerschmitt Bf 109. Image: Dane Penland, National Air and Space Museum

On May 12, 1944, Rall led his group against an American air raid. His pilots flew two different aircraft. Some flew Me 109s with engines equipped with special chargers to allow them to reach altitudes of 8,000 to 10,000 meters where they were able to attack the P-51 Mustang and P-47 Thunderbolts that protected Allied bomber units. Other pilots flew Fw 190s and attacked the lower-flying U.S. bomber aircraft. Rall shot down two Thunderbolts, but then other P-47s arrived. One of them fired at Rall’s Me 109. Bullets from a .50 caliber machine gun hit his cockpit, his engine, his cooler, and his left hand at the control stick, shooting his thumb. The glove donated to the Museum is the very glove worn by Rall during that engagement, and it clearly shows the damage from the machine gun round. Günther Rall bailed out and landed in a field. He was taken to a hospital and his left thumb amputated. Due to the onset of infections he was not able to fly for months.

Günther Rall’s glove with the torn thumb

Günther Rall’s glove with the torn thumb, result of the air attack on May 12, 1944.

The air battles of that day marked the beginning of a systematic U.S. offensive against the German fuel industry, one of the weakest links in the German war economy. The 8th and 9th USAAF with 886 bombers, and 980 accompanying fighters, flew attacks against refineries and production sites for synthetic fuel in the heart of Germany. Facing heavy German resistance, the U.S. lost 46 bombers and 12 fighters. On the German side, 28 pilots were killed and 26 wounded that day, among them was the entirety of Rall’s group. Later, Albert Speer, Reich Minister of Armament and War Production, would declare: “On that day, the fate of Germany’s technical warfare was decided.”

In November 1944, Rall returned to active duty. He spent the last months of the war with Fighter Wing 300, which mostly sat idly due to lack of fuel and supplies. At the end of the war, after 621 missions flown, 275 confirmed aerial victories, shot down eight times, and wounded three, Rall became a prisoner of war of the American Forces. Released in August 1945, he had to adjust to a civilian life and became a representative for the Siemens Company. In 1956, he joined the newly established Armed Forces of the Federal Republic in the rank of a Major of the Luftwaffe. He was put in charge of modifying the F-104 fighter jet for Luftwaffe’s requirements and worked his way to the position of Luftwaffe’s Inspector General, a rank he held from 1971 to 1974. That year, he was made the German military representative in NATO’s Military Committee at Brussels, with the rank of a Lieutenant General.

Photograph portrait of Günther Rall

Rall as Luftwaffe’s Inspector General in the early 1970s. Image: German Federal Archive, Bundesarchiv, Bild 183-J1112-0206-004 / CC-BY-SA 3.0

In 1977, Günther Rall visited a meeting of U.S. fighter pilots. While inquiring about the 1944 incident where he lost his thumb, he learned that he had encountered the notorious “Wolf Pack” on that fateful day in 1944, the 56th Fighter Group under Col. Hubert Hub Zemke. Zemke’s pilots were by far the most successful American fighter group in the European theatre, and Zemke himself was known as a supreme tactician. From that meeting, a close friendship developed between Rall, Hub Zemke, Zemke’s 2nd Lieutenant Robert “Shortie” Rankin, and other U.S. pilots. During his visits to the U.S., Rall frequently gave talks about his life as a pilot, often together with U.S. pilots like Hub Zemke or Chuck Yeager. In May 1996, he joined the Gathering of Eagles at the Museum and talked about his war time experiences. In 2003, he was made an honorary member of the prestigious Society of Experimental Test Pilots, and one year later published his memoirs, Mein Flugbuch [edited by Kurt Braatz, Moosburg/Germany: Edition NeunundzwanzigSechs]. In them, the third-highest scoring ace of all time said:

“Nothing is further from my mind than to join into the praise for the last Knights of the Air which you hear so often when people talk about World War II fighter pilots. The sober truth […] is that we fought each other for life and death, although we wanted nothing but to live, and that these fights became the more relentless the longer this terrible war lasted. […] War is not the continuation of politics with other means, but an infamy; it is the utter failure of political action.”

Photograph portrait of Günther Rall

Günther Rall (center) at the Museum during a National Air and Space Society lecture in May 1996.

Günther Rall died in 2009. The Museum plans to incorporate his glove, his diary, and his portrait in a new exhibition on World War II.

Evelyn Crellin is the curator for European Aviation in the Museum’s Aeronautics Department

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