Co-editor Hyman Goldsmith asked artist Martyl Langsdorf to come up with a design for the cover of the June 1947 edition of the Bulletin, the first issue published as a magazine rather than a newsletter. Martyl—as she was known professionally—was married to a physicist, Alexander Langsdorf, who worked on the Manhattan Project while at the University of Chicago.

At first the artist considered using the symbol for uranium. But as she listened to the scientists who had worked on the Bomb, as they passionately debated the consequences of the new technology and their responsibility to inform the public, she felt their sense of urgency. So she sketched a clock to suggest that we didn’t have much time left to get atomic weapons under control.

Graphic designer Michael Bierut reimagined the iconic image in 2007.

In the early days, Bulletin Editor Eugene Rabinowitch decided whether the hand should be moved. A scientist himself, fluent in Russian, and a leader in the international disarmament movement, he was in constant conversation with scientists and experts within and outside governments in many parts of the world. Based on these discussions, he decided where the clock hand should be set and explained his thinking in the Bulletin’s pages.

When Rabinowitch died in 1973, the Bulletin’s Board handled the responsibility. In 2008, the Science and Security Board was formally separated from the Governing Board and entrusted with the responsibility. Since that time the Science and Security Board has met twice a year to discuss world events and reset the clock as necessary. The board is made up of scientists and other experts with deep knowledge of nuclear technology and climate science, who often provide expert advice to governments and international agencies. They consult widely with their colleagues across a range of disciplines and also seek the views of the Bulletin’s Board of Sponsors, which includes nine Nobel laureates.

In 1991, with the end of the Cold War, the United States and the Soviet Union signed the Strategic Arms Reduction Treaty, the first treaty to provide for deep cuts to the two countries’ strategic nuclear weapons arsenals, prompting the Bulletin to set the clock hand to 17 minutes to midnight.