It is getting closer to midnight.

On Thursday, the group of scientists who orchestrate the Doomsday Clock, a symbolic instrument informing the public when the earth is facing imminent disaster, moved its minute hand from three to two and a half minutes before the final hour.

It was the closest the clock had been to midnight since 1953, the year after the United States and the Soviet Union conducted competing tests of the hydrogen bomb.

Though scientists decide on the clock’s position, it is not a scientific instrument, or even a physical one. The movement of its symbolic hands is decided upon by the Science and Security Board of the Bulletin of the Atomic Scientists. The organization introduced the clock on the cover of its June 1947 edition, placing it at seven minutes to midnight. Since then, it has moved closer to midnight and farther away, depending on the board’s conclusions.

Thursday’s announcement was made by Rachel Bronson, the executive director and publisher of the bulletin. She was assisted by the theoretical physicist Lawrence Krauss, the climate scientist and meteorologist David Titley, and the former United States ambassador Thomas Pickering.