The unified time standard will be known as “Coordinated Lunar Time (LTC),” the memo says.
A standardized time reference is needed because the moon has a weaker gravitational pull than Earth due to its smaller mass, meaning that time moves slightly faster on the moon than on Earth — on average, 58.7 microseconds per day, “with additional periodic variations,” the memo says.
The project, first reported by Reuters, will be important because “knowledge of time … is fundamental to the scientific discovery, economic development, and international collaboration that form the basis of U.S. leadership in space,” the memo said.
“The clocks run faster on the moon,” Catherine Heymans, the astronomer royal for Scotland and a professor of astrophysics at the University of Edinburgh, said in an interview. “This is one of the beauties of fundamental physics — crazy things happen.”
Heymans explained that “the way we define time on planet Earth is with an atomic clock.” Atomic clocks are affected by gravity, which means “if you took that same atomic clock up to the moon, then in 50 years it would be one second faster than the atomic clock on Earth.”
“So it’s a very small change in time” between the Earth and moon, she said, but as Einstein’s theories of relativity explain, time is “running faster on the moon than it is on Earth.” According to the theory, time moves differently depending on where you are in a gravity field, with time moving faster where gravity is weaker.
Timekeeping is an exact science for technologists — and in atomic time, a second is defined as 9,192,631,770 oscillations of a cesium atom.
Separately, Heymans notes that a day on the moon — to include a day and a night — is also different from a day on Earth. A lunar day is 29.5 Earth days, she said. “This means that on the moon, the sun is up for roughly two Earth weeks, and it’s then dark and nighttime for roughly the next two Earth weeks.”
The White House memo says one of the key reasons for the standardization of time is due to the fact that the United States plans to “return humans to the Moon and develop capabilities to enable an enduring presence.”
NASA’s Artemis moon program aims to realize the U.S. goal of returning astronauts to the moon for the first time in over 50 years. Artemis II aims to send a human crew around the moon, and its crew will include the first woman, the first African American and the first Canadian to fly on a moon mission. NASA hopes to launch Artemis III, involving a human moon landing, by September 2026.
The time standardization comes as China, India, Russia, Japan and others are also pushing for a greater presence in space — China, in particular, has said it aims to land its first astronauts on the moon before 2030. Private companies are also developing initiatives to send commercial spacecraft to the moon’s surface and orbit, for scientific research and mineral mining.
“U.S. leadership in defining a suitable standard — one that achieves the accuracy and resilience required for operating in the challenging lunar environment — will benefit all spacefaring nations,” the memo said, also noting that a “unified time standard will be foundational to these efforts.”
Last year, the European Space Agency issued its own memo outlining the “urgency of defining a common lunar reference time,”…
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