In recent years, as lunar exploration has attracted attention from countries around the world, how to define lunar standard time has become a new issue of concern to the scientific community. According to Hong Kong's South China Morning Post report on January 12, the Purple Mountain Observatory of the Chinese Academy of Sciences officially released the world's first "lunar timing software" last month, achieving precise conversion of moon and earth time.
According to the general theory of relativity, since the moon's gravity is only about one-sixth that of the Earth, time runs faster on the moon than on Earth, about 56 microseconds per day. These tiny errors can add up to have a serious impact on space missions that require precise timing.
To solve the problem of time conversion between the moon and the earth, researchers at the Purple Mountain Observatory built a model that takes into account the moon's weaker gravity and its movement in space, so that events on the moon can be accurately synchronized with clocks on the earth. Last month, the Purple Mountain Observatory officially released the lunar time and calendar product LTE440.
Photos of the moon taken by American astronaut Matthew Dominic on the International Space Station
According to the Chinese Academy of Sciences, one of the key links in defining and constructing lunar standard time is to clarify the correspondence between the lunar coordinate time and the solar system's center of mass mechanical time. As a result, lunar standard time can be converted to Earth time and Coordinated Universal Time, meeting the International Bureau of Weights and Measures' definition of lunar standard time that must be traceable to Earth time.
However, the conversion between lunar coordinate time and center-of-mass mechanical time is determined by the extremely complex multi-body motion of the moon and the dynamic gravitational field exerted on the moon by all celestial bodies in the solar system. Existing international conversion formulas all use series approximation theory, resulting in low accuracy of calculation results, cumbersome calculation processes, and a lack of directly usable products.
To this end, the research team used the most accurate orbit information of the sun, planets, main belt asteroids and Kuiper belt objects to achieve accurate conversion of lunar coordinate time and center-of-mass mechanical time. The cumulative error does not exceed 1/20000000 of a second even after 1,000 years. The research team further developed an end-to-end software package product, allowing users to obtain accurate conversion results of lunar coordinates in just one step.
Relevant research results have been published in the journal Astronomy and Astrophysics.
Jonathan McDowell, an astronomer at the Harvard-Smithsonian Center for Astrophysics in the United States, said that lunar time determination is becoming a real engineering need and can no longer be dealt with on a case-by-case basis based on Earth time as in the past.
He pointed out that in the navigation system of the spacecraft, microsecond-level errors can have a significant impact, thereby affecting the calculation results on the time scale of minutes. "If you want to use a GPS-like system on the moon - which we may need in a few years, especially for precise landing positions, you have to find a way to solve this problem."
McDowell said that although similar work is being done in the United States, he has not heard of "lunar timing software" that can be used directly. "This reflects the importance China attaches to the moon and is very open in sharing moon-related research," he said.
The South China Morning Post stated that in the past, the number of lunar missions was so small that the issue of time errors was almost "irrelevant" and engineers could individually correct each mission as needed based on Earth time. However, as lunar exploration becomes the focus of the world's aerospace field, more and more spacecraft and even manned spacecraft will go to the moon in the future, and it will be difficult to use temporary solutions to deal with the error problem.
In addition, setting time standards is not only important for coordinating lunar missions, but is also a symbol of political influence. The South China Morning Post cited, for example, the British decision in 1884 to set the site of the Greenwich Observatory as the base point of the prime meridian. This was not only for convenience, but also reflected Britain's dominant position in world navigation, trade and science at that time.