Context: Greenwich Mean Time (GMT) is an older standard, adopted starting with British railways in 1847. Using telescopes instead of atomic clocks, GMT was calibrated to the mean solar time at the Royal Observatory, Greenwich in the UK. Universal Time (UT) is the modern term for the international telescope-based system, adopted to replace "Greenwich Mean Time" in 1928 by the International Astronomical Union. Observations at the Greenwich Observatory itself ceased in 1954, though the location is still used as the basis for the coordinate system. Because the rotational period of Earth is not perfectly constant, the duration of a second would vary if calibrated to a telescope-based standard like GMT or UT—in which a second was defined as a fraction of a day or year. The terms "GMT" and "Greenwich Mean Time" are sometimes used informally to refer to UT or UTC.

Question: Which standard of time started with British Railways?
Answer: Greenwich Mean Time

Question: When was GMT adopted by British Railways?
Answer: 1847

Question: GMT used what instead of atomic clocks?
Answer: telescopes

Question: When did observations at the Greenwich Observatory cease?
Answer: 1954

Question: The terms GMT and Greenwich Mean Time are also used informally to refer to what?
Answer: UT or UTC

Question: In what year did British railways go into business?
Answer: Unanswerable

Question: What did the British railway stop using in 1954?
Answer: Unanswerable

Question: What did Greenwich Mean Time replace in 1847?
Answer: Unanswerable

Question: How constant is an atomic clock?
Answer: Unanswerable

Question: What happens when an atomic clock is not constant?
Answer: Unanswerable

Question: Which standard of time started with the International Astronomical Union?
Answer: Unanswerable

Question: When was GMT adopted by the International Astronomical Union?
Answer: Unanswerable

Question: The International Astronomical Union used what instead of atomic clocks?
Answer: Unanswerable

Question: When did observations at the Greenwich Observatory begin?
Answer: Unanswerable

Question: The term calibrated is also used informally to refer to what? 
Answer: Unanswerable