May 18, 2019

“Measuring Time by Astronomy or Physics” – History of Astronomy Talk – 23rd April 2019

Eddie Yeadon

Report by: Bobby Manoo

“I have no idea what time is and I am not going to attempt an explanation.” This was the opening line for Eddie Yeadon’s talk for the fourth session of the History of Astronomy talks this season. Eddie has a PhD in physics from Manchester University and has had a long career largely focused on optics and optical engineering.

Before getting into the details, Eddie explained that time is a dimension and can be measured.

In order to measure time, what is required is an interval of time which is reproducible. The rotation of the Earth defines the day and has been divided into hours, minutes and seconds. Measurement accuracy improved with the development of better clocks, and by the end of the 19th Century Greenwich Mean Time, measured by star transits, had been recognised internationally as Universal Time. By the 1920s, however, quartz crystal clocks were good enough to show that the Earth rotation was not uniform but varied randomly between small limits with a gradual slowing down due to tidal friction with the Moon. This meant that the second, as a unit of time, was varying as well and a new definition of a constant second was required.

The astronomical answer was to use the orbit of the Earth around the Sun, and a definition of the second as 1/31 556 925.975 of the length of the tropical year 1900 was adopted by the International Astronomical Union in 1955. This was extremely difficult to measure, though, and was based on historical data. After caesium atomic clocks were invented at the NPL in the mid-1950s, it was shown that the new Ephemeris Second was actually too short to match the Earth rotation “day”. Consequently a “leap second” has to be inserted occasionally to catch up. This extra second is irregular and has to be inserted into time-dependent computer systems manually and leads to severe problems particularly with GPS navigation and time stamping on financial transactions. The result is that GPS and other systems ignore the international UTC standard and adopt various time standards which do not invoke leap seconds.

A proposal to abandon leap seconds and have a continuous running of atomic clock time has been made and will be discussed again by the international committee in 2023. This would result in standard clock time very gradually drifting from mean solar time on the meridian of Greenwich.

Eddie has kindly provided some more details of his very informative and engaging talk. Please click on the link below for further details on his talk.

HOA-Detail-Talk-Eddie-Yeadon Download