4 ms·

It's an interesting situation when instruments or measurements become more precise, stable, or reliable than the reference material. And when someone (usually

It's an interesting situation when instruments or measurements become more precise, stable, or reliable than the reference material. And when someone (usually an individual) finally discovers that it has happened, or in some cases makes it so. >the ephemeris second is based on an astronomical ephemeris, which is a mathematical model of the solar system >the standard ephemeris was produced by Simon Newcomb in the late 1800s >he collected a vast amount of historical astronomical data to create his mathematical model >it remained the standard until the mid 1980s >in 1952 the international astronomical union changed the definition of time so that instead of being based on the rotation of the earth about its axis, it was based on the orbit of the earth around the sun >in the 1930s they had discovered that the earth’s rotation is not perfectly even: it slows down and speeds up slightly >clocks were now more precise than the rotation of the earth, so the ephemeris second was a new more precise standard of time

darkwater 3y ago
>in 1952 the international astronomical union changed the definition of time so that instead of being based on the rotation of the earth about its axis, it was based on the orbit of the earth around the sun >in the 1930s they had discovered that the earth’s rotation is not perfectly even: it slows down and speeds up slightly Yeah, I remember studying that back in high school but I wonder... what previous actual duration of a second they used? And also, being based on the rotation of Earth, what kind of data was the "vast amount of historical astronomical data" Newcomb collected? How can you reliably capture and store the length of time if you can only base it on the Earth rotation speed which varies over time? I would guess the data compared it to other natural phenomena?
- fanf2 3y ago
  When time was based on earth rotation, astronomers used “transit instruments” to observe when certain “clock stars” passed directly overhead. The clock stars had accurately known positions, so if you routinely record the time they pass overhead according to your observatory’s clock, then you can work out how accurate your clock is. Newcomb’s data would have been accurately timed observations, as many as he could get hold of, going back about two and a half centuries.

darkwater 3y ago

>in 1952 the international astronomical union changed the definition of time so that instead of being based on the rotation of the earth about its axis, it was based on the orbit of the earth around the sun >in the 1930s they had discovered that the earth’s rotation is not perfectly even: it slows down and speeds up slightly Yeah, I remember studying that back in high school but I wonder... what previous actual duration of a second they used? And also, being based on the rotation of Earth, what kind of data was the "vast amount of historical astronomical data" Newcomb collected? How can you reliably capture and store the length of time if you can only base it on the Earth rotation speed which varies over time? I would guess the data compared it to other natural phenomena?

fanf2 3y ago
When time was based on earth rotation, astronomers used “transit instruments” to observe when certain “clock stars” passed directly overhead. The clock stars had accurately known positions, so if you routinely record the time they pass overhead according to your observatory’s clock, then you can work out how accurate your clock is. Newcomb’s data would have been accurately timed observations, as many as he could get hold of, going back about two and a half centuries.