5 ms·

We already define time relative to 1970-01-01, relative to the mean solar noon in Greenwich. We should also define the length of a second to mean "at sea level

We already define time relative to 1970-01-01, relative to the mean solar noon in Greenwich. We should also define the length of a second to mean "at sea level in Greenwich".

tialaramex 5y ago
This just means the "length of a second" wanders around randomly and is thus useless. So now "a second" isn't any particular amount of time, and you've lost all ability to really measure time except in a vague inexact sense. The quest to eliminate irreproducibility from metrication is why SI did all that work to finally eliminate the kilogram prototype. The second was defined reproducibly decades ago because trying to define it based on our planet's inconstant spin was such a failure. The fact that the Earth's spin is not constant is perhaps frustrating, but like other facts just insisting it isn't true doesn't help you.
- kuschku 5y ago
  > This just means the "length of a second" wanders around randomly and is thus useless Why would it? My redefinition just means the length of a second is defined as a given number of cycles of caesium in a gravitational field equivalent to that of Greenwich. If you're in a different gravitational field, you'll just have to convert the values easily.
- fanf2 5y ago
  Right, but general relativity matters for how astronomers define their timescales, and how they are measured and maintained in practice: there are different scales for barycentric time (at the centre of gravity of the solar system), geocentric time (centre of the earth) and terrestrial time (on the surface of the rotating geoid). TAI is supposed to tick at the same rate as TT, so it depends on a model of an equal-gravitation version of sea level (the geoid) and the relative height of the time labs. In particular, NIST is a mile high in Colorado so their clocks tick a lot faster than (say) NPL by the Thames in west London.
  - tialaramex 5y ago
    Sure, the quest for Universal Time is despite the name necessarily a local quest so you can't meaningfully export your results to creatures orbiting a distant quasar as you can with the SI units. But the size of the discrepancy between Colorado and London is far smaller than the discrepancy Leap Seconds are concerned with right?
    - fanf2 5y ago
      Yeah, gravitational time dilation is about 1e-16 seconds per metre of elevation near the surface http://leapsecond.com/great2016a/index.htm http://leapsecond.com/great2016a/index.htm so Boulder is about 1e-13 fast. The rotation of the earth is stable to about 1e-9 http://leapsecond.com/ten/ http://leapsecond.com/ten/ which is also about the same as the rate difference between UT1 and UTC (0.2 ms / 86400).
      - sla29970 5y ago
        During the discussions leading to the inception of the SI second the bigwig standards folks nixed the consideration of relativity, basically paraphrased as "If you physicists do not stop arguing about relativity we will never redefine time in a way that removes it from the astronomers."
kempbellt 5y ago
Sea level when the tide is in, or out?
- fanf2 5y ago
  “sea level” is short for “mean sea level”
  - kempbellt 5y ago
    I mean, I feel like that's fairly obvious, and the playful pedantry of my comment in a discussion on the very definition of basic unit of measurement was overlooked. If we are going so far as to rigidly include "sea level" in the definition of "a second", it would probably help if we say that that we mean mean sea level. Know what I mean?
  - el_isma 5y ago
    Not trying to troll, but... What happens if/when global warming makes sea level rise? Do we have measurements that are still based on "sea level"?
    - _whiteCaps_ 5y ago
      https://tidesandcurrents.noaa.gov/sltrends/faq.html https://tidesandcurrents.noaa.gov/sltrends/faq.html > Tidal datums must be updated at least every 20-25 years due to global sea level rise. Some stations are more frequently updated due to high relative sea level trends.
CamperBob2 5y ago
These days, the length of a second is defined by the difference in frequency between two ground-state energy levels of a caesium-133 atom, which works out to 9192631770 cycles per second. No need to check tide tables.
- kuschku 5y ago
  That definition is still relative. It still requires as additional factor the place where the measurement is taken, due to Einstein's theories. That's why I proposed changing the second definition to 9192631770 cycles in a gravitational field equivalent to that of Greenwich.

We already define time relative to 1970-01-01, relative to the mean solar noon in Greenwich. We should also define the length of a second to mean "at sea level in Greenwich".

tialaramex 5y ago

This just means the "length of a second" wanders around randomly and is thus useless. So now "a second" isn't any particular amount of time, and you've lost all ability to really measure time except in a vague inexact sense. The quest to eliminate irreproducibility from metrication is why SI did all that work to finally eliminate the kilogram prototype. The second was defined reproducibly decades ago because trying to define it based on our planet's inconstant spin was such a failure. The fact that the Earth's spin is not constant is perhaps frustrating, but like other facts just insisting it isn't true doesn't help you.

kuschku 5y ago
> This just means the "length of a second" wanders around randomly and is thus useless Why would it? My redefinition just means the length of a second is defined as a given number of cycles of caesium in a gravitational field equivalent to that of Greenwich. If you're in a different gravitational field, you'll just have to convert the values easily.
fanf2 5y ago
Right, but general relativity matters for how astronomers define their timescales, and how they are measured and maintained in practice: there are different scales for barycentric time (at the centre of gravity of the solar system), geocentric time (centre of the earth) and terrestrial time (on the surface of the rotating geoid). TAI is supposed to tick at the same rate as TT, so it depends on a model of an equal-gravitation version of sea level (the geoid) and the relative height of the time labs. In particular, NIST is a mile high in Colorado so their clocks tick a lot faster than (say) NPL by the Thames in west London.
- tialaramex 5y ago
  Sure, the quest for Universal Time is despite the name necessarily a local quest so you can't meaningfully export your results to creatures orbiting a distant quasar as you can with the SI units. But the size of the discrepancy between Colorado and London is far smaller than the discrepancy Leap Seconds are concerned with right?
  - fanf2 5y ago
    Yeah, gravitational time dilation is about 1e-16 seconds per metre of elevation near the surface http://leapsecond.com/great2016a/index.htm http://leapsecond.com/great2016a/index.htm so Boulder is about 1e-13 fast. The rotation of the earth is stable to about 1e-9 http://leapsecond.com/ten/ http://leapsecond.com/ten/ which is also about the same as the rate difference between UT1 and UTC (0.2 ms / 86400).
    - sla29970 5y ago
      During the discussions leading to the inception of the SI second the bigwig standards folks nixed the consideration of relativity, basically paraphrased as "If you physicists do not stop arguing about relativity we will never redefine time in a way that removes it from the astronomers."

kempbellt 5y ago

Sea level when the tide is in, or out?

fanf2 5y ago
“sea level” is short for “mean sea level”
- kempbellt 5y ago
  I mean, I feel like that's fairly obvious, and the playful pedantry of my comment in a discussion on the very definition of basic unit of measurement was overlooked. If we are going so far as to rigidly include "sea level" in the definition of "a second", it would probably help if we say that that we mean mean sea level. Know what I mean?
- el_isma 5y ago
  Not trying to troll, but... What happens if/when global warming makes sea level rise? Do we have measurements that are still based on "sea level"?
  - _whiteCaps_ 5y ago
    https://tidesandcurrents.noaa.gov/sltrends/faq.html https://tidesandcurrents.noaa.gov/sltrends/faq.html > Tidal datums must be updated at least every 20-25 years due to global sea level rise. Some stations are more frequently updated due to high relative sea level trends.

CamperBob2 5y ago

These days, the length of a second is defined by the difference in frequency between two ground-state energy levels of a caesium-133 atom, which works out to 9192631770 cycles per second. No need to check tide tables.

kuschku 5y ago
That definition is still relative. It still requires as additional factor the place where the measurement is taken, due to Einstein's theories. That's why I proposed changing the second definition to 9192631770 cycles in a gravitational field equivalent to that of Greenwich.