On 2014-11-01 23:31, Steve Allen wrote:
In the appropriate contexts there are days of Terrestrial Time,
International Atomic Time, Barycentric Coordinate Time, Geocentric
Coordinate time, GPS system time, BeiDou system time, etc. Each of
those days is 86400 SI seconds in its own reference frame.
In other contexts there are days of Universal Time, Sidereal Time,
Ephemeris Time. Each of those days is 86400 of its own kind of
seconds.
I disagree. One wants to compare all these time scales with
each other, and comparison requires expression in the
/same/ unit, not in different units.
For instance, the differential rate d(TAI - UT1)/d(UT1) is
published as LOD by the IERS as a "dimensionless" number
with unit ms/d. To compute this, one must be able to
subtract the reading of UT1 from that of TAI, and to
compute the difference numerically one has to convert to
equal units. The rate is computed correctly /only/ if
one assumes that a second of TAI equals a second of UT1.
I agree that it can still make sense to use different
symbols for the same unit (such as s{TAI} and s{UT1});
it is similarly common practice to distinguish masses
of carbon dioxide from masses of carbon by different
unit symbols g{CO₂} and g{C} for the same unit gram.
Nevertheless, the BIPM seem to advise against such use
[SI brochure 2006, section 5.3.2, p 132]:
Units are never qualified by further information
about the nature of the quantity; any extra
information on the nature of the quantity should
be attached to the quantity symbol and not to the
unit symbol.
Reference:
[SI brochure 2006]:
http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf
Michael Deckers.
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