What is difference between "A1Gregorian" and just "Gregorian" to describe an epoch?

[ncc1701d]

What is difference between "A1Gregorian" and just "Gregorian" to describe an epoch?

What is difference between "A1Gregorian" and just "Gregorian" to describe an epoch?
or A1ModJulian verses UTCModJulian? never heard of "A1" before
thanks

 

[ADSWNJ]

See here: http://www.leapsecond.com/history/1968-Metrologia-v4-n4-Essen.pdf

Old document but still interesting. It's mind-blowing to think that we have 3 conflicting reasons to define a second:

1. As a way to determine midday. (I.e. linked to the variable rotation rate of the Earth).

2. As a way to determine winter from summer (I.e. linked to the variable rotation rate of the Earth around the Sun).

3. As a way to measure uniform time to something more atomic and physical (I.e. linked to the vibration of a cesium atom).


I'm not the expert here, but it looks like you have TAI weighted average atomic time, UTC which is TAI + #leap-secs (to keep midday at midday).

See this: http://gmat.sourceforge.net/doc/R2017a/GMATMathSpec.pdf

This suggests that A.1 is a fixed offset from TAI, but see this too:

[URL="https://www.usno.navy.mil/USNO/time/master-clock/international-time-scales-and-the-b.i.p.m"]https://www.usno.navy.mil/USNO/time/master-clock/international-time-scales-and-the-b.i.p.m which clarifies that A.1 is US reference time, and steered to coordinate to TAI.

So basically ... TAI and A.1 are the same, give or take a few nanoseconds.