10/15/2009

Do We Really Know What Time It Is?
 JD 2443144.5: is this unambiguous?
 Depends on required accuracy
 Full description:
 Time stamp Location of observer
 Motion of observer
 Time Scale used

Arnold Rotsp ySmithsonian AstrophysicalObservatoryy

2009‐10‐06

A.H.Rots, ADASS XIX Sapporo:When Time is of the Essence1

2009‐10‐06

A.H.Rots, ADASS XIX Sapporo:When Time is of the Essence

Absolute Time Uncertainty

Time Stamps

 Contributors:
 Instrumental delays
 Clock calibration
 Orbit ephemeris/geodetic position
 1 μs accuracy is feasible on spacecraft, Julian Date: changes at noon UTC
 Modified Julian Date: JD – 2400000.5
 ISO‐8601, as used in FITS and VO: ccyy‐mm‐ddThh:mm:ss[.sss…]
 No time zones
 Gregorius
gand ppre‐1582
5 dates Can only be used in the range
0001‐01‐01T00:00:00 to 9999‐12‐31T23:59:59

better on the ground Collect and record necessary metadatathat’s you responsibility (dare I mention STC?)

2009‐10‐06

A.H.Rots, ADASS XIX Sapporo:When Time is of the Essence23

2009‐10‐06

A.H.Rots, ADASS XIX Sapporo:When Time is of the Essence41

10/15/2009

Time Reference Positions

Earth‐based Time Scales Topocenter Easy, but hard to compare, unless low accuracy
 Geocenter Good for simultaneous measurements
 Up to 20 ms for ground‐based Barycentery For long‐term comparisons Up to 500 s

 TT: Terrestrial Time Official IAU time; on the rotating geoid; continues ET
 TAI: International Atomic Time Timekeepers’ time; TT – TAI = 32.184 s
 UTC: Universal Time Coordinated; GMT
 Distributed time,, with leapp seconds;; currentlyy 34 sbehind TAI
 GPS: GPS’s Time Scale TAI – GPS = 19 s

2009‐10‐06

A.H.Rots, ADASS XIX Sapporo:When Time is of the Essence

5Barycenter Correction

2009‐10‐06

A.H.Rots, ADASS XIX Sapporo:When Time is of the Essence

6Time Scales Realization

 Geometric path length delay (< 500 s)
 Shapiro delay (< 0.2 ms)

subtract leap seconds
currently 34 s

TAI

 Römer delay (typically < 0.1 ms)
 Eccentricity of earth orbit (< 2 ms)‐19 s+32.184 s

 These terms transform TT into TDBTT
2009‐10‐06

A.H.Rots, ADASS XIX Sapporo:When Time is of the Essence

UTC7

2009‐10‐06

GPSmainly sinusoidal
1.7 ms / yr

A.H.Rots, ADASS XIX Sapporo:When Time is of the Essence

TDB8210/15/2009

How Long is a Second?

Relativistic Trouble

9 7 the second was 1/31,556,925.9747/3 ,55 ,9 5 9747 of the Before 1967

 SI second: 9,192,631,770 cycles of radiation due to transition
between two hyperfine levels of 133Cs
 Clock at lower gravitational potential speeds up, clock

tropical year for 1900 January 0 at 12 h Ephemeris Time
 Until we changed to Julian epochs, we used Besselian
epochs, based on Tropical Years Trouble is, Tropical Years are getting shorter: 0.8 s percentury
 Julian epochs are based on a Julian Century: 36525 daysof 86400 s
 The second is now defined by Cesium atomic transition
2009‐10‐06

A.H.Rots, ADASS XIX Sapporo:When Time is of the Essence

9

Real Relativistic Trouble

in motion slows down
 A clock in LEO would lose 8.5 ms per year In LEO we don’t bother, just synchronize

2009‐10‐06

A.H.Rots, ADASS XIX Sapporo:When Time is of the Essence

10

Practical Time Scales Model But having a clock run synchronously at the barycenter

Terrestrial(as TDB does) has nasty consequences: fundamental
physical constants change Solution: Coordinate Time

Coordinate
time

Barycentric

TCG

4‐D transf

TCB

linear+2 s / cy

Synchronizedat 1977.0
1977 0 (UTC)linear‐0.5
0.5 s / yrTTsinusoidal
1.7 ms / yr

TDB

 TCG – faster than TT by 6.969290134 × 10−10
 TCB – faster than TDB by 1.550505 × 10−8 Both synchronized with TT and TDB at 1977.0 (UTC)

Dynamicaltime
2009‐10‐06

A.H.Rots, ADASS XIX Sapporo:When Time is of the Essence11

2009‐10‐06

A.H.Rots, ADASS XIX Sapporo:When Time is of the Essence12310/15/2009

2009‐10‐06

A.H.Rots, ADASS XIX Sapporo:When Time is of the Essence

134