Top Banner
Leap Seconds Dennis McCarthy U. S. Naval Observatory
21

Leap Seconds

Dec 31, 2015

Download

Documents

Leap Seconds. Dennis McCarthy U. S. Naval Observatory. Definition of Seconds. Rotational Second 1 / 86,400 of mean solar day Ephemeris Second First used in 1956 1/31,556,925.9747 of tropical year 1900 Length of year based on 19 th century astronomical observations. Atomic Second. - PowerPoint PPT Presentation
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Page 1: Leap Seconds

Leap Seconds

Dennis McCarthy

U. S. Naval Observatory

Page 2: Leap Seconds

Definition of Seconds

Rotational Second– 1 / 86,400 of mean solar day

Ephemeris Second– First used in 1956

– 1/31,556,925.9747 of tropical year 1900

– Length of year based on 19th century astronomical observations

Page 3: Leap Seconds

Atomic Second

SI second: 9,192,631,770 periods of the radiation corresponding to the transition between 2 hyperfine levels of the ground state of the Cesium 133 atom (adopted 1964)

Realizes the Ephemeris Second Frequency based on lunar observations

from1954.25 to 1958.25

SI second preserves the rotational second of mid-nineteenth century

Page 4: Leap Seconds

Time Scales

Rotational– UT1 is modern realization of historical

astronomical time scales including Mean Solar Time Greenwich Mean Time Greenwich Civil Time Universal Time (without suffixes) Weltzeit

Page 5: Leap Seconds

Time Scales (continued)

Atomic– TAI (International Atomic Time)

Follow-on from A.1 (maintained at USNO with input from 9 other laboratories

originally. - now only USNO) AM (at BIH with input from many laboratories) A3

– at BIH with input from 3 best laboratories– became AT (or TA) in 1969, TAI in 1971

others All atomic time scales were made equal to UT1

corrected for seasonal effects on 1 Jan 1958 0h 0m 0s may be considered modern realization of Ephemeris

Time (offset in epoch)

Page 6: Leap Seconds

Earth Rotation

Well documented deceleration– Tidal– Change in figure

Delta T

Year

1600 1700 1800 1900 2000

De

lta

T (

se

c.)

-60

-40

-20

0

20

40

60

80

Page 7: Leap Seconds

Historical Answers UTC (Coordinated Universal Time)

– Begun in 1960 as cooperative effort of U.S. Naval Observatory and Royal Greenwich Observatory to make coordinated changes to clocks

– in 1965 BIH defined UTC with respect to atomic time– Epoch and frequency adjusted to match UT1

corrected for seasonal variations Current UTC adopted beginning in 1972

– no changes in frequency– leap seconds so that |UTC-UT1| < 0.9 s

UTC consistent with previous definitions of legal time

Page 8: Leap Seconds

TAI-UTCTAI - UTC

Year

1970 1980 1990 2000

TA

I-U

TC

(se

c)

5

10

15

20

25

30

Adjustments in Epoch and Frequency

Adjustments in Epoch only (Leap Seconds)

TAI - UTC

Year

1962 1964 1966 1968 1970 1972 1974

TA

I-U

TC

(se

c)

2

4

6

8

10

12

14

Page 9: Leap Seconds

Causes for Concern

Frequency of leap seconds increasing– Increasing public annoyance

Software issues– Unpredictable– Continuous second counts: days with 86,401 seconds– Time stamping 23h 59m 60s

Communications problems– coordination of events during a leap second

Growth of systems based on independent time scales

Page 10: Leap Seconds

Things to Consider Navigation

– 1 second = 1/4 mile at the equator

Computer software– Continuous second counts? 61-second minute?

Communications– Maintain synchronization over the leap second?

Legal definitions– Mean solar time?

Religious observances– Sunrise, noon, sunset?

Page 11: Leap Seconds

Options

Status quo Discontinue leap seconds Use TAI Re-define second Increase tolerance for |UTC-UT1| Smooth over the leap second step Predictable periodic adjustment of UTC Conventional adjustment of UTC

– Every leap year? Every 10 years?– Predict leap seconds based on deceleration

model

Page 12: Leap Seconds

Status Quo

Pro:– No changes required– Minimize concerns of

celestial navigators Con:

– Frequency of leap seconds increasing

– Communications, software problems

– Growth of systems based on independent time scales

Projected Number of Leap Seconds per Year

Year1990 1995 2000 2005 2010 2015 2020 2025 2030 2035 2040 2045 2050

Num

ber o

f Lea

p Se

cond

s

0

1

2

0

1

2

Page 13: Leap Seconds

Discontinue Leap Seconds

Pro:– Eliminate causes for

concern

Con:– Unlimited growth of |

UTC-UT1|– Legal definitions of

time?

UTC-UT1

Year1980 2000 2020 2040 2060 2080 2100

UTC

-UT1

(sec

onds

)

0

20

40

60

80

100

120

140

Page 14: Leap Seconds

Use TAI

Pro:– Eliminate causes for concern

Con:– TAI must be more accessible– Legal definitions of time?

Similar to elimination of leap seconds TAI must be more accessible

Page 15: Leap Seconds

Redefine the Second

Pro– Fundamental Solution

Con– Require redefinition of physical units– Temporary solution

Correction to the Length of the Second

Year

2000 2100 2200

Cor

rect

ion

- cy

cles

/s

-300

-200

Page 16: Leap Seconds

Smooth Over Leap Second Step

Pro– Eliminates the “extra” second

Con– Requires seconds of different lengths– Date of adjustment unpredictable– Implementation?

58 59 0 1 260

58 59 0 1 2

LEAP

Page 17: Leap Seconds

Increase Tolerance for |UTC-UT1|

Pro– Easy to accomplish

Con– Larger discontinuities– DUT1 code limitations– Date of adjustment unpredictable– What is an acceptable limit?

Page 18: Leap Seconds

Periodic Adjustment of UTC

Pro– Date of adjustment is predictable

Con– Number of leap seconds remains

unpredictable– Large discontinuities

Page 19: Leap Seconds

Conventional Adjustment of UTC

Unknown number of leap seconds at predictable intervals

Known number of leap seconds at predictable intervals

– Pro»Date of adjustment is predictable

– Con» Number of leap seconds remains unpredictable» Large discontinuities possible» |UTC-UT1|1

– Pro»Date of adjustment and number of leap seconds predictable

– Con»Large discontinuities possible»|UTC-UT1|1

Page 20: Leap Seconds

UTC-UT1 with Various Leap Second Procedures (with Decadal Variations Modeled

Year

2000 2100 2200 2300 2400 2500 2600 2700

UT

C-U

T1

(sec

)

-20

-10

0

10

20

30

40

50

60

70

80

90

100

One sec at predicted intervalsPredicted no. of sec. every leap yearPredicted no. of sec. every 10 years

Conventional Adjustment of UTC

Page 21: Leap Seconds

What to do?

Question needs study– URSI– IAU– ITU-R