General Information About Earth orientation
The Conventions contains detailed information on definitions and models used to determine Earth Orientation.
for the latest updates to the Conventions.
Please note: This links to the 2003 IERS Conventions.
Please note: This links to the 1996 IERS Conventions.
Units commonly used for Earth Orientation
1 mas = 1 milliarcsecond = 1 thousandth of a second of arc (for polar motion & nutation);
1 ms = 1 millisecond = 1 thousandth of a second of time (for UT1);
1 mas equals an equatorial shift at the Earth's surface of 3.1 cm;
1 ms equals the time it takes for the Earth to spin 15 mas (46.5 cm at the equator)
Date and Time Definitions
Modified Julian Date (MJD):
MJD = Julian Date - 2,400,000.5 days
T, the Besselian date, is given by T = 2000.000 + (MJD - 51544.03) / 365.2422
UT2-UT1 = 0.022 sin(2*pi*T) - 0.012 cos(2*pi*T) - 0.006 sin(4*pi*T) + 0.007 cos(4*pi*T)
where pi = π and T is the date in Besselian years.
TT = TAI + 32.184 seconds
since 00h 00m UTC 01 Jan 2017, the cumulative leap second count is:
TAI-UTC = 37 seconds (exactly)
and GPS Time-UTC = 18 seconds
users may compute Delta T = TT - UT1 using the relation:
Delta T = 32.184 s + (TAI - UTC) - (UT1 - UTC)
(TAI - UTC) is the cumulative number of leap seconds
(UT1 - UTC) is a standard Earth Orientation product.
This is the IERS Gazette #13 (ASCII). Please note that the recommended routine does not interpolate UT1-UTC properly around a leap second.
Data series used in IERS Bulletin A
These are the raw observations which contribute to IERS Bulletin A.
Systematic corrections that are applied to Combination procedure.
Archive of changes to Bulletin A since 2007.
Past issues of Bulletin A are archived at Crustal Dynamics Data Information Center (CDDIS).
Plots of past EOP variations (including comparisons with atmospheric excitation).
IERS Bulletin C announces leap seconds. To learn what a leap second is, refer to the FAQ.
IERS Bulletin D announces UT1-UTC values.
values of DUT1 = (UT1-UTC) transmitted with time signals are published in IERS Bulletin D
to a precision of 0.1 s
previous issues of Bulletin D
are archived here.