#### Telescopic era values of Delta T

The above diagram displays the values of Delta T
for the telescopic era (1620 to the present) as tabulated on pages K8 and K9 of the current edition of
*The Astronomical Almanac*. Data are given for the beginning of each year. A simple
parabolic function used to estimate Delta T is also
plotted for comparison purposes. It takes the form
Delta T = – 20 + 32T^{2}
(Morrison & Stephenson, 2004) where T is the number of centuries since
1820. This function is based on the assumption that the length of the mean solar day has been
increasing by about 1.7 milliseconds per century.

#### Current values and short term predictions of Delta T

The above diagram shows the values of Delta T tabulated daily for the interval 2000 January 1 to the present as derived from the (UT1 - UTC) smoothed data provided in Bulletin B of the IERS, the International Earth Rotation and Reference Systems Service. It also shows the daily Delta T data and predictions provided with the USNO MICA v2.2.2 software package. The predictions used by MICA are occasionally updated; the data used here are those whose predictions of Delta T start on Julian Date 2455745.00. The current observed Delta T trend is increasing less rapidly than the MICA v2.2.2 predictions but more rapidly than the data derived from the (UT1 - TAI) predictions of the IERS Sub-bureau for Rapid Service and Predictions. The annual rate of change of Delta T since the beginning of 2010 is approximately 0.375 seconds per year.

#### Current values and longer term predictions of Delta T

The above diagram again shows the values of Delta T derived from Bulletin B of the IERS. Two sets of predictions are also provided for comparison purposes. The daily Delta T data and predictions are plotted for the interval 2000 to 2050 from MICA v2.2.2. The predictions derived from the IERS Sub-bureau for Rapid Service and Predictions are also plotted for the period 2006 April 1 to 2027 October 1 along with their uncertainties (vertical error bars). The current trend of observed Delta T data lies between the two sets of predictions. This diagram illustrates the problem faced by almanac producers when trying to estimate suitable values of Delta T for future almanacs.