Return the value of delta-T in units of seconds

Converting the Coordinated Universal Time (UTC) to the Ephemeris Time (ET) is complex. It is due to the unpredictable nature of the Earth's rotation, which is the basis for UTC, whereas ET was based on the more uniform orbital motion of the Earth around the Sun. The key to converting between these time scales lies in a value known as delta-T, which is the difference between a uniform time scale and one based on Earth's rotation. The conversion is handled as: ET = UTC + deltaT However, delta-T is not a constant value and cannot be calculated using a simple formula. The delta-T values are derived from the historical records and from direct observations. A series of polynomial expressions have been created to simplify the evaluation of delta-T. The calculated values are valid for the years from -1999 to +3000.

Usage

delta_t(year, month)

Arguments

year: Type in the year (integer between -1999 and 3000).
month: Type in the month (integer between 1 and 12).

Value

numeric: vector of numeric values

Details

More details: Morrison, L. and Stephenson, F. R., "Historical Values of the Earth's Clock Error ΔT and the Calculation of Eclipses", J. Hist. Astron., Vol. 35 Part 3, August 2004, No. 120, pp 327-336 (2004) Stephenson F.R., Historical Eclipses and Earth's Rotation, Cambridge Univ. Press, 1997

Examples

delta_t(1999, 10)
#> [1] 63.78768
delta_t(c(-200, 1610, 2030), c(1, 10, 12))
#> [1] 12791.65348   107.80766    78.25045