Polynomial approximations to Delta T, 1620(2000 AD

The difference between the uniform Dynamical Time and Universal Time, known as ?T (Delta T), is needed in several astronomical calculations, such as eclipses and occultations, or to reduce measured positions of minor planets to a uniform time scale for the aim of orbital determinations. As Universal Time is based on the variable rotation of the Earth, the quantity ?T reflects the irregularities of that rotation, so it varies slowly but rather irregularly with time. We have done some work on empirical formulae for approximating ?T and have derived a set of polynomials of the 4th order which are accurate to within +/-1 second for the period 1620-2000.