ΔT and Leap Seconds

ΔT

The orbital positions of the Sun, planets and Moon are usually calculated using Terrestrial Time (TT) —also called Terrestrial Dynamical Time or TD— because it’s a uniform time scale. However, world time zones and daily life are based on Universal Time (UT1). In order to convert eclipse and transit predictions from TT to UT1, the difference between these two time scales must be known. The parameter DeltaT (ΔT) is the arithmetic difference, in seconds, between the two as: ΔT = TT - UT1.
World time zones are actually based on Coordinated Universal Time (UTC). It’s an atomic time synchronized and adjusted to stay within 0.9 seconds of astronomically determined Universal Time (UT1). Occasionally, a leap second is added to UTC to keep it in sync with UT1, which changes due to variations in Earth’s rotation rate. UTC is synonymous with GMT, although GMT is no longer precisely defined by the scientific community; while Universal Time (UT1) is based on the rotation of Earth in relation to distant celestial objects (stars and quasars), UTC is based on International Atomic Time (TAI) and is adjusted to remain within one second of Universal Time (UT1) through the occasional addition of a leap second.

DeltaT (ΔT) seasonal variations from 1973 to today

DeltaT (ΔT) evolution from 1973 to today

Leap Seconds

A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (as measured by atomic clocks) and imprecise observed solar time (known as UT1 and which varies due to irregularities and long-term slowdown in the Earth’s rotation). The UTC time standard, which is widely used for international timekeeping and as the reference for civil time in most countries, uses precise "atomic time" and consequently would run ahead of observed solar time unless it is reset to UT1 as needed. The leap second facility exists to provide this adjustment.
Because the Earth’s rotation speed varies in response to climatic and geological events, UTC leap seconds are irregularly spaced and unpredictable. Insertion of each UTC leap second is usually decided about six months in advance by the International Earth Rotation and Reference Systems Service (IERS), to ensure that the difference between the UTC and UT1 readings will never exceed 0.9 seconds.
This practice has proved disruptive, particularly in the twenty-first century and especially in services that depend on precise time stamping or time-critical process control. The relevant international standards body has been debating whether or not to continue the practice with an increasing number of nations supporting its abolition.

Leap seconds evolution from 1972 to today