A number of time scales are used in the world of astronomy, some key ones are:
These time scales are related to each other and the following figure shows the mathematical relationships and the names ascribed to the differences between them e.g. DeltaT
Terrestrial time (TT)and Atomic time (TAI) are based on the SI second (defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom) and are independent of time as defined by rotation of the earth. The two time scales are related by a fixed constant offset of 32.184s. The terrestrial time scale is used as the yardstick for applying corrections for precession, nutation and star proper motion.
Coorindated universal time (UTC) is also based on the SI second and is always an integral number of seconds different to TAI. This is the timescale on which we govern our lives and is the base to which time zone and daylight saving time corrections are applied in order to get local civil time in the various time zones around the world.
Universal time is determined by the rotation of the earth and is gradually diverging from terrestrial time because earth rotation is gradually slowing and consequently the universal time second is getting longer when measured using the caesium oscillation based SI second. The difference between terrestrial time and universal time is known as DeltaT, the ASCOM astrometry namespace contains routines that should give good values of DeltaT for the next couple of years.
The fact that there is a difference in rate between UTC, which is based on atomic time, and UT, which is based on the rotation of earth and which governs when day and night actually occur, requres management. This is effected by introducing occasional leap seconds into UTC in order to keep it within plus or minus 0.9 seconds of UT. This approach ensures that our civil time, despite being based on the SI second always stays close to universal time as defined by the rotation of the earth.
Please note that DeltaT and DeltaUT change in opposite senses to each other. E.g. if DeltaT increases in value and DeltaUT happens to be positive, then DeltaUT will decrease from positive through zero to a negative value. Eventually it will become less than -0.5 and a leap second will be added to UTC in June or December, which will add a whole second to DeltaUT, making it positive again, with a value around +0.5. The leap second addition does not affect DeltaT at all because it is only influenced by the earth's rotation and is independent of UTC.
These relationships are shown in the following figure.