Word origin · etymology
Mathematical notation read aloud.
Delta is the Greek letter conventionally used for a difference; T denotes time. Here the difference is TT minus UT1.
How we know · epistemology
Measured and reconstructed quantity
The difference between TT and UT1 depends on knowledge of Earth’s rotation. Recent determinations and distant historical reconstructions have different uncertainties; many decimal places do not remove that distinction.
Connected concepts
Terms used in this explanation
These links connect subjects and terminology. They do not establish a shared historical origin.
Sources & provenance
Use the citations beside each section to distinguish a calculation convention, a historical teaching and an editorial example.