Delta T
Delta T (ΔT) is the time difference between Universal Time (UT) and Terrestrial Time (TT), used in astronomical calculations to account for irregularities in Earth's rotation. It has been increasing gradually due to tidal braking and other factors, and historical values are derived from ancient eclipse records.
Background
- **ΔT (Delta T)** is the difference between Terrestrial Time (TT, a uniform atomic-clock time) and Universal Time (UT1, which tracks Earth's actual rotation). Because Earth's rotation is gradually slowing due to tidal friction, ΔT increases over time.
- This matters for **historical astronomy, eclipses, and calendar reconstruction**: to know exactly when and where a solar eclipse was visible centuries ago, you need ΔT to convert between modern uniform time and the "real" time kept by Earth's rotation in that era.
- ΔT cannot be predicted far into the future or past purely by theory; it must be derived from historical records of eclipses, occultations, and other astronomical observations. Before the 17th century, values are uncertain and based on sparse data.
- The value is published annually by the International Earth Rotation and Reference Systems Service (IERS). As of the early 2020s, ΔT is about 69–70 seconds and increasing by roughly 0.5–0.7 seconds per decade.