UT1 is basically Earth’s actual rotation time. Think of it as “astronomical clock time,” determined by how far the Earth has spun relative to the stars. It’s the modern, high-precision descendant of old Greenwich Mean Time (GMT), except UT1 wobbles slightly because Earth’s rotation isn’t perfectly steady — tides, earthquakes, and atmosphere drag the planet around just enough to make it drift by milliseconds each day.

In short:

• UT1 = “True” Earth-rotation time.
• It defines what the angle of the Earth is right now (used for sidereal time, Local Apparent Sidereal Time, etc.).
• It’s not uniform — it speeds up and slows down unpredictably.
• The difference between atomic time and UT1 is tracked as ΔUT1, which is kept within ±0.9 s by adding or removing leap seconds to UTC.