I\'m confused by the ust of the term \"UT\" in the description of time scales us

Question

I'm confused by the ust of the term "UT" in the description of time scales used by the JPL HORIZONS system.

Their manual states that

UT is Universal Time This can mean one of two non-uniform time-scales based on the rotation of the Earth. For this program, prior to 1962, UT means UT1. After 1962, UT means UTC or "Coordinated Universal Time".

and the key attached to the tool's output says

Prior to 1962, times are UT1. Dates thereafter are UTC.

My understanding is that UTC has leap seconds, so that there should be an extra second at the end of a day on which a leap second was added, but the intervals reported by HORIZONS lacks these, and look more like UT1:

2012-Jun-30 23:59:58.000 2456109.499976852
2012-Jun-30 23:59:58.667 2456109.499984568
2012-Jun-30 23:59:59.333 2456109.499992284
2012-Jul-01 00:00:00.000 2456109.500000000
2012-Jul-01 00:00:00.667 2456109.500007716
2012-Jul-01 00:00:01.333 2456109.500015432
2012-Jul-01 00:00:02.000 2456109.500023148

Even more confusingly, the data reported do in fact behave as if the times are UTC. For example the reported azimuth of Pluto at Greenwich for the times above changes by 0.0028

Explanation / Answer

You raise two issues: ?T, and the HORIZONS timescales. Let us tackle each in turn.

1. What is Delta-T?

You are correct that HORIZONS is using a confusing term here.

What the HORIZONS menu calls Delta-T is an entirely different quantity than the ?T you will see defined and used in many other references on astronomy. Briefly:

What HORIZONS calls Delta-T tallies the diverging difference between how time behaves down at the bottom of the (fairly) temporally stable environment of the solar system barycenter, and how time behaves up here on Earth as we speed up and slow down each year as our orbit takes us close to and then away from the Sun. Their idea of CT is a clock at the solar system barycenter, their UT means UTC, and

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