Hi Victor, Yes, you’re right – my point is about much more than the simple difference defined by Delta T. Firstly, Delta T is the difference between two very important time-scales: TT (was called TDT), which is based on the SI second, and UT1, which is based on the Earth’s rotation.We use UT1 because it corresponds to the Earth’s rotation. But because it’s irregular it can’t be represented on a clock, which is why we use UTC (and insert occasional leap seconds like just happened today). But if we aren’t worried these 1-second adjustments we can use UTC as if it were UT1. We need TT because it corresponds to the clockwork of the universe and, more importantly, can be measured using the SI seconds of atomic time (using another timescale called TAI). It’s easier to understand my issue if we work backwards through the standard positional calculation and just concentrate on stars. We’ll ignore precession, nutation, proper motion, aberration, refraction, and so on. To calculate the azimuth and altitude of a star with a known Right Ascension and Declination, we apply Formulae 13.5 and 13.6 from Meeus (1998), or Equations 11.43-3 and 11.43-4 from Seidelmann (1992). These formulae take the following arguments: - The local hour angle - The observer’s latitude and longitude - The star’s declination To calculate the local hour angle we need: - Greenwich sidereal time - The star’s right ascension And lastly, to calculate Greenwich sidereal time we need: - The time in UT But we have already decided that we can substitute UTC for UT1 with < 1 sec error. So, thinking about the way the sky view changes in Stellarium when you switch to different Delta T models, it’s pretty clear that: - The observer’s location didn’t change - The RA and Dec of the stars didn’t change, and - The UTC time didn’t change. - Therefore the position of the stars in the sky should NOT change And that’s my point: TO CALCULATE THE ALTITUDE AND AZIMUTH OF A CELESTIAL BODY WITH KNOWN RA AND DEC DOES NOT REQUIRE THE USE OF DELTA T. (Apologies for the capitalisation; plain text doesn’t give me much scope for highlighting this important fact.) The many corrections that *do* use Delta T (precession, nutation, proper motion, aberration, etc.), which we ignored, are too small to see. Here is a summary of the full star position algorithm from Seidelmann, showing those corrections and their time arguments: 1a: Calculate date in TDT 1b: Convert to centuries 1c: Calculate mean anomaly of Earth (timescale = TDT centuries) 1e: Convert to centuries since Julian Date 2000.0 2: Determine position and velocity of Earth, and position of Sun (timescale = TDT) 3: Calculate star’s space motion (timescale = TDT) 4: Calculate star’s barycentric position (timescale = TDT) 5: Calculate star’s geocentric position (timescale = TDT) 6: Apply relativistic deflection of light (timescale = TDT) 7: Apply aberration of light 8: Apply precession (timescale = TDT) 9: Apply nutation (timescale = TDT) 10: Calculate apparent RA and Dec (timescale = TDT) 11. Convert to altitude and azimuth (timescale = UT1) 12. Apply refraction Steps 1 to 10 are all about converting the catalogue RA and Dec of a star into an apparent RA and Dec. But the cumulative effect of all of these calculations is probably only a fraction of a degree. Much too small to see in Stellarium. Although TDT is the time argument, it isn’t used to calculate the final azimuth and altitude. Only UT1 is used for that step (at least, if we ignore precession/nutation and polar motion). The modern equivalent shown in Figure 5.1 of the IERS Conventions (2010), or Figure 1 of Wallace (2006) uses a similar algorithm, with a cleaner separation between Earth rotation (UT1) and precession/nutation. Sorry about the long email, but I hope it explains things properly. Cheers, Frank Meeus (1998). Astronomical Algorithms Seidelmann (1992). Explanatory Supplement to the Astronomical Almanac Wallace (2006). Example application of the IAU 2000 resolutions concerning Earth orientation and rotation On Mon, Jun 29, 2015 at 12:35 AM, Victor Reijs