| Astronomers do not need leap seconds, because even with this adjustment UTC cannot be used to determine anything in astronomy. Astronomers need either true time, which is TAI, to be used in computing the positions of celestial bodies, and they need for observations the so-called Sidereal Time, which is not a time but the angle between a coordinate system attached to the Earth and an inertial system of coordinates attached to distant celestial objects that have negligible angular movement (in the past those were distant stars, now they are distant galaxies or quasars). The Sidereal Time can be computed in a complex way from TAI, because it is determined by the periodic rotation and precession of the Earth and by various superposed periodic or random movements. The UTC is not adjusted to match the current true rotation angle of the Earth, which you can measure by looking up to the stars, but it is adjusted to match within 1 second a fictitious angle that would be the rotation angle of the Earth-Sun direction corresponding to an Earth that would rotate uniformly both around itself and around the Sun, so that the duration of a day would have been constant. In reality, the duration of a Solar day, i.e. the time between 2 consecutive noons, varies a lot during the year, by a large fraction of an hour (by about a half of hour peak-to-peak), so using UTC directly for estimating the position of the Sun gives a very big error, of many minutes of hour. So what you need for astronomy is to know the current TAI and you need a Sidereal Time calculator, which you need for knowing in what direction to point your telescope, to find a given celestial object. UTC cannot be used directly in astronomy, but only after passing either explicitly or implicitly through TAI. The fact that astronomical almanacs are published using UTC in their tables is obfuscating this, because the values in the tables have not been computed using UTC, but everything has been converted to UTC to match the time that is presumably shown by the watch or clock that the almanac user may have. |