When it comes to the full astronomical day – a single rotation of planet Earth in which the hour hand moves twice around a standard clock – some of this year’s shortest are happening in July and Aug. This week has seen the shortest days of the year so far. According to data posted Friday from the US Naval Observatory and the International Earth Rotation and Reference Systems Service, Wednesday’s rotation was about 1.34 milliseconds less than 24 hours. This isn’t completely out of the ordinary: Our world’s spins have been faster than usual lately. The average day has mostly shortened over the past decade, and within the past five years or so, the full rotation has clocked in at a hair less than 24 hours more often than not. Factors driving the change include movements at Earth’s core, atmospheric changes and the moon’s position.But long-term trends do not suggest that the days will shorten in perpetuity. In fact, it is just the opposite. For many millenniums, the days have been growing longer. A Tyrannosaurus rex that lived 70 million years ago would have experienced an average daily rotation of about 23 1/2 hours, studies have found. The lengthening trend is expected to continue.The main reason has to do with energy lost from the tides. The moon’s gravitational tug is responsible for tidal changes on Earth. Tidal currents heat the ocean, dissipating energy, which slows the rotation of Earth and lets the moon move farther as time passes.