sun - Is the length of the day increasing?

In Frontiers of Astronomy, Fred Hoyle advanced an idea from E.E.R.Holmberg that although the Earth's day was originally much shorter than it is now, and has lengthened owing to tidal friction, that further increases in the length of the day would not occur because of resonance effects in the atmosphere caused by the gravitational field of the sun.

I haven't heard that suggested anywhere else, though, and wondered whether it was really the case, or is the length of the day increasing?

Answer

The length of a day is increasing slowly. The rate is very slow, about 0.0017 seconds per century.

The length of SI day is based upon the mean solar day between 1750 and 1892. A mean day nowadays (!) is about 0.002 seconds longer, which means that we accumulate a difference of about 0.6 seconds every year. This is why we have leap seconds (leap second is when we stop our clocks for one second to let the earth "catch up"). From the Wikipedia article on leap seconds:

The leap second adjustment (which is approximately 0.6 seconds per year) is necessary because of the difference between the length of the SI day (based on the mean solar day between 1750 and 1892) and the length of the current mean solar day (which is about 0.002 seconds longer). The difference between these two will increase with time, but only by 0.0017 seconds per century. In other words, the adjustment is required because we have decoupled the definition of the second from the current rotational period of the Earth. The actual rotational period varies due to unpredictable factors such as the motion of mass within Earth, and has to be observed rather than computed.

Also see: ΔT.