Summer is ending here in the shadow of the Odenwald in Germany. The transition to autumn is not gradual and ill-defined, as it tends to be in Ireland, but nearly always happens suddenly, on or about the same week every year. Over a very brief period, the dry, hot, sunny, humid days give way to much cooler weather with almost daily bouts of heavy rain.
And once the rain starts in autumn, they say, it will not stop again until the following spring - not an observation to be taken literally but a fair assessment of the situation nonetheless. Meanwhile the trees, noticeably day by day, are turning from their slightly faded green into a rich kaleidoscope of reds and oranges and browns. And the days themselves are growing short.
After the summer solstice, the time of sunset advances a little every day. The daily change is very gradual at first but as the months progress, the evenings shorten at an ever-increasing rate, until by around this time of year, sunset becomes earlier by a full 17 minutes every week.
Later we will see the rate of advance begin to ease again, and the change from week to week will again be very gentle as we approach the winter solstice in December. But at present, the evenings shorten more rapidly, day by day, than at any other time throughout the autumn.
But there is another sense in which September days are short. The 24-hour day by which our lives are regulated is based on the average length of time between two consecutive transits of the Sun across the same meridian - if you like, the average length of time from noon to noon.
But this interval is not exactly constant: the length of the solar day, as it is called, varies rhythmically throughout the year - one of the reasons being that the Earth moves slightly faster in its orbit at certain times, when it is closer to the Sun, than it does at others. The solar day is at its longest around Christmas - at about 24 hours and 30 seconds, and the very shortest solar day of the year, a mere 23 hours, 59 minutes and 38 seconds, occurs just now, on or about September 17th.
And there are fluctuations in the length of day over longer periods as well, as the Earth, for various reasons, increases or slows down its rate of spin. Careful measurements have shown, for example, that in the last 100 years or so the average day was at its shortest in the 1860s, when it was eight thousands of a second less than in the early 1900s: from 1910 until about 1930 the Earth accelerated, shortening the average day, and then slowed down again to provide the longest average days in recent times during the early 1970s.