Midsummer Day, by popular tradition, is June 24th, the feastday of St John the Baptist. It is noteworthy for the wealth of superstition that surrounds it, much of it going back to pagan times. Others, however - particularly those who like to frolic in diaphanous deshabille around the rocks of Stonehenge or other neolithic monuments - tend to think of Midsummer Day as being today, June 21st - the summer solstice.
"Solstice" comes from the Latin words sol and sistit. The latter word means "stands", so the combination might be rendered "sun stands still" - something which at first glance would appear to be a sheer impossibility. Yet anyone willing to take the trouble to observe the rising or the setting sun for a week or two around this time of year, can capture the illusion.
If you were to choose a vantage point that gives an unobstructed view of the horizon, and from there to note the place at which the rising sun appears each morning of the year, you would see that the rising-point travels up and down the horizon over the 12-month period. In Ireland, the sun rises almost due south-east around the winter solstice in December, and in the north-east at this time of year.
The rate of change of position of the rising-point is greatest at the vernal and autumnal equinoxes. In March the sun rises farther and farther north on the horizon with every passing day, and the difference from day to day is greater than it is a month before or after. The same rapid movement of the rising-point is evident in late September, except that then the rising-point advances southwards. By contrast, as the summer solstice approaches, the daily change in the position of the rising-point is very small - except for a few days around June 21st. It hardly moves at all; the rising-point stands still.
The process can be compared to the rhythmic movement of a pendulum that takes a year to swing from side to side and back again. The swinging weight would move most rapidly when near the centre of its zone of travel, but would slow down as it reached the limit of its arc on either side - and stop entirely for an instant at the turning point.
The solstice has another, much more visible significance; it marks the longest day of the year - the day with the longest interval between sunrise and sunset. From today the hours of daylight will again diminish, and the days will become shorter as sunrise and sunset converge to their closest point in six months' time.