June 20 was the summer solstice, the shortest night of the year. However, the latest sunset of the year did not happen that day, but some days later, depending on latitude. For example, in north Spain this latest sunset took place on June 27; in north Ireland, on June 24.
Why does this happen? The Earth goes around itself in 23 hours and 56 minutes, but in that time it also moves around the sun and will need about 4 minutes more on average to rotate a little more and return to the same orientation with respect to it (that’s why we say that the day has 24 hours).
The speed at which the Earth moves around the Sun is not always the same. Around January 4 it was going at about 110,700 kilometers/hour and on July 4 it will go at about 103,500. This is because the path is not a perfect circumference. This difference in velocities, understood by the great Kepler four hundred years ago, is what makes the evenings continue to grow a little longer and is also the reason that summer in the northern hemisphere lasts about five days longer than in the south.
Jorge Mira (University of Santiago de Compostela)