Summer twilight

At this time of year, with most of the summer before us, it seems paradoxical that the days are already shortening and the year progressing inexorably towards winter. But with last Saturday’s summer solstice, that is exactly what is happening. Once again the holly king and the oak king, rulers of the solar year, have fought their solstice battle, and the holly king now rules over these next six months of the declining sun.

Although considered to be the first day of summer, when the sun reaches its farthest point north of the equator, there is a seasonal temperature lag, which means that we have to wait for July and August for the hottest days. The cause of this lag is the role of the land and sea as natural storage heaters, absorbing the sun’s energy and gradually releasing it back into the atmosphere as heat. The oceans take longer to heat up compared to the land, and at this time of year are still relatively cool due to low spring temperatures.

But we can enjoy the long days and short nights while we wait for the dog days of summer. In fact, strictly speaking, there is no real night since twilight continues throughout the whole period from dawn until dusk. Twilight occurs when there is no direct sun but refracted sunlight still provides some light. Astronomically, morning twilight begins when the sun is 18° below the horizon, and ends in the evening when the sun drops back to 18° below the horizon. At the winter solstice, morning twilight lasts only from six until 8 am, and evening twilight from a quarter to four until 6 pm. But at the summer solstice evening twilight does not start until 9.20 pm and continues right through the night, ending at 4.30 in the morning.

Some animals are most active during twilight and these are referred to as crepuscular, the word derived from the Latin crepusculum, meaning twilight. Bats, for example, hunt during the twilight hours, thereby avoiding diurnal predators, although the species that emerge earliest from their daytime roosts are the fastest flyers and most able to avoid diurnal predators. The exceptional daytime vision of diurnal hunters such as kestrels diminishes rapidly as dusk light levels fall. By contrast, the large eyes of owls function well in poor light, and little owls in particular are crepuscular predators.

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