Blog: The Solstices

Date: 6 November 2019

Copyright © David Boettcher 2005 - 2023 all rights reserved.

The solstices are notable events in the solar calendar. Here I explain what the word solstice means, and why the winter solstice has been a source of both dread and celebration for many thousands of years.

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The Solstices and Flaming June

The two solstices are notable events in the solar calendar. They are known as the shortest and longest days of the year, and the winter solstice, the shortest day with the smallest number of hours of hours of daylight, has been celebrated for millennia - for good reason!

The two solstices occur around 21 June and 21 December each year. In the northern hemisphere these are the summer and winter solstices respectively, in the southern hemisphere they are the other way round.

The word solstice comes from the Latin words sol for sun and stit meaning stop or stand, which has the same Latin root as the stet that editors write when they change their mind and want some text that they have deleted to remain. The Latin solstitium became the English solstice, which literally means that the sun stops or stands still.

On the solstices the sun doesn't actually stop or stand still in space; it stands still on the horizon at sun rise and sun set. The solstices occur at the limits of the progression through the year of the position of sunrise and sunset along the eastern and western horizons.

If you watch where the sun rises on the eastern horizon, or sets on the western horizon, throughout the year, you will notice that in the winter it rises and sets further to the south than it does in the summer, and that as the seasons change the position of sunrise and sunset gradually cycles between two extremes. The figure here is a sketch of where the sun rises on the eastern horizon at each solstice. The position of sun rise and sun set appears to march along the horizon from midsummer to mid winter. When it gets to a solstice, the position of sunrise and sunset stops marching in one direction, appears to stand still for, and then starts marching in the other direction.

Summer Solstice

In the northern hemisphere, as spring turns into summer, you might have noticed that the sun rises on the eastern horizon further to the north. This means that if you note the point at which the sun rises, it gradually moves left from your perspective as the days get longer. When the point at which the sun rises stops moving to the north, or to your left, that is the summer solstice.

The summer solstice gives the day with the greatest number of daylight hours, for which reason it is called the longest day, although of course it remains 24 hours long in total; the longest day really means the day with the greatest number of daylight hours.

The summer solstice is sometimes called midsummer day, although it occurs at what is generally reckoned to be near to the start of summer. In Britain, July and August are usually the hottest months of the year. June is frequently disappointing as prevailing westerly winds, which are often absent in April and May, bring warmer but wetter air in from the Atlantic.

The summer solstice marks the day from which the days begin to get shorter, and then cooler as autumn and winter approaches. In the northern hemisphere it occurs on or around 21 June. But there is usually plenty of summer to come - enjoy it while it's here!

Flaming June?

With the arrival of June comes frequently wet, showery and often thundery weather. What happened to “Flaming June”, an often-heard phrase that seems to promise that there should be scorching sunshine for the month of June.

Flaming June is actually the name of an 1895 painting by Lord Frederic Leighton, not a statement about the weather for the month of June, which is often not flaming - sometimes far from it.

The arrival in Britain of wet weather in June after a spell of dry weather in the spring is due to a meteorological phenomenon known as the “return of the westerlies”, or sometimes even the “European monsoon.” In the spring, clear blue skies, dry weather and cold nights are due to dry easterly winds, which are dry because they don't cross an ocean. The North Sea is actually very small and shallow compared to the Atlantic ocean, so it can store very little heat and winds from the east don't pick up much heat or moisture as they cross it.

Around June the wind switches back to more normal westerly winds. These blow across the Atlantic ocean, which is not only vast in area, it is also very deep, which means it can store a lot of heat during the summer months. Westerly winds pick up a lot of heat and moisture as they cross it, bringing warm damp air to Europe, which turns to clouds and rain. The westerlies keep Europe warmer in the winter than it would otherwise be, but also wetter whenever they blow, which is often in June.

Winter Solstice

The winter solstice is, in my view, by far the more important, because it marks the point at which the days stop getting shorter and start to lengthen again, promising the end of winter and the arrival of spring. On a technical note, of course a day remains 24 hours long, the length of the day refers to the number of daylight hours.

From its rise furthest to the left or north on midsummer's day, the point at which it rises gradually moves to the right on the horizon, towards the south, as the year moves on towards winter. The further south that it rises, the lower it is at its zenith in the sky at mid day, and the shorter and colder the days are. The same thing happens at sunset; as winter draws on the sun sets gradually further south on the western horizon.

It is easy to image an early human watching this happening and becoming worried. As sunrise and sunset move along the horizon and the days get shorter and colder, without any knowledge of celestial mechanics and how the earth moves around the sun, they would worry that eventually the sun might not rise and the earth would be plunged into freezing darkness. Which of course is what does happen inside the Arctic circle . . .

But assuming that our early human is watching this from somewhere south of the Arctic circle, one day the sun would stop rising and setting further south on the horizon. It pauses, and then starts rising and setting further north. What a huge relief that would have been! The days would gradually start to lengthen and warmth would return. In time spring would come; grass and crops would start to grow again, and trees would put on leaves.

The day on which the sunrise and sunset stop marching south on the horizons and start marching north is called the day on which the sun stands still: the solstice. And knowing that the sun has stopped marching south, has turned round, and is marching north towards longer and warmer days is a great reason to celebrate! Happy solstice!

Earth's Distance From The Sun

Johannes Kepler was probably the first person to realise that the earth's orbit around the sun was elliptical rather than circular, with the sun at one of the foci of the ellipse.

The earth's greatest distance from the sun is just over 94.5 million miles at a point called the aphelion, from the Greek apo, meaning away, and helios for the sun. The aphelion occurs during the northern hemisphere's summer, a few weeks after the summer solstice.

The earth's least distance from the sun is just over 91.4 million miles at a point called the perihelion. Unfortunately this is rather unmemorably from the Greek peri, meaning around, and helios for the sun. Using the inverse square law, solar radiation at the earth at perihelion is about (94.5/91.4)² = 1.069, or nearly 7%, more intense than it is at aphelion.

The earth's orbital speed is fastest when it is at perihelion and slowest at aphelion, meaning that the northern hemisphere summer is slightly longer than the southern.