Summer Solstice

Image shows the Earth’s tilt towards the Sun.

The summer solstice (or estival solstice), also known as midsummer, occurs when one of the Earth’s poles has its maximum tilt toward the Sun. It happens twice yearly, once in each hemisphere (Northern and Southern). For that hemisphere, the summer solstice is when the Sun reaches its highest position in the sky and is the day with the longest period of daylight. At the pole, there is continuous daylight around the summer solstice. On the summer solstice, Earth’s maximum axial tilt toward the Sun is 23.44°. Likewise, the Sun’s declination from the celestial equator is 23.44°.

The summer solstice occurs during summer This is the June solstice in the Northern Hemisphere and the December solstice in the Southern Hemisphere. Depending on the shift of the calendar, the summer solstice occurs sometime between June 20 and June 22 in the Northern Hemisphere and between December 20 and December 23 in the Southern Hemisphere. The same dates in the opposite hemisphere are referred to as the winter solstice. 

Since prehistory, the summer solstice has been seen as a significant time of year in many cultures, and has been marked by festivals and rituals. Traditionally, in many temperate regions (especially Europe), the summer solstice is seen as the middle of summer and referred to as “midsummer”. Today, however, in some countries and calendars it is seen as the beginning of summer.

What is a Solstice?

A one-year exposure, showing sun trails for the entire year 2014. The Winter Sun is located at the bottom of the image whilst the Summer Sun is at the nighest point.
Image taken in Sashegy, Buda, Budapest, Hungary.

solstice is an event occurring when the Sun appears to reach its most northerly or southerly excursion relative to the celestial equator on the celestial sphere. Two solstices occur annually, around June 21 and December 21. The seasons of the year are determined by reference to both the solstices and the equinoxes.

The term solstice can also be used in a broader sense, as the day when this occurs. The day of a solstice in either hemisphere has either the most sunlight of the year (summer solstice) or the least sunlight of the year (winter solstice) for any place other than the Equator. Alternative terms, with no ambiguity as to which hemisphere is the context, are “June solstice” and “December solstice”, referring to the months in which they take place every year. 

The word solstice is derived from the Latin sol (“sun”) and sistere (“to stand still”), because at the solstices, the Sun’s declinationappears to “stand still”; that is, the seasonal movement of the Sun’s daily path (as seen from Earth) stops at a northern or southern limit before reversing direction.

Solstice Determination

Unlike the equinox, the solstice time is not easy to determine. The changes in solar declination become smaller as the Sun gets closer to its maximum/minimum declination. The days before and after the solstice, the declination speed is less than 30 arcseconds per day which is less than ​160 of the angular size of the Sun, or the equivalent to just 2 seconds of right ascension.

This difference is hardly detectable with indirect viewing based devices like sextant equipped with a vernier, and impossible with more traditional tools like a gnomon or an astrolabe. It is also hard to detect the changes on sunrise/sunset azimuth due to the atmospheric refraction changes. Those accuracy issues render it impossible to determine the solstice day based on observations made within the 3 (or even 5) days surrounding the solstice without the use of more complex tools.

Accounts do not survive but Greek astronomers must have used an approximation method based on interpolation, which is still used by some amateurs. This method consists of recording the declination angle at noon during some days before and after the solstice, trying to find two separate days with the same declination. When those two days are found, the halfway time between both noons is estimated solstice time. An interval of 45 days has been postulated as the best one to achieve up to a quarter-day precision, in the solstice determination. In 2012, the journal DIO found that accuracy of one or two hours with balanced errors can be attained by observing the Sun’s equal altitudes about S = twenty degrees (or d = about 20 days) before and after the summer solstice because the average of the two times will be early by q arc minutes where q is (πe cosA)/3 times the square of S in degrees (e = earth orbit eccentricity, A = earth’s perihelion or Sun’s apogee), and the noise in the result will be about 41 hours divided by d if the eye’s sharpness is taken as one arc minute.

Astronomical almanacs define the solstices as the moments when the Sun passes through the solstitial colure, i.e. the times when the apparent geocentric longitude of the Sun is equal to 90° (summer solstice) or 270° (winter solstice). The dates of the solstice varies each year and may occur a day earlier or later depending on the time zone. The solstices always occur between June 20 and 22 and between December 20 and 23 with the 21st and 22nd being the most common dates.

Midsummer

Midsummer is the period of time centered upon the summer solstice, and more specifically the northern European celebrations that accompany the actual solstice or take place on a day between June 19 and June 25 and the preceding evening. The exact dates vary between different cultures. The undivided Christian Church designated June 24 as the feast day of the early Christian martyr St John the Baptist, and the observance of St John’s Day begins the evening before, known as St John’s Eve.

These are commemorated by many Christian denominations, such as the Roman Catholic Church, Lutheran Churches, and Anglican Communion. In Sweden the Midsummer is such an important festivity that there have been serious discussions to make the Midsummer’s Eve into the National Day of Sweden, instead of June 6. It may also be referred to as St. Hans Day.

Nativity of St. John the Baptist

Midsummer/Saint John’s Day bonfire with festivities in front of a Christian calvary shrine in Brittany, 1893

Saint John’s Day, the feast day of Saint John the Baptist, was established by the undivided Christian Church in the 4th century AD, in honour of the birth of the Saint John the Baptist, which the Gospel of Luke records as being sixth months before Jesus. As the Western Christian Churches mark the birth of Jesus on December 25, Christmas, the Feast of Saint John (Saint John’s Day) was established at midsummer, exactly sixth months before the former feast.

Professor Éamonn Ó Carragáin, University College Cork

By the sixth century, this solar cycle was completed by balancing Christ’s conception and birth against the conception and birth of his count, John the Baptist. Such a relationship between Christ and his cousin was amply justified by the imagery of scripture. The Baptist was conceived six months before Christ (Luke 1:76); he was not himself the light, but was to give testimony concerning the light (John 1:8–9). Thus John’s conception was celebrated on the eighth kalends of October (24 September: near the autumn equinox) and his birth on the eighth kalends of July (24 June: near the Summer solstice). If Christ’s conception and birth took place on the ‘growing days’, it was fitting that John the Baptist’s should take place on the ‘lessening days’ (‘diebus decrescentibus’), for the Baptist himself had proclaimed that ‘he must increase; but I must decrease’ (John 3:30). By the late sixth century, the Nativity of John the Baptist (24 June) had become an important feast, counterbalancing at midsummer the midwinter feast of Christmas.

Within Christian theology, this carries significance as John the Baptist “was understood to be preparing the way for Jesus”, with John 3:30 stating “He must increase, but I must decrease”; this is symbolized in the fact that the “sun begins to diminish at the summer solstice and eventually increases at the winter solstice.” By the 6th century AD, several churches were dedicated in the honour of Saint John the Baptist and a vigil, Saint John’s Eve, was added to the feast day of Saint John the Baptist and Christian priests held three Masses in churches for the celebration.

Wales

In Wales it is called Gŵyl Ifan, or Gŵyl Ifan Ganol Haf (St John’s of Midsummer) to distinguish it from Gŵyl Ifan Ganol Gaeaf (St John’s of Midwinter, the feast of John the Evangelist). Great agricultural fairs used to be held at this time, along with merriment and dancing. A bonfire was also kept this night. With the advent of non-conformist beliefs on the Welsh socio-political culture, this (among so many other similar festivals) suffered greatly, and its observance finally died out in south-east Wales by the end of the 19th century. However, since 1977, a folk-dance revival started in Cardiff, and is held now annually on this feast day.

Bryn Celli Ddu

Bryn Celli Ddu/brʌn keɪli ði/ is a prehistoric site on the Welsh island of Anglesey located near Llanddaniel Fab. Its name means ‘the mound in the dark grove’. It was archaeologically excavated between 1928 and 1929. Visitors can get inside the mound through a stone passage to the burial chamber, and it is the centrepiece of a major Neolithic Scheduled Monument in the care of Cadw. The presence of a mysterious pillar within the burial chamber, the reproduction of the ‘Pattern Stone’, carved with sinuous serpentine designs, and the fact that the site was once a henge with a stone circle, and may have been used to plot the date of the summer solstice have all attracted much interest.

Bryn Celli Ddu is generally considered to be one of the finest passage tombs in Wales. Unlike many stone chambered tombs, this not only has a complete passage and burial chamber, but is also buried under a mound or cairn, although this was re-instated following its excavation in 1929. As it now stands, the passage is 8.4 m (28 ft) long, the first 3.4 m (11 ft) being unroofed with a pair of portal stones. The main passage runs between vertical slab rocked walls roofed by a series of stone lintels. The mound, being substantially smaller than as originally made, no longer completely encloses the burial chamber, so the back wall is open to the air, allowing some natural light in.

Free-standing inside the burial chamber is a smooth pillar of blueschist, a metamorphic rock, some 2 m (6.6 ft) high, with a very rounded shape.

Beyond the back wall of the chamber, in a location that would once have been within the mound, is a replica of the ‘Pattern Stone’. This was found buried under the mound, and has been put standing up in what is thought to have been its original location at a time when the site was a henge rather than a tomb. The patterns take the form of sinuous serpentine shapes that wind around both sides of the stone. Inside the tomb another stone has a small spiral pattern chipped into it, although its authenticity has been questioned.

Outside the tomb, a ring of kerbstones shows the original extent of the mound, and they also follow the line of the ditch of the earlier henge monument. Three of the stones, visible within the cairn mound, are thought to be from the stone circle of that time.

The passage is roughly aligned with the Summer Solstice sunrise, such that for some weeks around the summer solstice, sunlight can find its way through to the back wall of the burial chamber.

The monument is part of a cluster of Neolithic and Bronze Age features. Two further cairns have been identified just to the south of Bryn Celli Ddu, while in the field immediately to the west is a standing stone, and a rock outcrop with cupmarks carved into it.

 

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