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December 21 2018 – the solstice

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This 12 months, the northern hemisphere winter solstice will fall on 21 December in Britain.  On this date there are the fewest hours of daylight and the Solar is at its lowest most elevation.

The origin of the phrase solstice is from two Latin phrases:  sol, which suggests Solar, and sistere, to face nonetheless. On the time of the winter solstice the Solar stops getting steadily decrease within the noon sky on successive days, seems to face nonetheless on the identical peak for a couple of days, after which begins to get larger within the sky. The reverse is true for the summer time solstice. At the moment, the Solar stops getting larger within the noon sky on successive days, seems to face nonetheless on the identical peak for a couple of days, after which begins to get decrease within the sky.

The graph above reveals the utmost elevation of the Solar, measured in levels throughout December 2018. This information is for Manchester which lies at a latitude of 53.5 levels latitude North.  

The truth that the Solar’s elevation modifications very slowly within the days across the solstice additionally signifies that the overall hours of daylight change little or no. That is proven within the desk under, which supplies the dawn and sundown occasions and the quantity of daylight in hours, minutes and seconds for Manchester.

Information from Time and Date (2018).

The information additionally present that, though the winter solstice happens on the date with the least daylight, the date with earliest sundown in Manchester is about 10 days earlier. The explanation for that is mentioned in my earlier submit: The darker mornings 

Exact astronomical definition of the solstice

The diagram under reveals the Earth’s orbit across the Solar. For readability the sizes of the Earth and Solar have been tremendously exaggerated in relation to the Earth-Solar distance.

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Seasons

  • Throughout June, marked as Awithin the diagram, the Earth’s North Pole is tilted in the direction of the Solar and the times are longer within the northern hemisphere.
  • Throughout December, marked as Cwithin the diagram, the Earth’s South Pole is tilted in the direction of the Solar and days are longer within the southern hemisphere.
  • At factors Band D, often known as the equinoxes, neither pole is tilted in the direction of the Solar and the quantities of daylight within the northern and southern hemisphere are equal.

 

The exact astronomical definition of the December solstice (the winter solstice within the northern hemisphere) is the actual time limit when the North Pole is tilted furthest away from the Solar. Many individuals assume that the solstice at all times happens on the 21st, however the precise dates and occasions for this occasion over the following 4 years are given within the desk under – in GMT, in Beijing (which is eight hours forward of GMT) and in Honolulu, Hawaii (which is 10 hours behind GMT).

 

For the small variety of individuals residing in American Samoa within the mid-Pacific, which is 11 hours behind GMT, the December solstice will happen at 11:02 pm on 20 December within the 12 months 2020.

Significance of the solstices to early man

Each the winter and summer time solstices have been of nice significance to early man, and lots of prehistoric websites have been constructed to rejoice them. Essentially the most well-known of those is Stonehenge in England, which consists of a horseshoe association of stones inside a big stone circle, constructed between 4 and 5 thousand years in the past. It’s an incredible feat of development by stone age man; the stone circle is over 30 metres in diameter and its largest stones are greater than 9 metres tall and weigh over 25 tonnes.

Stonehenge

Picture from Wikimedia commons

The horseshoe association consists of 5 arches referred to as triliths, every made up of three stones.  The open aspect of the horseshoe factors north east in the direction of a massive stone 80 metres away from the primary circle. At the moment this huge stone is named ‘The Heel Stone’.

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Stonehenge_Heelstone

Picture from Wikimedia commons

For a couple of days both aspect of the June solstice, somebody standing within the centre of the horseshoe and dealing with north east will see the Solar rise over the Heel Stone.

Heel Stone Sunrise

How dawn on the summer time solstice at Stonehenge would have sorted the monument’s development.

Additionally, for a couple of days both aspect of the December solstice, somebody standing on the Heel Stone dealing with south west will see the Solar set in the midst of the central trilith. Thus, Stonehenge marks each dawn on the summer time solstice and sundown on the winter solstice.

How Stonehenge initially seemed from above

 

It’s superb that prehistoric man constructed such a big monument to line up with the solstices. Nonetheless, these would have been main occasions for a individuals residing outside with solely pure daylight and whose lives have been tied in to the cycle of the seasons. The arrival of the winter solstice signified that though the times would begin to get longer there would very doubtless be a interval of colder climate the place meals can be briefly provide for these hunter-gatherers.

Over 38,000 discarded animal bones have been discovered at Durrington Partitions, a big Neolithic settlement three.2 km away from Stonehenge. These stays have been dated to be round four,500 years outdated and symbolize at the very least 1,000 animals, of which 90% have been pigs and the remainder cattle.

Many of the pigs had been killed at about 9 months outdated. For the reason that piglets have been most likely born within the early spring, they have been all slaughtered across the time of the solstice. A number of the bones nonetheless had meat hooked up suggesting that they have been being eaten at a time of loads which suggests that individuals have been gathering for feasts right now of 12 months.

 

Fashionable teams with historical origins equivalent to Druids and Pagans, who revere the pure world greater than most fashionable people, rejoice the winter solstice at Stonehenge. Annually there’s a gathering of a number of thousand who really rejoice the primary dawn after the winter solstice, regardless that Stonehenge is aligned for sundown.

To forestall injury to such an vital historical monument it’s not usually attainable to get proper as much as the stones. Nonetheless, English Heritage, the charity which manages the location, opens it up yearly from round 07:30 am to 10:00 am within the morning for the solstice celebrations, giving individuals a uncommon probability to rise up shut.

Senior Druid King Arthur Pendragon at Stonehenge. To the Druids the winter solstice is taken into account extra vital than its summer time counterpart because it marks the ‘re-birth’ of the solar

For the BBC report on the 2017 Stonehenge winter solstice celebrations click on on the hyperlink under.

https://www.bbc.co.uk/news/uk-england-wiltshire-42453915

The southern hemisphere

To these of you who dwell within the southern hemisphere the December solstice is the summer time solstice, when the noon Solar is at its highest within the sky. After the solstice the days begin getting steadily shorter and the nights steadily longer, though the change doesn’t actually turn out to be noticeable till July.

Observe

 

Though nonetheless in widespread use, the time period Greenwich Imply Time (GMT) is not utilized by astronomers.   As an alternative, they use two totally different occasions which agree with one another to inside 1 second:

  • Common Time, typically abbreviated to UT1, is the imply photo voltaic time, the time decided by the rising and setting of the Solar on the Greenwich Meridian, zero levels longitude.
  • Co-ordinated Common Time, normally abbreviated to UTC, is the time measured by atomic clocks and is saved to inside 1 second of UT1 by the addition of leap seconds.

In widespread use, GMT is commonly taken to be the identical as UTC, which is the method I’ve taken for this submit. Nonetheless, it may also be taken to imply UT1.  GMT is often prevented in exact writing, as a result of it’s not at all times clear whether or not UTC or UT1 is supposed, and since timekeeping legal guidelines normally consult with UTC.


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