夏至（the summer solstice)是24节气中的一个重要节气，意味着夏天的正式开始。
夏至日太阳几乎直射北回归线，北半球白昼最长，北极圈及其以北的地区太阳整日不落，形成面积最大的极昼区。此日过后，阳光直射位置向南移动，北半球白昼逐渐减短，日出、日落的方位也开始往南移动。在北回归线及其以北地区，正午太阳高度亦日渐降低，日影逐渐被拉长。但由于太阳辐射到地面的能量仍比地面向空中发散的多，所以在短时间内气温继续升高。
“至”也有极点的意思。在传统的阴阳理论中，夏至是阳气生发到极点，开始收藏的节气。
What is the summer solstice?
The summer solstice is the longest day of the year - the day in which we can enjoy approximately 17 hours of daylight.
It might seem like good news, but for those of us in the northern hemisphere this is the time when the sun's path stops moving northward in the sky and the days begin to slowly shorten.
When is it?
In the northern hemisphere, summer solstice takes place between June 20 and 22.This year it's today, Monday, June 20. The sun rose at 4.45am and sunset will happen at 10.34pm.
The winter solstice (the shortest day of the year) falls between December 20 and 22 in the UK. This year it's on Wednesday, December 21.
Here's a list of the dates and times of solstices & equinoxes this year.
Vernal Equinox (Spring) March 20 2016 04:30 GMT
Summer Solstice (Summer) June 20 2016 22:34 GMT
Autumnal Equinox (Fall) September 22 2016 14:21 GMT
Winter Solstice (Winter) December 21 2016 10:44 GMT
So the days will be shorter now?
Yes, but we won't notice a difference for a while. The shortest day of the year isn't until December - known as the winter solstice.
The June solstice happens when the tilt of Earth's axis is most inclined towards the sun, directly over the Tropic of Cancer, so that's why we get the most daylight of the year.
During the winter solstice, the Earth's axis is tilted furthest away from the sun directly over the Tropic of Capricorn bringing only a few hours of daylight.
The shortest day of the year lasts for 7 hours 49 minutes and 41 seconds in Britain. This day is 8 hours, 49 minutes shorter than the June solstice.
Read everything you need to know about winter solstice here.
Why is Stonehenge so significant?
Stonehenge in Avebury, Wiltshire is the most popular place for Pagans to celebrate the longest day because it famously aligns to the solstices. The rising sun only reaches the middle of the stones one day of the year when it shines on the central alter.
Built in three phases between 3,000 B.C. and 1,600 B.C Stonehenge's exact purpose still remains a mystery. The stones were brought from very long distances – the bluestones from the Preseli Hills more than 150 miles away, and the sarsens probably from the Marlborough Downs, 19 miles to the north.
This year, the midsummer solstice is being celebrated at Stonehenge on Saturday into Sunday and at the Avebury stone circle from Friday until Monday.
The English Heritage-run site is expected around 20,000 visitors.
What's all the fuss about?
The day marks the ancient middle of summer, even though we haven't had the hottest day.
It has significance for pagans who have always believed that midsummer day holds a special power. Midsummer's eve was believed to be a time when the veil between this world and the next is at its thinnest, and when fairies were though to be at their most powerful.
Over the centuries, the June solstice has inspired many festivals and midsummer celebrations involving bonfires, picnics, singing, watching the sun rise and Maypole dancing. Many towns and villages across Britain still mark the day.
One ritual was the lighting of fires, heralding the start of shorter days, although this doesn't really happen anymore. The idea was that flames would keep the dark away.
It's also a 'strawberry' moon...
For the first time since 1967 the summer solstice coincides with a rare ‘strawberry’ moon and – clouds willing – the 17 hours of sunlight will give way to a bright moonlit sky.
Despite the name, the moon will not appear pink or red, although it may glow a warm amber. The romantic label was coined by the Algonquin tribes of North America who believed June’s full moon signalled the beginning of the strawberry picking season.
Did you know?
Traditionally, this summer solstice period fell between the planting and harvesting of crops, leaving people who worked the land time to relax. This is why June became the traditional month for weddings.
The first (or only) full moon in June is called the 'honey moon' because many believed it was the best time to take honey from beehives.
