Every year, there are two solstices: one in the winter and one in the summer. The summer solstice happens when the Earth's axis is most inclined towards the sun and directly above the Tropic of Cancer. The winter solstice falls on the same day in June as the summer solstice does in December. These events can only happen once every year because the axial tilt of the earth changes over time due to tectonics and other processes.
The angle between the Earth's axis and its orbit around the Sun is called "obliquity". It is different at each location on Earth, but on average it is 23.4 degrees. When obliquity is 0 degrees, the Earth is considered to be in perihelion, which means it is closest to the sun. When obliquity is 90 degrees, the Earth is in aphelion, which means it is farthest from the sun.
During a full moon, when the center of mass of the Moon is behind the Earth, we see a shadow cast onto the surface. At least part of this shadow is visible from anywhere on Earth during a full moon. This is because the axis about which the shadow rotates is not vertical, but rather tilted by about 5.5 degrees to the plane of the Earth's orbit around the Sun. So even though the center of mass of the Moon is located beyond the Earth's atmosphere, parts of it still block out sunlight during a full moon.
When the Earth's axis is pointing straight toward the Sun, the solstices occur. During Earth's orbit, this occurs twice a year. On June 21, the north pole is inclined 23.5 degrees toward the sun, and the northern hemisphere celebrates the summer solstice, the longest day of the year. On December 21, the south pole is inclined 23.5 degrees away from the sun, and the southern hemisphere experiences its winter solstice, the shortest day of the year.
The axis of the Earth is not perfectly perpendicular to its surface; instead, it makes an angle with the ground called the latitude. All around the world, the highest points are found in regions where the rocks are tilted under their own weight, as opposed to being pushed over by pressure from above. These high places are called mountain ranges. The direction that these range walls point is called a compass point; each one is separated from its neighbors by 90 degrees (or a full circle).
At the center of each compass point is a location where the rock has been forced upward against the force of gravity; this happens when there are gaps in the range wall below it. The gap allows water to flow in and create lakes or ponds where the rock is dry land elsewhere. This center location is called a promontory. During the summer solstice, most of North America is at a latitude where there are no mountains nearby that would cause the area to be tilted either way.
The summer solstice happens when the earth's axis is at its most tilted towards the sun, causing the day to last longer than the night. On this day, we get the longest time of daylight since the sun is at its highest point in the sky. The Summer Solstice occurs on June 21st this year.
The shortest day of the year is January 20th, known as New Year's Day. This is because it is the first day of the civil year and many people will be celebrating with family and friends. The winter solstice is when the days start getting longer again; December 21st is the end of the Christmas season and the beginning of Advent.
The ancients marked the beginning of spring and the end of winter by observing the tides. At low tide, you can go fishing, while at high tide you should stay away from sea water because it is too dangerous. They also used astronomical observations such as the vernal equinox and the summer solstice to determine when to plant crops and animals feed.
Modern science has confirmed that the ancient knowledge was correct: The tides are caused by gravitational forces that vary depending on the position of the moon and the earth. Plants and animals spread out across the planet according to these forces so they can find the sunlight and moisture they need to survive.
Solstices occur because the Earth's axis of rotation is tilted by approximately 23.4 degrees relative to the Earth's orbit around the sun. At the same time that the Northern Hemisphere experiences its summer solstice (which always occurs around June 21), the Southern Hemisphere experiences its winter solstice. The opposite is true in January: at the winter solstice, the Northern Hemisphere is experiencing spring and the Southern Hemisphere is experiencing fall.
The exact date of the summer solstice will always be on either June 20 or June 28, depending on which day is longer. If it is the former, then we have the summer solstice; if it is the latter, then we have the autumnal equinox. Because these events are tied to the position of the Earth around its own axis, they do not depend on the location of the sun but instead take place at any time between April 19 and October 22.
During a solar eclipse, the moon blocks out the sun, causing darkness where you can see the shadow of the moon. At the summer solstice, there is no part of the earth where the sun is completely hidden at any time during the day. All over the northern hemisphere, people see some form of eclipse every day from about June 20 to July 27. These eclipses are all partial eclipses, meaning that some part of the sun is still visible after sunset.
The first day of summer (June solstice) is when the sun is directly over the Tropic of Cancer. This is known as "astronomical summer" or "summer solstice." The second day of summer (July 2nd) is when the sun is directly over the Arctic Circle.
The third day of summer (August 3rd) is when the sun is directly over the Antarctic Circle.
The fourth day of summer (September 23rd) is when the sun is directly over the North Pole.
The fifth day of summer (October 28th) is when the sun is directly over the South Pole.
Summer in the Northern Hemisphere ends on October 30th; in the Southern Hemisphere it starts now.
The summer solstice occurs between June 20 and June 22, depending on the calendar change. In the Southern Hemisphere, the December solstice occurs between December 20 and December 23. In the opposite hemisphere, the same dates are known as the winter solstice. The summer and winter solstices always occur on the same days of the month and usually around the same time, but they do not have to.
In Europe and North America, the summer solstice is when the sun is highest in the sky and temperatures are at their peak. But in Australia and New Zealand, due to our distance from the sun, the summer solstice is when the sun is lowest in the sky and temperatures begin to drop off after that first hot day of summer.
An important thing to note is that although it is called the "summer" solstice in the Southern Hemisphere, there is no such thing as a "winter" solstice there because of how far away we are from the sun. Instead, the term "solstice" refers to the fact that it is the longest day of the year and the shortest night.
In addition to being high in the sky, the summer solstice also happens to be when the earth is closest to the sun. So even though it's not as warm as other times of the year, you will still get some sunlight during the day and night will not last very long.