There are two solstices every year: One in June and one in December. The June solstice marks the longest day north of the equator and the shortest day south of it.
The June solstice is the moment in which the Sun reaches its northernmost point in the sky during the year. After this, it begins to move south from there.
Longest Day in the North
In June, the Northern Hemisphere is tilted towards the Sun and receives more sunlight during the course of a day. The North Pole’s tilt toward the Sun increases towards the solstice, so this event marks the longest day of the year at high latitudes.
The effects of Earth’s axis tilt are most noticeable in places that are further away from the equator. In tropical locations, the longest day of the year is still just a hair over 12 hours. In the temperate zone, it is significantly longer; and places within the Arctic Circle experience Midnight Sun or polar days when the Sun does not set at night.
Shortest Day in the South
On 21st June, the sun’s rays fall vertically over the Tropic of Cancer. This position of the Earth is called the Summer Solstice. On this day, it is considered the longest daylight time in the Northern Hemisphere, but in the Southern Hemisphere, it is considered the shortest daylight time. The longest day in the Southern Hemisphere is December 22nd, which is the winter solstice.
On December 22, the Tropic of Capricorn receives direct sunlight as the south pole tilts towards it. As the sun’s rays fall vertically at the Tropic of Capricorn (231/2° s), a larger portion of the southern hemisphere gets light. Therefore, it’s summer in the southern hemisphere with longer days and shorter nights. The reverse happens in the northern hemisphere. This position of the earth is called the winter solstice.