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 in the south.
The June solstice is the moment the Sun is directly above the Tropic of Cancer in the Northern Hemisphere. This is the northernmost latitude it reaches during the year. After the solstice, it begins moving south again.
Longest Day in the North
Since the Northern Hemisphere is tilted toward the Sun in June, it receives more sunlight during the course of a day. The North Pole's tilt toward the Sun is greatest at the solstice, so this event marks the longest day of the year north of the equator.
This effect is greatest in locations that are farther away from the equator. In tropical areas, the longest day is just a little longer than 12 hours; in the temperate zone, it is significantly longer; and places within the Arctic Circle experience Midnight Sun or polar day, when the Sun does not set at night.
Shortest Day in the South
Conversely, the day of the June solstice is the shortest day of the year in the Southern Hemisphere. Here, too, the effect is greater the farther a location is away from the equator.
Places within the Antarctic Circle experience polar night, when the Sun does not rise at all.
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Why Is It Called a “Solstice?”
During a year, the subsolar point—the spot on the Earth's surface directly beneath the Sun—slowly moves along a north-south axis. Having reached its southernmost point at the December solstice, it stops and starts moving northward until it crosses the equator on the day of the March equinox. At the June solstice, which marks the northernmost point of its journey, it stops again to start its journey back toward the south.
This is how the solstices got their name: the term comes from the Latin words sol and sister, meaning “Sun” and “to stand still”.
Initially, the naming arose from observations of how the Sun’s apparent path across the sky changes slightly from one day to the next, which is caused by the same process as the subsolar point's movement described above.
In the months leading up to the June solstice, the position of sunrise and sunset creeps northward. On the day of the solstice, it reaches its northernmost point. After that, the daily path of the Sun across the sky begins to creep southward again.
Earth: The living planet
The Sun: Our home star
Why Does the Sun Move North and South?
The subsolar point moves north and south during the year because the Earth’s axis is tilted at an angle of about 23.4° in relation to the ecliptic, an imaginary plane created by Earth’s path around the Sun. In June, the Northern Hemisphere is tilted toward the Sun, and the subsolar point is north of the equator. As the Earth travels toward the opposite side of its orbit, which it reaches in December, the Southern Hemisphere gradually receives more sunlight, and the subsolar point travels south.
The Solstices and the Seasons
The June solstice marks the start of summer in the Northern Hemisphere and the start of winter in the Southern Hemisphere, according to one definition.
Equinox and solstice dates—years 1-2149
Sunrise and Sunset Times Lag Behind
The longest day of the year is commonly associated with the earliest sunrise and the latest sunset of the year. However, in most locations, the earliest sunrise happens a few days before the solstice, while the latest sunset occurs some days after it.
The June Solstice in the Calendar
Even though most people consider June 21 as the date of the June solstice, it can happen anytime between June 20 and June 22, depending on the time zone. June 22 solstices are rare—the last June 22 solstice took place in 1975, and there won't be another one until 2203.
Note: All dates refer to Coordinated Universal Time (UTC). Local dates may vary depending on the time zone.
Why Does the Date Vary?
The date of the equinoxes and solstices varies because a year in our calendar does not exactly match the length of the tropical year—the time it takes the Earth to complete an orbit around the Sun.
Today's Gregorian calendar has 365 days in a common year and 366 days in a leap year. However, our planet takes about 365.242199 days to orbit the Sun. This means that the timing of the equinoxes and solstices slowly drifts apart from the Gregorian calendar, and the solstice happens about 6 hours later each year. Eventually, the accumulated lag becomes so large that it falls on the following date.
To realign the calendar with the tropical year, a leap day is introduced (nearly) every four years. When this happens, the equinox and solstice dates shift back to the earlier date again.
Other factors influencing the timing of the equinoxes and solstices include variations in the length of a tropical year and in the orbital and daily rotational motion of the Earth, such as the “wobble” in the Earth's axis (precession).
Source: Time and Date
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