The sky — astronomy

Solstice: The Longest and Shortest Day

A solstice is the moment when the Sun reaches its northernmost or southernmost point relative to the celestial equator. It happens twice a year, around 21 June and 21 December. The cause is Earth’s axial tilt of about 23.4°, not a change in distance from the Sun.

What a solstice is

A solstice is the moment when the Sun reaches its greatest angular distance north or south of the celestial equator. That distance is about 23.4°, the same as the tilt of Earth’s axis. The word comes from the Latin sol (Sun) and sistere (to stand still): in the days around a solstice the Sun’s noon height barely changes, as if it paused before turning back.

There are two solstices each year, around 20–21 June and around 21–22 December. A solstice is an exact instant, not a whole day. When people say “the longest day,” they mean the calendar day on which that instant falls. The date can shift slightly from year to year because the calendar year does not exactly match the astronomical year and leap years correct for this. From place to place, it can differ because of time zones.

Why a day is longest or shortest

The seasons, and the changing length of daylight, come from Earth’s axial tilt. The axis points in nearly the same direction in space as Earth orbits the Sun, so for part of the year the northern hemisphere leans toward the Sun and for part of the year it leans away.

A common misconception is that summer happens when Earth is closer to the Sun. In fact Earth is closest to the Sun in early January, in the middle of northern winter. The change in distance has a small effect compared with the tilt.

The Sun’s highest and lowest points

Over a year the Sun traces a path across the sky called the ecliptic. It is tilted relative to the celestial equator, which is why the Sun climbs higher and sinks lower in the sky through the year.

• At the June solstice the Sun is at its northernmost point. For northern observers it rises far to the northeast, reaches its highest noon position, and the day is longest. At that moment the Sun is directly overhead at the Tropic of Cancer, about 23.4° north latitude.
• At the December solstice the Sun is at its southernmost point. In the northern hemisphere it stays low, its daily arc is short, and the day is shortest. It is overhead at the Tropic of Capricorn, about 23.4° south.

Halfway between, when the Sun crosses the celestial equator, come the equinoxes. Together, these four points frame the seasons.

Northern and southern hemispheres

The solstices are opposite in the two hemispheres. When the northern hemisphere has its summer solstice in June, the southern one has its winter solstice, and the reverse in December. In Australia or Chile, the longest day is around December.

How much day length changes depends on latitude:

• At the equator, day length stays close to 12 hours all year, and solstices barely change it.
• The farther you are from the equator, the bigger the gap between the longest and shortest day.
• North of the Arctic Circle (about 66.5° N), the Sun does not set around the June solstice, the midnight Sun, and does not rise around the December solstice, the polar night. It is the other way round south of the Antarctic Circle.

A detail that is often missed

The shortest day does not coincide with the earliest sunset, nor the longest day with the latest. At mid-northern latitudes the earliest sunset comes in early December and the latest sunrise in early January. The reason is that solar time, as shown by a sundial, drifts away from the even time of our clocks during the year, an effect called the equation of time. The total length of daylight, however, really is smallest at the December solstice.

Solstices in ancient monuments

Many ancient societies built with these moments in mind. The study of such links between structures and the sky is called archaeoastronomy.

• Stonehenge in England has a solar axis. At the June solstice the Sun rises over the Heel Stone along the monument’s main avenue, and at the December solstice the sunset lines up with the same axis in the opposite direction. The monument was built in stages thousands of years ago.
• Newgrange in Ireland is a stone passage tomb about 5,000 years old. Around the December solstice sunrise, a beam of light enters through an opening above the entrance and lights the inner chamber for a short time.

We may never fully know what these alignments meant to their builders. Researchers are careful here: the alignment can be measured, while its meaning remains a subject of study.

Festivals around the solstices

Solstices have long been occasions for celebration. These are cultural traditions, not astronomical facts:

• In northern Europe the winter solstice is linked with Yule and the summer solstice with Midsummer, marked with bonfires and flowers.
• In China, the Dongzhi festival around the December solstice is a time for family gatherings.
• In the Andes, Inti Raymi honors the Sun around the June solstice, which is winter in the southern hemisphere.
• In Bulgaria, Enyovden on 24 June is tied in folk tradition to midsummer and herb gathering.

In short

Solstices show how Earth is tilted relative to the Sun. They are the clearest visible sign of the yearly rhythm of light: from the shortest day, after which daylight begins to grow, to the longest, after which it slowly fades. For the bigger picture, see also the Sun.

Positions in Astronativa are astronomically exact. The interpretation is for inspiration. · Glossary · Terms · Privacy