Solstice and Equinox: How Astronomical Events Shape the Seasons

Solstice and Equinox: Astronomical Events That Define the Change of Seasons

Our planet is constantly in motion, rotating around its own axis and traveling along its path around the Sun. It is precisely these movements that create remarkable astronomical phenomena that humanity has observed for thousands of years — solstices and equinoxes. These events not only determine the change of seasons but are also deeply rooted in the cultures of peoples around the world.

What Is a Solstice

A solstice is an astronomical event when the Sun at noon reaches its maximum or minimum height above the horizon. The name of this phenomenon comes from the Old Russian word “solntsevorot” and is explained by the fact that for several days before and after the event, the height of the midday Sun hardly changes — it seems to freeze in one place. This phenomenon occurs twice a year: in summer, around June 20–21, and in winter, around December 21–22. The cause of solstices lies in the specifics of Earth’s motion — our planet rotates around its axis at an angle of approximately 23.4 degrees to the plane of its orbit.

Winter Solstice — the Longest Night of the Year

On the day of the winter solstice, residents of the Northern Hemisphere experience the longest night and the shortest daylight hours of the year. The Sun occupies its lowest position in the sky, and in some northern regions it hardly rises above the horizon. In 2026, the winter solstice will occur on December 21 at 23:49 Moscow time. The length of daylight will vary depending on geographical location — from a complete absence of daylight in Murmansk (polar night) to about nine hours in the southern regions of Russia. This event marks the beginning of astronomical winter, which will last until the spring equinox. Already the day after the solstice, daylight hours begin to gradually increase, adding about seven minutes of daylight by early January.

Summer Solstice — a Festival of Light

The summer solstice occurs between June 20 and 21 and is characterized by the longest day of the year. At this moment, the Sun rises to its maximum height above the horizon, illuminating the Northern Hemisphere most intensively. At the latitude of Moscow, daylight lasts up to 17 hours and 33 minutes. In 2026, the winter solstice will occur on June 21 at 11:24 Moscow time. Interestingly, during the summer solstice observers get the impression that the Sun freezes at its zenith and remains motionless throughout the day. This event symbolizes the beginning of astronomical summer and has been considered a special time endowed with magical power since ancient times.

Equinox — When Day Equals Night

An equinox is an astronomical phenomenon when the Sun crosses the celestial equator, moving from one hemisphere to the other. On these days, the length of day and night across the entire planet is nearly the same and amounts to about 12 hours.

Spring Equinox

Around March 20–21, the spring equinox occurs, when the Sun moves from the southern celestial hemisphere to the northern one. This event marks the astronomical beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere. After this date, days in northern latitudes become longer than nights, and nature awakens from its winter sleep. In 2026, the spring equinox will occur on March 20, opening a period of active growth and renewal in nature.

Autumn Equinox

The autumn equinox occurs around September 22–23, when the Sun makes its reverse movement — from the Northern Hemisphere to the Southern Hemisphere. This moment marks the beginning of astronomical autumn in the Northern Hemisphere. After the autumn equinox, nights become longer than days, air temperatures gradually decrease, and nature prepares for the winter period.

Why These Phenomena Occur

The main reason for the change of seasons and the existence of solstices and equinoxes lies in the tilt of Earth’s axis. Our planet rotates not vertically but at an angle of about 23.4 degrees relative to the plane of its orbit. At the same time, the axis of rotation remains almost parallel to itself throughout the entire annual journey around the Sun. Because of this tilt, different hemispheres of Earth receive unequal amounts of sunlight and heat at different times of the year. When the Northern Hemisphere is maximally tilted toward the Sun, summer begins there and the summer solstice occurs. At the same time, winter and the winter solstice take place in the Southern Hemisphere. Earth’s orbit is also not a perfect circle — it has an elliptical shape. This additionally affects astronomical cycles and explains why the dates of solstices and equinoxes can shift slightly from year to year.

How the Phenomena Differ Between Hemispheres

A remarkable feature of these astronomical events is that they occur simultaneously in both hemispheres but have opposite meanings. When the winter solstice with the longest night takes place in the Northern Hemisphere, the summer solstice with the longest day occurs at the same moment in the Southern Hemisphere. The same principle applies to equinoxes: the spring equinox in the northern part of the planet corresponds to the autumn equinox in the southern part, and vice versa. Thus, while one half of Earth welcomes spring, the other bids farewell to summer and prepares for autumn.

Cultural Significance and Traditions

For thousands of years, the days of solstices and equinoxes have held special significance for humanity. Ancient civilizations built monumental structures oriented toward the position of the Sun on these key dates. The famous Stonehenge in the United Kingdom and Newgrange in Ireland are vivid examples of such architecture.

Slavic Traditions

Ancient Slavs associated the winter solstice with the birth of the sun god Kolyada, who was meant to oppose the dark forces of winter. The celebration was accompanied by the lighting of bonfires symbolizing the rebirth of light and round dances around them. Young people went from house to house singing carols and received treats in return. The summer solstice among the Slavs was the time of celebrating Kupala — a festival dedicated to the flourishing of nature and the power of fire and water. It was believed that on this night water acquired healing properties, and herbs were filled with special strength.

Festivals of Other Peoples

The ancient Romans celebrated the winter solstice with the festival of Saturnalia, which lasted from December 17 to 23. During this time, all work ceased, feasts were held, and people exchanged gifts. The Scandinavian peoples celebrated Yule — a midwinter festival symbolizing the return of light. The tradition of burning a large log, which was supposed to smolder for several days, symbolized the extension of sunlight. In Iran, Shab-e Yalda is still celebrated — a festival on the longest night of the year, when families gather together and eat pomegranates and watermelons, symbolizing dawn and life.

Interesting Facts

The date of the winter solstice is not constant. Julius Caesar in 45 BCE set it on December 25, but due to discrepancies between the calendar and the solar year, it gradually shifted. The Gregorian calendar corrected this situation, although scientists note that the date will shift by about one day every three thousand years. A full moon on the day of the winter solstice occurs extremely rarely. Since 1793, such a coincidence has happened only about ten times, making this event a truly unique astronomical phenomenon. In some regions of Earth located beyond the Arctic Circle, a polar night occurs during the winter solstice, when the Sun does not appear above the horizon for more than 24 hours. In Murmansk, for example, the polar night lasts about 40 days.

Impact on Nature and Climate

Solstices and equinoxes have a significant impact on natural cycles. After the winter solstice, daylight hours begin to increase, triggering biological processes of nature’s awakening, although the coldest months are still ahead. The spring equinox serves as a signal for many plants and animals — a period of active growth begins, birds migrate, and animals awaken from hibernation. The summer solstice marks the peak of the growing season, while the autumn equinox signals the need to prepare for winter.

Conclusion

Solstices and equinoxes are not just astronomical phenomena, but important milestones of the annual cycle that influence climate, nature, and human culture. Understanding these processes helps us more deeply realize our place in the cosmic dance of planets and stars, as well as maintain a connection with the ancient traditions of our ancestors, who observed the movement of the celestial body with reverence.