Summer Begins
Summer begins in the Northern Hemisphere around June 21, marked by the summer solstice, when the sun reaches its highest point in the sky and daylight hours are longest. This astronomical event occurs due to the tilt of the Earth's axis and its elliptical orbit around the sun. The summer solstice is one of four key astronomical markers of the seasons, alongside the vernal equinox, autumnal equinox, and winter solstice. Culturally, summer is often associated with agriculture, as it represents the peak growing season, benefiting from longer days and warmer temperatures.
In addition to its scientific significance, summer has rich cultural implications, inspiring festivals and traditions that celebrate the season’s warmth and abundance. For example, Midsummer Day on June 24 has historical roots in ancient solar rites. The symbolism of summer, often depicted through art and folklore as a woman with grain and a sickle, reflects humanity's longstanding connection to the cycles of nature. In the Southern Hemisphere, summer occurs from December 21 to March 21, showcasing the global nature of this season, despite differing seasonal experiences.
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Summer Begins
In the Northern Hemisphere, summer begins on approximately June 21 of every year. The precise moment at which the sun reaches the summer solstice, officially marking the change of season, differs slightly from one year to the next because of the multitude of oscillations and wobbling motions that Earth manifests during its daily rotation on its axis and its annual elliptical orbit around the sun.
Each of the four seasons has a precise astronomical beginning. The ecliptic, the plane in which the sun appears to revolve around Earth and in which Earth actually revolves around the sun, is divided into four 90-degree sections. Each section starts with a definite point: two equinoxes and two solstices. Summer begins at the summer solstice (from the Latin solstitium, derived from sol, “sun,” and sistere, “to stand still”), which takes place midway between the vernal equinox, marking the start of spring, and the autumnal equinox, marking the start of fall. The period of time in which the sun traverses the 90-degree section from the summer solstice to the autumnal equinox is termed the season of summer.
Summer is also said, somewhat anachronistically, to begin when the sun enters the zodiac sign of Cancer, the crab. In the night sky, the constellations of the zodiac approximately coincide with the ecliptic; as such, ancient Babylonian astronomers divided the ecliptic into twelve sections, each measuring 30 degrees celestial longitude and corresponding roughly to one of the zodiac constellations. This system was later taken up by the ancient Greeks, who fixed the zero point of celestial longitude—the beginning of the section corresponding to Aries—as the position of the sun on the vernal equinox, which by extension meant that the summer solstice began when the sun entered the section corresponding to Cancer (and the winter solstice began when the sun entered Capricorn; hence the names "tropic of Cancer" and "tropic of Capricorn" for the lines of latitude defining the northern and southern limits of the tropics, respectively). The second-century Greek astronomer Hipparchus, who is credited with discovering the phenomenon of axial precession and the resulting retrograde motion of the equinoctial points, correctly predicted the gradual cyclical displacement of the constellations in regard to those points. After a slow-motion cycle now known to last 25,784 years, the constellations will again be in the same positions as in Hipparchus's time, when the summer solstice was found in Cancer. In the meantime, precession has caused the summer solstice to shift into Gemini and then, as of 1990, into Taurus.
Around June 21 each year, the sun reaches its greatest northern declination (astronomical corollary of terrestrial latitude). At this point in time, the period of daylight is at its longest, and the period of nighttime is at its shortest. During the course of summer, the sun's northern declination constantly decreases, and the days gradually shorten while the nights gradually lengthen. As observed from the terrestrial equator, the sun crosses closer and closer to the celestial equator until it reaches the autumnal equinox, located at a declination of 0 degrees and a right ascension (corollary of longitude) of 12 hours—originally the beginning of Libra, now in Virgo. The season of summer then terminates.
At the summer solstice, when the North Pole reaches its greatest degree of tilt towards the sun, the amount of solar radiation absorbed by the Northern Hemisphere is at its highest, and so summer is the hottest season of the year. In the Southern Hemisphere, the seasons are reversed, since when Earth's North Pole is tilted towards the sun, the South Pole is tilted away from it. Astronomical summer in the Southern Hemisphere starts around December 21, the same day as the winter solstice in the north, and ends around March 21.
Because of the extended days of sunshine and the resulting period of high temperatures, summer is the prime growing season for plant life. Like the other seasons, it has influenced the development of mythology and folklore. For example, Midsummer Day (June 24), with its elaborate solar rites, was an especially important festival in ancient and medieval times. There are still many other festivals and events also associated with summer, taking their impetus in whole or in part from the characteristic climate of the season. In ancient and medieval art, summer was frequently personified as a woman carrying sheaves of grain and a sickle.
Bibliography
Franklin, Anna. Midsummer: Magical Celebrations of the Summer Solstice. Llewellyn Publications, 2003.
Grieser, Justin. "Summer Has Arrived—5 Things to Know about the Longest Day of the Year." The Washington Post, 21 June 2018, www.washingtonpost.com/news/capital-weather-gang/wp/2018/06/20/summer-solstice-is-thursday-5-things-to-know-about-the-longest-day-of-the-year/. Accessed 1 May 2024.
Hughes, David W. "Neolithic and Early Bronze Age Skywatchers and the Precession of the Equinox." Journal of the British Astronomical Association, vol. 115, no. 1, 2005, pp. 29–35. Academic Search Complete, search.ebscohost.com/login.aspx?direct=true&db=a9h&AN=15861479&site=ehost-live. Accessed 21 June 2018.
Kaler, James B. "Measuring the Sky: A Quick Guide to the Celestial Sphere." James B. (Jim) Kaler, U of Illinois, stars.astro.illinois.edu/celsph.html. Accessed 1 May 2024.