[Graphic Source: © Copyright 2005, Eric G. Canali, former Floor Operations Manager of the original Buhl Planetarium and Institute of Popular Science (a.k.a. Buhl Science Center - Pittsburgh's science and technology museum from 1939 to 1991), and Founder of the South Hills Backyard Astronomers amateur astronomy club; permission granted for only non-profit use with credit to author.]
By Glenn A. Walsh
Reporting for SpaceWatchtower
Late Sunday night, Summer begins in the Northern Hemisphere of Earth, while at the same time, Winter begins in the Southern Hemisphere.
2021 Summer Solstice
For 2021, the season of Summer begins at Earth's Northern Hemisphere's Summer Solstice (and the season of Winter begins at the Southern Hemisphere's Winter Solstice) at the moment of the June Solstice: Sunday Evening, 2021 June 20 at 11:32 p.m. Eastern Daylight Saving Time (EDT) / June 21, 3:32 Coordinated Universal Time (UTC).
In etymology, the word solstice comes from the Latin terms Sol (Sun) and sistere (to stand still). In ancient times, astronomers / astrologers / priests recognized that on one day of the year (in the Northern Hemisphere, on or near the day we now call June 21), the Sun would appear to stand-still as Sol reaches its highest point in the sky for the entire year. The motion of the Sun's apparent path in the sky (what is known astronomically, today, as the Sun's declination) would cease on this day, before appearing to reverse direction.
Although the Summer months in the Northern Hemisphere are known for the year's warmest weather, the Earth is actually at the point in its orbit farthest from the Sun (astronomically known as the point of aphelion) around July 5; the Earth's closest approach to the Sun (perihelion) each year is around January 2. Hence, in general, the distance from the Earth to the Sun is not the major factor determining the heat of Summer or the cold of Winter.
Earth is farther from the Sun during our Spring and Summer seasons,
people in Earth's Northern Hemisphere actually benefit from a few extra
days of warmth (on average), than the number of days in the Autumn and
Winter seasons of the year. When Earth is closer to the Sun, the Earth
travels faster in its elliptical orbit around the Sun (during the Autumn
and Winter months); and, when Earth is farther than average from the
Sun (during the Spring and Summer seasons) the Earth travels a little
more slowly --- again, this refers to the Northern Hemisphere. Hence, the Spring and Summer seasons, in the Northern Hemisphere, have a few more days than the Autumn and Winter seasons.
In fact, Jay Pasachoff, Field Memorial Professor of Astronomy at Williams College and author of widely-used, college astronomy text-books, has precisely calculated the duration of each season, in the Northern Hemisphere:
* Summer: 93 days, 15 hours
* Spring: 92 days, 19 hours
* Autumn / Fall: 89 days, 20 hours
* Winter: 89 days, 0 hours
Solar radiation, and hence the heat from the Sun, depends on the length of daylight and the angle of the Sun above the horizon. The tilt of the planet's axis toward the Sun determines the additional and more direct solar radiation received by a planet's Northern or Southern Hemisphere, and hence, the warmer season of the respective hemisphere.
While the Sun does have motions, it is actually the motion of the Earth tilted on its axis, away from the plane of the ecliptic (Earth's orbital plane around the Sun), while revolving around the Sun, that causes the Earth's seasons. As of this June Solstice, the tilt of Earth's axis is 23.4365 degrees / 23 degrees, 26 minutes, 11.4 seconds. Hence, as the Earth arrives at the point in its orbit around the Sun, when the north polar axis is most directly inclined toward the Sun, this marks the Summer Solstice in the Northern Hemisphere and the Winter Solstice in the Southern Hemisphere.
Alternately, the Winter Solstice in the Northern Hemisphere (the Winter Solstice is always on or near December 21) occurs when the Earth reaches the point in its orbit when the North Pole is most directly inclined away from the Sun. And, conversely, at this time Summer begins in the planet's Southern Hemisphere.
For Earth observers at precisely 23.4365 degrees / 23 degrees, 26 minutes, 11.4 seconds North Latitude at the moment of June Solstice, the Sun will appear to shine directly overhead. The line around the Earth at 23.4365 degrees / 23 degrees, 26 minutes, 11.4 seconds North Latitude is known as the Tropic of Cancer. Likewise, at 23.4365 degrees / 23 degrees, 26 minutes, 11.4 seconds South Latitude is located the Tropic of Capricorn, where the Sun appears directly overhead at the moment of the December Solstice.
However, as the tilt of the Earth is dynamic, and changes minutely over the years, the location of the Tropic lines also change. Currently, these Tropic lines are moving north at the rate of 0.47 arc-seconds / 49.21 feet / 15 meters per year.
The names Tropic of Cancer and Tropic of Capricorn were coined in the last centuries B.C., when the Sun would appear in the Constellation Cancer the Crab on the June Solstice and in the Constellation Capricornus the Horned Goat on the December Solstice. However today, hours after the June Solstice, the Sun enters the Constellation Gemini the Twins, 30 degrees from Cancer. And at the December Solstice, the Sun is now in the Constellation Sagittarius the Archer.
This is due to “Precession of the Equinoxes” of Earth, which is analogous to the wobbling of a spinning top. In the case of the Earth, this 25,772-year wobble causes observers to view the Sun in different parts of the sky over the centuries, at the same time of year while remaining in the same geographical location. As the Earth wobbles over the centuries, the north pole star also changes. Currently, Polaris is our north pole star; around A.D. 13,700, Vega will be our north pole star, due to the Precession of the Equinoxes.
No matter which hemisphere, the day of the Summer Solstice always has the most hours and minutes of daylight (the length of time between sunrise and sunset) for the year, while the Winter Solstice always has the least number of hours and minutes of daylight for the year. The exact number of hours and minutes of daylight, for a particular location, depends on the locale's geographic latitude on the Earth. Astronomers, amateur ("ham") radio operators, and long-distance radio enthusiasts (“radio DXers”), all of whom mostly depend on non-daylight hours to ply their craft, often prefer the days closer to the Winter Solstice.
The Vernal Equinox, when the season of Spring begins in the Northern Hemisphere (and the season of Autumn begins in the Southern Hemisphere), occurs between the Winter and Summer Solstices when the Earth reaches the point in its orbit around the Sun when the Earth's axis is inclined neither toward nor away from the Sun. Likewise, when the Earth reaches the point in its orbit around the Sun, between the Summer and Winter Solstices, when the Earth's axis is inclined neither toward nor away from the Sun, this is known as the Autumnal Equinox (beginning of Fall or Autumn) in the Northern Hemisphere; at this time Spring begins in the Southern Hemisphere. And, half-way between the beginning points of each season are Cross-Quarter Days, each related to traditional holidays: Groundhog Day (February 2), May Day (May 1), Lammas Day (traditionally, the first harvest festival of the year on August 1), and Halloween (October 31).
In ancient times, the Summer Solstice was known as Mid-Summer Day, in early calendars observed around June 24. At that time, May 1 to August 1 (i.e. the two Cross-Quarter Days) was considered the season of Summer. Such early European celebrations were pre-Christian in origin. Many will associate this ancient holiday with the famous William Shakespeare play, “A Midsummer Night's Dream.” Some speculate that the play was written for the Queen of England, to celebrate the Feast Day of Saint John.
As with the Roman Catholic Church's decision to Christianize the pagan Winter Solstice festivals with the introduction of Christmas Day on December 25 (by an early calendar, December 25 was reckoned as the Winter Solstice), the Church began to associate the Mid-Summer festivals with the Nativity of Saint John the Baptist on June 24. In the Christian Bible, the Gospel of Saint Luke implies that Saint John was born six months before the birth of Jesus, although no specific birth dates are given.
The most famous celebration of the Summer Solstice occurs each year at the Stonehenge pre-historic monument in England. Constructed between 3,000 B.C. and 1,600 B.C. in three phases, the actual purpose of the landmark is still unclear. However, it seems to have been associated with burials, originally. It was also used as a type of astronomical observatory, particularly for observing the Sun, which was important to help early cultures make annual decisions regarding agriculture.
Stonehenge is known as a way for pre-historic peoples to mark both the Summer and Winter Solstices. From inside the monument, a viewer facing northeast can watch the Sun rise (weather-permitting) above a stone outside the main circle of rocks, known as the Heel Stone, on the day of the Summer Solstice in the Northern Hemisphere. Although today, due to serious erosion of the stones, visitors on the Summer Solstice can only walk around the landmark from a short distance away during this annual event.
Although not as prominent as Stonehenge, a calendar ring using smaller rocks was also constructed at Nabta Playa in southern Egypt, perhaps as early as 7,000 years ago! As with Stonehenge, some stones aligned with sunrise on the day of the Summer Solstice.
Today, a Stonehenge-like event occurs each year at the University of Wyoming (UW) Art Museum in Laramie, Wyoming, free-of-charge to the general public. At 12:00 Noon Mountain Daylight Saving Time (MDT) / 2:00 p.m. EDT / 18:00 UTC on the day of the Summer Solstice, visitors can see a single beam of sunlight shine through a solar tube in the ceiling of the UW Art Museum's Rotunda Gallery; the beam of sunlight then shines onto a 1923 Peace Silver Dollar embedded in the floor of the Museum's Rotunda Gallery. Visitors are encouraged to arrive at the museum by 11:30 a.m. MDT / 1:30 p.m. EDT / 17:30 UTC, to view this rather unique architectural feature.
The bright Star Spica (Alpha Virginis), the brightest star in the Constellation Virgo the Virgin and the 16th brightest star in Earth's night sky (Apparent Visual Magnitude: + 0.97), may have helped develop another one of civilization's early calendars. A calendar of ancient Armenia used the year's first sighting of Spica in the dawn sky, a few days before the Summer Solstice, to mark the beginning of the New Year for this particular calendar. The development of this calendar somewhat coincided with the beginning of agriculture in Armenia.
Like clock-work, a well-known asterism (pattern of stars in the sky, not officially recognized as a constellation) of three stars shaped as a triangle is visible nearly overhead around local midnight during the Summer months (weather-permitting). And logically, as Star Trek's Mr. Spock might say, this asterism is known as the Summer Triangle!
Three of the brightest stars in the Summer sky constitute the Summer Triangle ---
Vega (Alpha Lyrae - brightest star in the Constellation Lyra the Harp); brightest of the three stars and closest to the zenith (highest point in the sky);
Altair (Alpha Aquilae - denotes the eagle eye and brightest star in the Constellation Aquila the Eagle); second brightest star of the trio;
Deneb (Alpha Cygni - denotes the tail star, is the brightest star in the Constellation Cygnus the Swan, and is the “head” star of the asterism known as the Northern Cross).
The term Summer Triangle was popularized in the 1950s by American author H.A. Rey and British astronomer Patrick Moore, although constellation guidebooks mention this triangle of stars as far back as 1913. And, during World War II, military navigators referred to this asterism as the “Navigator's Triangle.”
Regardless of city light pollution, the three bright stars of the Summer Triangle should be visible to nearly everyone in Earth's Northern Hemisphere (weather-permitting). So, just look overhead late-evening or early-morning throughout the Summer for these annual visitors to our Summer sky!
Internet Links to Additional Information ---
Link 1 >>> http://scienceworld.wolfram.com/astronomy/SummerSolstice.html
Link 2 >>> http://en.wikipedia.org/wiki/Summer_solstice
Season of Summer: Link >>> http://en.wikipedia.org/wiki/Summer
History of Mid-Summer: Link >>> http://en.wikipedia.org/wiki/Midsummer
Summer "Solstice Day" Annual Free-of-Charge Day (With Snowballs !), 1985 to 1991, at the original Buhl Planetarium and Institute of Popular Science (a.k.a. Buhl Science Center), Pittsburgh's science and technology museum from 1939 to 1991:
Link >>> http://spacewatchtower.blogspot.com/2015/06/snowballs-on-first-day-of-summer.html
Stonehenge: Link >>> https://en.wikipedia.org/wiki/Stonehenge
News Release - University of Wyoming Stonehenge-type event:
Link >>> https://www.uwyo.edu/uw/news/2018/06/uw-art-museum-to-celebrate-summer-solstice-june-21.html
Star Spica: Link >>> https://en.wikipedia.org/wiki/Spica
Precession of the Equinoxes: Link >>> https://en.wikipedia.org/wiki/Axial_precession
Tropic of Cancer: Link >>> https://en.wikipedia.org/wiki/Tropic_of_Cancer
Tropic of Capricorn: Link >>> https://en.wikipedia.org/wiki/Tropic_of_Capricorn
Summer Triangle: Link >>> https://en.wikipedia.org/wiki/Summer_Triangle
Friday, 2021 June 18.
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Glenn A. Walsh, Informal Science Educator &
Link >>> http://buhlplanetarium2.tripod.com/weblog/spacewatchtower/gaw/
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Project Director, Friends of the Zeiss: Link >>> http://buhlplanetarium.tripod.com/fotz/
SpaceWatchtower Editor / Author: Link >>> http://spacewatchtower.blogspot.com/
Formerly Astronomical Observatory Coordinator & Planetarium Lecturer, original Buhl Planetarium & Institute of Popular Science (a.k.a. Buhl Science Center), Pittsburgh's science & technology museum from 1939 to 1991.
Formerly Trustee, Andrew Carnegie Free Library and Music Hall, Pittsburgh suburb of Carnegie, Pennsylvania.
Author of History Web Sites on the Internet --
* Buhl Planetarium, Pittsburgh: Link >>> http://www.planetarium.cc Buhl Observatory: Link >>> http://spacewatchtower.blogspot.com/2016/11/75th-anniversary-americas-5th-public.html
* Adler Planetarium, Chicago: Link >>> http://adlerplanetarium.tripod.com
* Astronomer, Educator, Optician John A. Brashear: Link >>> http://johnbrashear.tripod.com
* Andrew Carnegie & Carnegie Libraries: Link >>> http://www.andrewcarnegie.cc