(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
This morning, Summer begins in the
Northern Hemisphere of Earth, while at the same time, Winter begins
in the Southern Hemisphere.
For 2019, 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: Friday Morning, 2019 June 21 at 11:54
a.m. Eastern Daylight Saving Time (EDT) / 15:54 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.
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 the
2019 June Solstice, this tilt of Earth's axis is precisely 23.43676
degrees / 23 degrees, 26 minutes, 12.3 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
23.43676 degrees / 23 degrees, 26 minutes, 12.3 seconds North
Latitude at the moment of June Solstice, the Sun will appear to shine
directly overhead. The line around the Earth at 23.43676 degrees / 23
degrees, 26 minutes, 12.3 seconds North Latitude is known as the
Tropic of Cancer. Likewise, at 23.43676 degrees / 23 degrees, 26
minutes, 12.3 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 and long-distance radio
enthusiasts, both 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 16
th
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!
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
Related Blog Posts ---
"Science Experiments Children & Teens Can Do At Home !" 2018 June 5.
"Snowballs on the First Day of Summer!" 2015 June 21.
Source: Glenn A. Walsh Reporting for SpaceWatchtower, a project of Friends of the Zeiss.
Friday, 2019 June 21.
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gaw
Glenn A. Walsh, Informal Science Educator & Communicator:
<
http://buhlplanetarium2.tripod.com/weblog/spacewatchtower/gaw/ >
Electronic Mail: <
gawalsh@planetarium.cc >
Project Director, Friends of the Zeiss: <
http://buhlplanetarium.tripod.com/fotz/ >
SpaceWatchtower Editor / Author: <
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:
<
http://www.planetarium.cc >
* Adler Planetarium, Chicago:
<
http://adlerplanetarium.tripod.com >
* Astronomer, Educator, Optician John A. Brashear:
<
http://johnbrashear.tripod.com >
* Andrew Carnegie & Carnegie Libraries:
<
http://www.andrewcarnegie.cc >
