This diagram shows the position of the Earth, in relation to the Sun,
at the time of the Winter Solstice, as well as the other solstice and
equinoxes of the year, in Earth's Northern Hemisphere.
[Graphic Source: ©1999,
Eric G. Canali, former Floor Operations Manager of the original Buhl
Planetarium & Institute of Popular Science (a.k.a. Buhl Science Center ), Pittsburgh's science & 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
The season of Winter, in the Northern
Hemisphere of Earth, begins at the moment of the Winter / December
Solstice, late Tuesday Morning, 2021 December 21 at 10:59 a.m.
Eastern Standard Time (EST) / 15:59 Coordinated Universal Time (UTC).
This moment also marks the astronomical beginning of the Summer
season in the Southern Hemisphere.
This year's Winter Solstice marks the
53rd anniversary of the launch of Apollo 8, the first
human mission to the Moon. Apollo 8 astronauts Frank Borman, James
Lovell, and William Anders launched from Cape Canaveral, Florida on
1968 December 21 at 7:51 a.m. EST / 12:51 UTC, entered lunar orbit
early on the morning of Christmas Eve, orbited the Moon ten times,
and returned to Earth on 1968 December 27.
Almost exactly 24 hours after the
Winter Solstice will mark the peak time for the annual Ursid Meteor
Shower. This meteor shower peaks Wednesday Morning, 2021 December 22
at 11:00 a.m. EST / 16:00 UTC. Although, with the bright, Long-Nights Full Moon occurring just a few days (Saturday Evening, 2021 December 18 at 11:35 p.m. EST /
December 19, 4:35 UTC) before the Ursids Meteor Shower peak, Ursid meteors may be more difficult to find this year.
Winter Solstice
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 one day of the year when the
Sun would appear to reach its lowest 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, and the Sun would appear to stand-still,
before reversing direction.
With our Gregorian
Calendar, this usually occurs on, or very close to, December 21. In
ancient times, when people used the Julian Calendar, the Winter
Solstice was on, or very close to, December 25, what we now know as
Christmas Day. Mid-Winter festivals, at the time of the Winter
Solstice, were common in ancient times. Instead of competing with
these traditions, the early Roman Catholic Church Christianized the
Winter festivals by observing the birth of Jesus Christ on December
25 (the actual birth date of Jesus was probably in late Summer or
early Autumn).
Today, we know
that, while the Sun does have motions, it is actually the motion of
the Earth, tilted on its axis 23.44 degrees from the plane of our
Solar System while revolving around the Sun, that causes the Earth's
seasons. Hence, as the Earth arrives at the point in its orbit around
the Sun, where the south polar axis is most directly inclined toward
the Sun (thus, the Sun appears at its lowest point for the
year in the Northern Hemisphere sky) around December 21, this marks
the Winter Solstice in the Northern Hemisphere (and the Summer
Solstice in the Southern Hemisphere).
Alternately around
June 21, the Summer Solstice marks the beginning of Summer in the
Northern Hemisphere (and this date also marks the Winter Solstice,
which is the beginning of Winter in the Southern Hemisphere) as the
Earth reaches the point in its orbit where the north polar axis is
most directly inclined toward the Sun.
The day of the
December Solstice is the only time of the year when the Sun reaches
the point of Local Solar Noon at the South Pole. Conversely, it is
also the only time of the year when Local Solar Midnight occurs at
the North Pole. And, of course, it is the reverse during the June
Solstice: the only time the Sun reaches the point of Local Solar Noon
at the North Pole and the only time when Local Solar Midnight occurs
at the South Pole.
Although the
Winter months in the Northern Hemisphere are known for the year's
coldest weather, the Earth is actually at the point in its orbit
closest to the Sun (astronomically known as the point of Perihelion)
on or very near January 2. The Earth is farthest from the Sun, each
year shortly after the Northern Hemisphere's Summer Solstice, on or
very near July 5 (the point of Aphelion). Over a half-year's time between Earth Perihelion and Earth Aphelion, the difference in distance between the Sun and Earth varies by about 3.2 million miles.
Solar radiation,
and hence heat from the Sun, to warm an Earth hemisphere depends on
the length of daylight and the angle of the Sun above the horizon. Earth receives about 7 per-cent more solar radiation from the Sun during the time of Earth Perihelion in January, than at the time of Earth Aphelion in July. However, 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.
The Earth's Perihelion in January, and Aphelion in July, are due to the
elliptical nature of the Earth's orbit around the Sun. Perihelion and Aphelion would not occur if the Earth's orbit was a true circle.
Since the Earth is
closest to the Sun near the beginning of the Northern Hemisphere's
Winter Season, the Earth, then, moves faster in its orbit around the
Sun than it moves in July, making the Northern Hemisphere's Winter a
shorter season than Summer. Winter will last for only 89 days, while
this past-Summer lasted nearly 93 days. This is one of the observed
consequences of Johannes Kepler's Laws of Planetary Motion, which he
published at the beginning of the 17th century.
The day of the
Winter Solstice is known as the “shortest day of the year” and
the “longest night of the year” as the Sun shines on the Northern
Hemisphere for the shortest length of time for the entire year, on
this day. For this reason, Homeless Persons' Memorial Day is
commemorated on December 21.
Since the Summer Solstice in June, the number of daylight hours have slowly diminished each day, with the night-time hours progressively increasing each day. This has benefited astronomers (to view planets and stars), amateur / ham radio operators (to communicate with other ham operators around the world), and long-distance (DX) radio enthusiasts (to receive AM / medium-wave and short-wave radio stations from around the country or around the world), who need the lack of solar radiation to ply their respective craft. Once we reach the Winter Solstice, the number of daylight hours will, now, slowly increase each day, with the night-time hours declining each day--until, once again, this reverses on the Summer Solstice.
Interestingly, the
climate of a locale in the Southern Hemisphere is, on average,
slightly milder than a location at the same latitude in the Northern
Hemisphere, because the Southern Hemisphere has significantly more
ocean water and much less land. Water warms-up and cools-down more
slowly than does land. The only exception is the Antarctic Continent,
which is colder than the Northern Hemisphere's Arctic region,
possibly because most of the Arctic region is covered with water
(although, often frozen water on the surface, but liquid water
beneath the ice) while Antarctica is mostly a land mass.
On
the Winter Solstice, the Sun appears
(from Earth's perspective) to be in the constellation
Sagittarius—that is, if you could view the stars behind the
Sun on the Winter Solstice, you would see the stars of Sagittarius.
Previously, just a few days earlier, the Sun had appeared to be in
the constellation Ophiuchus. The change, when the Sun appeared
to move from Sagittarius to Ophiuchus, occurred on December 17.
However, a couple thousand years ago,
the Sun would have appeared to be in the constellation
Capricornus during the Winter Solstice. And, about 150 years from
now, the Sun will appear to be in the constellation Ophiuchus
during the Winter Solstice.
This apparent change is known as
Precession of the Equinoxes or Axial Precession. This is a slow
“wobble” of the Earth's axis, which causes the background stars
or constellations that the Sun appears in to change over an
approximately 25,771.5 year-cycle. This cycle runs through 12
traditional constellations of the zodiac, plus the constellation
Ophiuchus, comprising the constellations along the ecliptic.
Precession also causes the North Star
to change over the approximately 25,771.5 year-cycle. Today, Polaris
is known as the North Star, which has been used for ages by
navigators. However, at the time Egypt constructed the Great Pyramid,
architects used Thuban, the North Star at that time, to align the
pyramid. And, about 12,000 years from today, Vega will be the North
Star.
Although for the year, December 21, for
Earth's Northern Hemisphere, has the fewest number of daylight hours
and the most night-time hours, it may be surprising to some that this
date does not have the latest sunrise time nor the earliest sunset
time for the year. This is also true for the June 21 solstice in the Southern
Hemisphere.
Depending on a location's latitude, the
latest sunrise time actually occurs a few days after the respective
solstice, while the earliest sunset time occurs a few days before the
day of the solstice. These time differences are due to, what scientists
call, the Equation of Time (the Equation of Time is graphically
displayed on most world globes as a figure “8”, known as the
Analemma).
The U.S. Naval Observatory in
Washington defines the Equation of Time: "the Equation of Time
is the difference apparent solar time minus mean solar
time". Apparent solar time, which is somewhat variable,
directly tracks the motion of the Sun and can be measured using a
sundial. Mean solar time measures solar motion if the Sun's motion
was uniform; it is measured by an accurate clock which does .not vary
in time measurement.
The Sun's motion does vary throughout
the year. Hence, the latest sunrise time and the earliest sunset time
do not occur on the actual day of the solstice.
Moon Before Yule or Long-Nights Moon
Late
last night (Saturday Evening, 2021 December 18 at 11:35 p.m. EST /
December 19, 4:35 UTC), was the Primary Moon Phase of Full Moon for
the month of December. Each year, the December Full Moon is known as
the Long-Nights Moon due to the great length of time the Full Moon is
visible in the sky this time of year, compared to the much shorter
time the Sun is in the sky at this time of year.
Many years, the
December Full Moon is also known as the Moon Before Yule, when the
Full Moon occurs before Christmas Day. Originally, Yule referred to the Winter Solstice, celebrated by Germanic peoples. Again, Yule was another mid-Winter festival which was eventually Christianized, so that today people refer to Yule as Christmas.
Of
course, it can easily be understood why the Full Moon may be longer
in the sky than the Sun this time of year, since the Winter Solstice
marks the day with the shortest amount of daylight and longest
duration of night. This means that early this week there will be
about 14 and one-half hours of moon-light, with only about 9 and
one-half hours of sunlight, weather-permitting.
In addition, this time of year the Moon
climbs to its highest altitude in the sky for the year, what
scientists call a high declination, while the Sun has the lowest
declination for the year. The Sun, Moon, and planets all travel along
and near a line known as the ecliptic. Near and during the season of
Winter, the ecliptic arcs low across the sky in the daytime, because
the Earth's North Pole is pointed away from the Sun this time of
year. Alternately, the Full Moon on the other side of our planet
benefits from the ecliptic arcing high across the sky during the
night-time hours this time of year.
The December Full Moon is also very
prominent, particularly when this high, bright Moon shines on a
snowy-white field, which may justify the name Big Moon.
The December Full Moon was also known
to Native Americans as the Cold Moon. Other names given to the
December Full Moon have been reported by the Farmers' Almanac
(Oak Moon) and The American Boy's Book of Signs, Signals and
Symbols published in 1918 for use by the Boy Scouts (Wolves Moon
and Big Moon).
Of course Cold Moon refers to the cold temperatures
that begin with the start of the Winter season this month. And, the
Moon Before Yule was used by the Christian settlers to refer to the
Full Moon before Christmas Day (Yule being an early religious
festival observed by Germanic peoples, later absorbed and equated
with Christmas); of course, this name would not be used during years
when the December Full Moon is after Christmas Day. With the longest
night of the year occurring near the Winter Solstice, this justifies
the term Long-Nights Moon, as the Full Moon is visible all-night
long.
In the Southern Hemisphere, the December Full Moon is known
as the Strawberry Moon, Honey Moon, and Rose Moon.
Ursid Meteor Shower
Almost 24 hours
after the Winter Solstice comes the peak of the annual Ursid Meteor
Shower, which actually begins on December 17 and usually lasts about
a week ending December 24, 25, or 26. The Ursids seem to comprise a
narrow stream of debris originating from Comet Tuttle. Hence, it is
difficult to see Ursid meteors outside of a 12-hour window before and
after the peak, where possibly 12 meteors per-hour could be seen,
under ideal conditions.
The Ursid Meteor
Shower is so-named because most meteors appear to radiate from a
point near the Star Beta Ursae Minoris (apparent meteor shower
radiant) in the Constellation Ursa Minor (better known as the
asterism the “Little Dipper”), which is the brightest star in the
bowl of the Little Dipper. Some people call these meteors “Ursids,”
in an attempt to emphasize that their apparent radiant is Ursa Minor,
not Ursa Major (the asterism the “Big Dipper”).
However, you
should not, necessarily, be looking only at the Little Dipper when
looking for meteors in this shower. Meteors can appear in any part of
the sky at any time (although a meteor's tail may tend to point back
toward the radiant).
Of course meteor
showers, like all celestial observations, are weather-permitting.
If there are more than a few clouds in the sky, meteors will be much
more difficult to find. Clear skies are not always available in the
skies of late Autumn and early Winter. And, it is always best to get
away from city lights, for the opportunity to see the smaller, dimmer
meteors. A bright Moon in the sky will also make it more difficult to view the smaller, dimmer meteors. As always, the best time to view any meteor shower is
between local midnight and local dawn, when the Earth is actually
rotating into the stream of meteoric debris.
Binoculars and
telescopes are not very useful for finding meteors. Meteors streak
across the sky in a very short period of time, far too short to aim
binoculars or a telescope. So, the best way to view a meteor shower
is to lie on a blanket or beach towel on the ground, or use a
reclining a chair, outdoors in an area with a good view of the entire
sky (with few obstructions such as buildings, trees, or hills), and
keep scanning the entire sky.
So, if you go out
to see the Ursid Meteor Shower, start looking for meteors around
local midnight, or perhaps a little later. Make sure you have a good
site where you can see most of the sky, and that sky is relatively
clear. Be sure to dress properly for the early morning temperatures,
now that we are at the very beginning of Winter.
And, you want to
go out ahead of time, before you actually start looking for meteors,
to get your eyes accustomed to the dark sky. Dark-adapting your eyes
for meteor-watching could take up to a half-hour.
Internet Links to Additional
Information ---
Winter Solstice:
Link 1 >>>
http://scienceworld.wolfram.com/astronomy/WinterSolstice.html
Link
2 >>> http://en.wikipedia.org/wiki/Winter
Solstice:
Link >>> http://en.wikipedia.org/wiki/Solstice
Popular
Winter Planetarium Sky Shows Shown at Pittsburgh's original Buhl
Planetarium and Institute of Popular Science (1939 to 1991),
including full scripts of each show:
The Star of Bethlehem
>>>
http://buhlplanetarium3.tripod.com/skyshow/bethlehem/
The
Stars of Winter >>>
http://buhlplanetarium3.tripod.com/skyshow/winter/
Calendar
Formats ---
Gregorian
Calendar: Link >>>
http://en.wikipedia.org/wiki/Gregorian_calendar
Julian Calendar: Link >>>
http://en.wikipedia.org/wiki/Julian_calendar
Primary Moon Phase of Full Moon: Link >>> http://en.wikipedia.org/wiki/Full_moon
Full Moon names ---
Link 1 >>> http://earthsky.org/astronomy-essentials/full-moon-names
Link 2 >>> https://en.wikipedia.org/wiki/Full_moon#Full_moon_names Link 3 >>> http://www.farmersalmanac.com/full-moon-names/
Ursid
Meteor Shower: Link >>>
http://en.wikipedia.org/wiki/UrsidsA
Homeless
Persons' Memorial Day:
Link >>>
http://nationalhomeless.org/about-us/projects/memorial-day/
Source:
Glenn A. Walsh Reporting for SpaceWatchtower,
a project of Friends
of the Zeiss
Sunday, 2021 December 19.
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gaw
Glenn A. Walsh, Informal Science Educator &
Communicator:
Link >>> http://buhlplanetarium2.tripod.com/weblog/spacewatchtower/gaw/
Electronic
Mail: < gawalsh@planetarium.cc >
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
* Other Walsh Authored Blog &
Web-Sites: Link >>>
https://buhlplanetarium.tripod.com/gawweb.html
