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 Manager of Pittsburgh's original Buhl Planetarium and Institute of Popular Science 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, Thursday, 2017 December 21 at 11:28 a.m. Eastern
Standard Time (EST) / 16:28 Coordinated Universal Time (UTC)---the moment of the posting of this blog-post. This
moment also marks the astronomical beginning of the Summer season in
the Southern Hemisphere.
Almost 24 hours later, Friday
morning will mark the peak time for the annual Ursid Meteor
Shower; of course dark skies are needed to actually see meteors. This meteor shower peaks Friday, 2017 December 22 at 10:00
a.m. EST / 15:00 UTC.
Winter Solstice 2017
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 September).
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).
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.
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 is 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.
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, 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.
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. 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.
Special Thanks: Eric G. Canali, former Floor Manager of Pittsburgh's original Buhl Planetarium and Institute of Popular Science and Founder of the South Hills Backyard Astronomers amateur astronomy club.
Special Thanks: Eric G. Canali, former Floor Manager of Pittsburgh's original Buhl Planetarium and Institute of Popular Science and Founder of the South Hills Backyard Astronomers amateur astronomy club.
Internet Links to Additional Information ---
More on the Winter Solstice:
Link 1 >>> http://scienceworld.wolfram.com/astronomy/WinterSolstice.html
Link 2 >>> http://en.wikipedia.org/wiki/Winter
More on a 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/
More on calendars ---
Gregorian Calendar: Link >>> http://en.wikipedia.org/wiki/Gregorian_calendar
Julian Calendar: Link >>> http://en.wikipedia.org/wiki/Julian_calendar
More on the Ursid Meteor Shower: Link >>> http://en.wikipedia.org/wiki/UrsidsA
More on the 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.
2017 December 21.
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Glenn A. Walsh, Project Director, Friends of the Zeiss: < http://buhlplanetarium.tripod.com/fotz/ >
& SpaceWatchtower Editor / Author: < http://buhlplanetarium2.tripod.com/weblog/spacewatchtower/gaw/ >
Electronic Mail - < gawalsh@planetarium.cc >
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Author of History Web Sites on the Internet --
* Buhl Planetarium, Pittsburgh:
< http://www.planetarium.
* Adler Planetarium, Chicago:
< http://adlerplanetarium.
* Astronomer, Educator, Optician John A. Brashear:
< http://johnbrashear.tripod.com >
* Andrew Carnegie & Carnegie Libraries:
< http://www.andrewcarnegie.
* Civil War Museum of Andrew Carnegie Free Library:
< http://garespypost.tripod.com >
* Duquesne Incline cable-car railway, Pittsburgh:
< http://inclinedplane.tripod.
* Public Transit:
< http://andrewcarnegie2.tripod.
