Thursday, December 20, 2018

Winter Begins Friday; Full Moon & Ursid Meteors Peak Saturday
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 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, Friday Afternoon, 2018 December 21 at 5:23 p.m. Eastern Standard Time (EST) / 22:23 Coordinated Universal Time (UTC). This moment also marks the astronomical beginning of the Summer season in the Southern Hemisphere.

Almost 24 hours later, Saturday Afternoon 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 Saturday Afternoon, 2018 December 22 at 4:00 p.m. EST / 21:00 UTC.

Plus, this year the December Full Moon occurs on Saturday just after Noon in the Eastern Standard Time Zone of the Northern Hemisphere: Saturday Afternoon, 2018 December 22, 12:49 p.m. EST / 17:49 UTC. AND, it was 50 years ago on Friday (1968 December 21 at 7:51:00 a.m. EST / 12:51:00 UTC) that humans began their first voyage to the Moon, when Apollo 8 was launched for a successful mission orbiting the Moon

                                               Winter Solstice 2018

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.

                                            December Full Moon

In our sky, the Moon lines-up opposite the Sun during the Full Moon phase, both appearing to travel along the Ecliptic, the apparent path on the celestial sphere where we find the major planets of our Solar System as well as the Sun and the Moon. This is due to the fact that all of these major bodies travel within a particular plane of the Solar System.

When we view the Moon in our sky, it appears to do the opposite of what the Sun does. When the Sun is high in the sky near the Summer Solstice in June, the Moon is low in the sky. This is because at the Summer Solstice, the North Pole is tilted the maximum extent (23.44 degrees) toward the Sun and away from the Moon.

When the Sun is low in the sky near the time of the Winter Solstice in December (when the North Pole is tilted the maximum extent, 23.44 degrees, away from the Sun and toward the Moon), the Northern Hemisphere receives the lowest number of sunlit hours for the year. Then, the Moon is high in the sky and a Full Moon at this time is bright and appears in the sky for the longest length of time for the year.

The December Full Moon in the Northern Hemisphere of Earth was known to Native Americans as the Cold Moon or the Long Nights Moon, and sometimes also referred to as the Moon Before Yule. 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, rising approximately at sunset and setting approximately at sunrise. And, this month's Moon is high in the Northern Hemisphere sky, as this is the time of the year that the Sun is the lowest in the sky; traveling high in the sky also means the Moon stays in the sky longer.

A couple centuries ago, when night artificial lighting had little effect and the December Full Moon brightened a snowy field, one might see how some people may refer to this as a Big Moon.

In the Southern Hemisphere, the December Full Moon is known as the Strawberry Moon, Honey Moon, and Rose Moon.

Special Thanks: Eric G. Canali, former Floor Operations 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 >>>
Link 2 >>>

More on a Solstice: Link >>>

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 >>>
The Stars of Winter >>>

More on calendars ---
       Gregorian Calendar: Link >>>
       Julian Calendar: Link >>>

More on the Ursid Meteor Shower: Link >>>

More on the Homeless Persons' Memorial Day:
Link >>>

More on the Full Moon: Link >>>

More on Full Moon names ---
Link 1 >>>
Link 2 >>>
Link 3 >>>
Image of the Full Moon photographed by the Apollo 11 astronauts in July of 1969, during the spacecraft's trans-Earth journey homeward after the first landing of astronauts on the Moon, approximately 10,000 nautical miles from the Earth
(Image Source: NASA):
Link >>>

Source: Glenn A. Walsh Reporting for SpaceWatchtower, a project of Friends of the Zeiss.
              Thursday, 2018 December 20.

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Glenn A. Walsh --- < >
Electronic Mail: < >
Project Director, Friends of the Zeiss: < >
SpaceWatchtower Editor / Author: < >
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 of the Andrew Carnegie Free Library and Music Hall, Pittsburgh suburb of Carnegie, Pennsylvania.
Author of History Web Sites on the Internet --
* Buhl Planetarium, Pittsburgh:
  < >
* Adler Planetarium, Chicago:
  < >
* Astronomer, Educator, Optician John A. Brashear:
  < >
* Andrew Carnegie & Carnegie Libraries:
  < >


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