THE SOUTHWORTH PLANETARIUM
70 Falmouth Street      Portland, Maine 04103
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43.6667° N    70.2667° W  Altitude:  10 feet below sea level Founded
January 1970
2021-2022: CXLI
"Behind it all is surely an idea so simple, so beautiful, that when we
grasp it - in a decade, a century, or a millennium - we will all say to
each other, how could it have been otherwise? How could we have been so
stupid?"  John Archibald Wheeler

THE DAILY ASTRONOMER
Wednesday, June 22, 2022
On the Solstice Part II



*RISE/SET TIMES*

One would assume that the earliest sunset and latest sunrise would both
occur on the summer solstice. And, it certainly would if we were living on
a perfectly spherical, uniformly dense planet that traveled along a
perfectly circular orbit. The astronomical reality -like reality, in
general- is a bit more complicated. Our planet is an *oblate spheroid*,
meaning that its equatorial diameter is greater -by about 43km/27 miles-
than its polar diameter. It bulges slightly around the middle.

Moreover, the landmass is not evenly distributed over its surface.
Approximately 68% of Earth's landmass is located in the northern
hemisphere, the Southern Hemisphere contains only thirty-two percent. This
significant difference has induced a slight distortion along Earth, making
it a bit pear-shaped. Another complication relates to Earth's orbit. Our
world travels along an elliptical, as opposed to circular orbit. Its path
is more akin to an oval, albeit one that closely resembles a circle.* Earth
is almost at aphelion, its point of greatest distance** and so is moving
more slowly in its orbit than it does when it is closer. The Sun's rise and
set times are affected by this change because of the delay the Sun
experiences in crossing the meridian, or reaching true noon.

Here, the earliest sunrise occurs on June 15th, when the Sun rises at 4:59
a.m.
The latest sunrise happens on June 27th, when the Sun sets at 8:27 p.m.

However, the summer solstice does mark the day of greatest daylight
duration: 15 hours, 27 minutes. The daylight length will be just about the
same today and tomorrow. On June 24th, the length of daylight will equal 15
hours, 26 minutes. Not a substantial change, but the beginning of a slow,
but inexorable decline in daylight duration that will continue until the
winter solstice.

*THE NAME*

Solstice means "Sun still" because on either solstice, Summer (June) or
winter (December), the Sun appears to linger at its position for 2-3 days
before moving away. This "stillness" represents a perfect example of
"extreme behavior," or behavior observed at an extreme position. Regard the
path of a ball following a parabolic orbit away from and then toward the
ground:


[image: Scene-01-1.jpg]

Were one to watch the ball very closely, one would notice that it appears
to linger for just a moment at the maximum height before beginning its
descent.  When moving under the influence of Earth's gravity, the ball's
vertical velocity is zero at the uppermost point, hence the momentary
delay.

The Sun's rise/set position doesn't seem to change noticeably during the
first two -three days  after either solstice.        A curious historical
note:      During the ancient Roman holiday of Saturnalia, which occurred
around the winter solstice, they celebrated a day entitled  *Dies Natalis
Solis Invicti, *"The Birthday of the Unconquered Sun."   They celebrated
this holiday on December 25th, the first day when the Sun's rise position
is first seen to be visibly north of the solstice location: a clear
indication that another solar cycle had commenced.     Early Christians
chose this date to commemorate another birth, that of Christ.  Hence, the
chosen date for Christmas. The first recorded Christmas celebration was
held in Rome in 336 AD.


To summarize, the daylight duration will not decrease, but the sunsets will
be progressively later for the next few days.  The sun started rising later
last week.    While we're hoping for a scorching hot summer rife with fun
the frolic, we acknowledge that the Sun's altitude is now starting to
decrease.    And, since the less said about that the better, we'll address
a different astronomical topic tomorrow.

*Astronomer use eccentricity to measure an ellipse's departure from
circularity.     An ellipse's eccentricity range varies from 0 to 1, the
latter not inclusive.    An ellipse with an eccentricity equal to zero is a
circle.   When its eccentricity equals one, it becomes a parabola.
 Earth's orbital eccentricity is currently 0.016.  (Yes, it does change,
but over an immensely long time period.)

**Earth reaches aphelion this year on July 4, 2022 at 3:10 a.m.

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