THE SOUTHWORTH PLANETARIUM
70 Falmouth Street Portland, Maine 04103
(207) 780-4249 usm.maine.edu/planet
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:
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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