THE SOUTHWORTH PLANETARIUM
70 Falmouth Street  Portland, Maine 04103
(207) 780-4249   www.usm.maine.edu/southworth-planetarium
43.67° N 70.27° W
Founded January 1970
2022-2023: XCIX
Sunrise: 5:00 a.m.
Sunset: 8:20 p.m.
Civil twilight begins: 4:25 a.m.
Civil twilight ends: 8:56 p.m.
Sun's host constellation: Taurus the Bull
Lunar phase: Waning gibbous (85% illuminated)
Moon rise: 12:12 a.m. (6/8/2023)
Moon set: 9:37 p.m. (6/8/2023)
Julian date: 2460102.29
"Sic itur ad astra"
(thus one journeys to the stars)
-Virgil



THE DAILY ASTRONOMER
Wednesday, June 7, 2023
June 2023 Night Sky Calendar Part II


FRIDAY, JUNE 16: MOON NEAR MERCURY
One could almost describe this as an 'academic event,' meaning one that is essentially invisible. Academically, it might be worth mentioning, but the sight will be difficult, if not impossible to observe. Mercury appears to pass between the waning crescent moon (3% illuminated) and Aldebaran, the brightest star in Taurus the Bull. It is unfortunate that these three bodies will appear so close to the Sun as they'll be so beautifully aligned.

SATURDAY, JUNE 17: NEW MOON
Beginning of lunation

WEDNESDAY, JUNE 21: SUMMER (JUNE) SOLSTICE (GOLD EVENT!!!)
10:57 a.m.

THE HIGHEST SUN
Each day, the Sun attains its highest point in a northern hemisphere's observer's sky when it crosses the meridian, the arc that connects due south and due north while passing through the zenith, the point directly overhead.    As the tilted Earth migratres around the Sun, the latter's meridian altitude varies slightly each day.       Today, when the Sun crosses the meridian, an event called upper culmination, it will attain its maximum altitude for the entire year.   

To determine this altitude, we must first explain the celestial equator, the imaginary projection of Earth's equator onto the celestial sphere, a beautiful term for the night sky.   Envision this celestial equator as a ring poised high above Earth and circling around the equator.     An equatorial observer would see this ring pass directly overhead.    However, a polar observer would see this ring running directly along the horizon.       Observers between the pole and equator would see it -if they could see it- at various altitudes depending on the observer's location.      Calculating this altitude is straightforward.      At the equator, the celestial equator intersects the zenith and so is 90 degrees above due south.   At the pole, the Sun is 0 degrees above due south.     The latitude of the equator is 0, whereas the polar latitude equals 90.         We can now establish a relation:

The celestial equator's angle above due south (for the northern hemisphere) equals 90 minus the observer's latitude. 

Portland's latitude is about 43 degrees.  (We're rounding down to make the math lovelier.) So, the celestial equator would pass 47 degrees above due south.        

We also know that the Sun's position oscillates between 23.5 degrees north of the celestial equator and 23.5 degrees south of the celestial equator.      These values make sense because Earth's tilt from the vertical equals 23.5 degrees.     The Sun is at 23.5 N of the celestial equator on the summer (June) solstice, so its angle above due south on the solstice will be 47 +23.5 = 70.5 degrees.      The Sun cannot ascend to any higher position at this location.

At the Tropic of Cancer, located at 23.5 degrees north, the celestial equator will pass  66.5 degrees due south.   On the summer solstice, the Sun will reach the zenith when crossing the meridian (66.5 + 23.5 = 90 degrees).        The Tropic of Cancer is the northernmost location at which the Sun can occupy the zenith.  


THE RISING AND SETTING SUN

To track the Sun's changing position throughout the year, an observer won't observe the Sun at noon.   Instead, he/she will watch the point along the horizon where the Sun either rises or sets or both.          On both the vernal and autumnal equinoxes, the Sun rises due east and sets due west.     However, during the spring, the Sun's rise position migrates farther north until it reaches its maximum position north of east on the summer solstice.     The Sun's rise position will then start a gradual, but inexorable migration toward the south.    After the autumnal equinox, the Sun's rise position will migrate progressively farther south until it reaches the maximum position south of east on the winter solstice.  After the solstice, the rise position resumes the northern progression again until the summer solstice.  

sunset-position.png

The Sun's path through the sky is longest on the summer solstice and shortest on the winter solstice.   Since the rotation rate remains constant each day throughout the year, the Sun's time in our sky is at a maximum on the summer solstice, but is at a minimum on the winter solstice.

Here in Portland, the duration of daylight on the summer solstice is approximately 15 hours, 26 minutes.   Conversely, on the winter solstice, the Sun only remains above the horizon for 8 hours and 56 minutes.

However, the Sunrise and Sunset times are not at their earliest or latest on the summer solstice

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. 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 sunset 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:


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 and frolic, we acknowledge that the Sun's altitude is now starting to decrease.   

THURSDAY, JUNE 22: MOON, VENUS AND MARS WITHIN A FIVE DEGREE DIAMETER CIRCLE (SILVER EVENT!!)
Look into the western evening sky tonight and you will find the waxing crescent moon (17% illuminated) close to brilliantly bright Venus and ruddy red Mars.      Looming above them all is none other than Leo, the voraciously hungry lion who is eager to devour them all in one bloated-throat swallow.         Venture out after 9 p.m. to behold these beautiful empyreal spheres.  They will all set by 11 p.m.

MONDAY, JUNE 26:  FIRST QUARTER MOON

Tomorrow -and it will actually be tomorrow-  planets and stars



*Astronomers 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.)



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