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: CXL
"But thy eternal summer shall not fade."
-William Shakespeare
THE DAILY ASTRONOMER
Tuesday, June 21, 2022
On the Solstice Part I
Summer is now less than twelve hours old. At precisely 5:13 a.m. EDT, the furiously fast Earth reached the point in its orbit at which the northern hemisphere is inclined as closely to the Sun as possible. Consequently, astronomical summer has not begun in the northern hemisphere, while astronomical winter has started in the southern hemisphere. We northern hemisphere denizens will remain enveloped in summer's comforting embrace until 9:03 p.m. on Thursday, September 22, 2022, the precise moment of the autumnal equinox.
We celebrate summer's long-awaited arrival with a review about the phenomena related to the solstice and how they affect us and others.
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.
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. Much more on this topic tomorrow.
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