THE SOUTHWORTH PLANETARIUM70 Falmouth Street Portland, Maine 04103
43.6667° N 70.2667° W
Altitude: 10 feet below sea level
Founded January 1970
2021-2022: VI
"We don't stop playing because we grow old. We grow old because we stop playing."
-George Bernard Shaw
THE DAILY ASTRONOMER
Thursday, September 9, 2021
Restless Sky
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Happy 23rd Birthday, Infernally Adorable Young Adult!
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The one lesson astronomy teaches us all: nothing remains the same. Everything changes, sometimes rapidly, other times slowly. Examine the sky on successive nights. Although the constellations and planets appear to be unchanged and to occupy more or less the same positions relative to the setting Sun, alterations have occurred during the intervening time period so that each night sky is unique.
The moon experiences the greatest shift, for it moves half a degree an hour, or twelve degrees a day. As its location changes, so, too, will its illumination percentage. For instance, the moon was new on September 6th and is currently progressing through the waxing crescent phase. One can see it low in the western early evening sky. Each night it appears to grow larger as its illuminated face turns slowly toward back toward us. Every 29.5 days, the moon completes another phase cycle as it goes from western evening sky re-birth to gradual demise in the pre-dawn east.
Other alterations are not as readily noticeable. During a 24-hour period, the Sun moves along its ecliptic path by approximately one degree. Meanwhile, Earth's orientation relative to the Sun nudges slightly either up or down depending on the hemisphere. Presently, the Sun's diurnal (daily) path through the sky is becoming both shorter and lower and shall continue to do so until the winter solstice. The combined motions change the sunrise/sunset times by just 1 - 2 minutes. These are the alterations we tend to only notice over many weeks.
Planets, follow their own path through the stars, hence the word "planet," which derives from 'planetes,' meaning "wanderer." Planetary motions are not created equal: the inferior planets, Mercury and Venus, dart back and forth between the western evening and eastern morning skies at a frenetic pace by virtue of their close proximity to the Sun. The outer planets meander through the firmament more gradually. Old man Saturn requires nearly thirty years to complete one orbit and so trudges lethargically among the stars. Yet, even that "trudge" amounts to slightly more than 21,000 miles per hour.
Some changes are so subtle as to pass unnoticed even over the course of a human lifetime. Presently, Polaris serves as our north star as it is almost precisely aligned with Earth's north celestial pole. However, Earth's precessional wobble, induced primarily by the gravitational influences of the Sun and moon, will draw the north celestial pole away from Polaris and toward other stars. In 2,000 years, Errai, a star in Cepheus the King, will sit at the night sky's apex. Thirteen thousand years from now, the brilliant star Vega, the brightest star in the Summer Triangle, shall become our north star. Yet, during our lifetime and the lifetimes of our closest descendants, Polaris shall remain the axis about which the stars appear to turn.
On far larger timescales, even the constellations will dissipate like cloud forms owing to proper stellar motions: the independent movements of stars within the galaxy. Stellar motion is rapid! The Sun races through the galaxy at 143 miles per second and the surrounding stars travel at comparable speeds. The stars are so remote, however, that they scarcely seem to move a micrometer over many centuries. Even the Stonehenge builders gazed upon the same star patterns that adorn our modern sky. However, over tens of thousands of years, the constellations will become distorted due to these proper motions. Notice in the above graphic that Orion's belt will more or less maintain its shape while the constellation's other features become bent and contorted. (This graphic does not take into account the distinct possibility that Betelgeuse, Orion's eastern shoulder star, could go supernova within the next 100,000 years....or even in the next few minutes.)
We derive great comfort from the constellations. Some are circumpolar and so always remain in our sky. Others dissolve into the dusk at the same times each year only to return to the pre-dawn sky a couple of months later. Throughout our lives, we can expect these transitions to occur and recur. We should bear in mind, however, that change is the only constant, even if that change is beyond our perception. The philosopher Heraclitus explained that we never step in the same river twice. If only he knew that the night sky we behold tonight will never again be precisely the same.
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