StarWatch: Moravian College Astronomy

SEPTEMBER 2020

SEPTEMBER STAR MAP | MOON PHASE CALENDAR | STARWATCH INDEX | NIGHT SKY NOTEBOOK

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Current Geomagnetic Field:

lunar phases

1255 SEPTEMBER 6, 2020: Strolling Along the Suburban Milky Way: If you looked at the moon this past weekend, the sunlight reflecting off its surface was on the left, a sure sign that Luna was on the wane. That’s great news if you are interested in viewing starscapes, and particularly, the summer Milky Way, our home galaxy, which favors September by arching overhead just after it becomes dark. I’ve only seen the Milky Way on one occasion from Moravian College’s Sky Deck located in center city Bethlehem, but if you live just five to 10 miles from Allentown, Bethlehem, or Easton, it is in these first hours after dark that our galaxy will be the easiest to spot because of its overhead location. You’ll also notice three bright stars near your zenith, the Great Summer Triangle. The brightest is Vega, the alpha star of Lyra the Lyre. If you’re facing east, Altair, the brightest star of Aquila the Eagle, is to your right. The faintest of the triad, Deneb, is the most prominent luminary of Cygnus the Swan. Cygnus, from a suburban locale, is seen primarily as a cross with the Milky Way appearing as a fuzzy path of elongated mist following its staff. You’ll notice its appearance is mottled with dark splotches, areas where dust from countless supernovas during the nearly 13-billion-year history of our galaxy have blotted out the stars that lie in back of this dross. As your eye wanders to either side of the zenith, the Milky Way seems to fade, the result of light pollution from the East Coast megalopolis in which we live. Moving southwest in my neighborhood, trees are intersected past Aquila and that part of the sky is lost; however, if your horizons are unobstructed enough, you might be able to perceive Sagittarius, the (Centaur) Archer, a grouping of stars that in September looks more like a teapot pouring its hot beverage onto the southwestern horizon, an apt reminder that cooler weather is on our doorsteps. Finding Sagittarius allows you a view towards the center of the Milky Way, about 27,000 light years distant. Its location is just above and to the right of the spout of the teapot. Consider the fact that near its center, our galaxy contains a black hole “weighing in” at about four million suns. Following the path of the Milky Way to the northeast, I can sometimes detect its haziness up to a sideways, W-shaped constellation named Cassiopeia, the Queen of Ethiopia. Take the “W” and twist it counterclockwise by almost a quarter of a turn, and you’ve got it correctly positioned for an evening September sky. It is really a chair created by a fainter star near the middle of the “W.” Without padding and constructed with a crooked back, Cassiopeia is definitely not reclining on a La-Z-Boy, but rather the chair is serving as part of her punishment for wanting to sacrifice her daughter, Andromeda, to a hideous monster called the Kraken, also known as the constellation of Cetus the Whale. Just below the chair’s back can be found the Double Cluster of Perseus, one of my favorite star groupings of the nighttime sky. Below the Double Cluster is Perseus himself looking like an upside-down “V,” his curved legs extending outward towards the east. By now the Milky Way has faded to invisibility in the extra glow of light pollution and haze found nearer to the horizon. Look for a really bright star, Capella, below and to Perseus’ left, and you’ll have the first harbinger of autumn and the winter sky to come.

1256

SEPTEMBER 13, 2020: The Greatest Triangle of Them All

When the French philosopher, Auguste Comte, predicted in 1835 that we would never know the true chemical nature of the stars, little did he know that 30 years later the burgeoning field of spectroscopy would begin to reveal the intimate details of a star’s gaseous composition. Comte also did not envision that just three years later, the German astronomer and mathematician, Friedrich Bessel, would determine the distance to the star, 61 Cygni, by measuring its parallax or displacement due to Earth’s annual revolution around the sun. These two investigations shaped the groundwork for understanding the true nature of the stars. By the early twentieth century, it was understood that the varying intensities of the dark absorption lines embedded within the rainbow of colors found in the spectrum of a star’s white light was also the blueprint to understanding the temperatures of the stars being observed. These dark lines were created by the gases in a star’s atmosphere filtering specific wavelengths of energy unique to their atomic structure. Improvements in making parallax measurements created enough data to show that some stars were distant and very bright while others were close and faint. Understanding the absolute luminosity of these stars and their temperatures allowed Ejnar Hertzsprung (Danish) and Henry Norris Russell (American) to create independently the HR diagram, the Rosetta Stone for the eventual understanding of stellar evolution and the introduction of such terms into the lexicon as red giants, main sequence stars like our sun, and white dwarfs. What does all of this have to do with the Great Summer Triangle, a group of three bright stars found overhead right after dark? Plenty… Venture outdoors this week and take a look at this asterism, a group of stars as famous as a constellation, but not officially recognized as such by professional astronomers. Vega is the brightest. It’s a hydrogen burning main sequence star about 25 light years distant. One light year equals the distance that light travels in a single year, about 5.9 trillion miles. Through binoculars and telescopes, it has a distinct bluish tint, a star with a surface temperature of nearly 18,000 degrees F. Blue represents the hotter end of the stellar temperature sequence. Red stars are the coolest. Next in brightness comes Altair, another hydrogen burning main sequence star, more whitish but without the vibrancy of Vega. At 17 light years distant, its slightly warmer hue is indicative of a lower surface temperature of 13,000 degrees F. The difference in luminosity between Vega and Altair also tells astronomers that Altair is a less massive star than Vega, converting hydrogen into helium at a less ferocious rate. Altair will also outlive its more luminous and massive rival. The big surprise, however, is Deneb, the faintest and most distant star of the triad. The fact that it is still bright, but spaced from our sun at a minimum distance of 1500 light years, means that it must also be the most luminous of the trio. Vega and Altair are about 50 and 10 times more brilliant than Sol respectively, but Deneb is one of the truly great supergiant stars of our galaxy, blistering the heavens with conservatively 50,000 times the luminosity of our sun. Had Auguste Comte been correct, we might, like eighteenth and early nineteenth century astronomers, still be considering the stars to be nearly all the same, with the brightest luminaries representing those that were closest and the faintest ones located at the greatest distances. Although I have a great deal of respect for the critical thinking abilities that philosophy develops in its disciples, it is no match for the predictive truths revealed through the dogged, investigative powers of the scientific process.

1257 SEPTEMBER 20, 2020: Say Goodbye to the Sun: My favorite day of the year was originally the summer solstice, the time of high sun and the longest day of the year, but when both of my grandfathers passed away on that date, it lost some of its appeal; and I switched to Memorial Day. It probably should have been my original choice because May is the transition month between the last cold of winter and the summer’s radiant warmth. For a public school astronomy teacher, it also held the expectation of the solstice, as well as the approach of the summer break. Solstice is a wonderful word. From the Latin, it literally means “sun still,” the time when Sol reaches its greatest deviation north or south of the Earth’s equator. It virtually “hangs” in the same high midday location for about one month. The sun is cresting at summer solstice or reaching its nadir at winter solstice all because of its annual path in the sky, fashioned by the tilted Earth’s orbital motion. The Earth’s axial inclination causes the equator to be angled at 23.5 degrees to the plane of its orbit, the ecliptic, which is where the sun lies. The sun glides along this path reaching its high point above the equator in June, producing those endless, warm days of summer which seem to go on forever, as well as the freezing and shadowed short days of winter, which at least to me, seem even longer. There has to be a transition period, a moment when the change from high to low sun is at its greatest, and that time is now. On Tuesday, September 22 at 6:32 p.m. EDT, the sun reaches its midpoint between the extremes of the seasons, the autumnal equinox. Again from the Latin, equinox means “equal nights.” Day and night are equal as the sun transitions from favoring the northern hemisphere to spreading more of its warmth into the southern hemisphere by crossing over the equator on its southward slide. Despite my gloominess over the colder weather that belies our present warmth, the few weeks surrounding the equinoxes, both autumnal and vernal, offer the best opportunity to witness some of the most rapid changes that can take place in the heavens. By October 5, two weeks after the autumnal equinox, the sun will be over five degrees lower in the sky at noon, and the sunlit day will be shorter by 35 minutes for people living at 40 degrees north latitude, Moravian College’s location. The effects are more pronounced northward and less noticeable southward. Fairbanks, Alaska loses about seven minutes of light each day surrounding the fall equinox. One positive aspect of the autumnal months is the seasonal lag in temperatures. During the spring and summer, the northern hemisphere absorbs more energy than it radiates back into space. Temperatures along the East Coast reach their yearly maximums around the third week in July which causes the effects of summer to spill over well into the fall. The big chill really does not happen until sometime in November. Halloween can still be pretty balmy, but by Thanksgiving, those pleasantly warm Indian summer days are gone and won’t be returning until late April or early May. Say goodbye to the sun. The time of the long shadows is rapidly approaching which will cause the “winter people” to rejoice.

1258 SEPTEMBER 27, 2020: Harvest Moon This Week: Have you been watching the moon lately? My Moravian astronomy students have, as part of their initiation into instrumentation and observing. Last Monday, it was a midsized waxing crescent gracing the southwest after sundown. By Thursday, it was a day past first quarter with more than half of the nearside illuminated by the sun in a waxing gibbous phase. I don’t recall any four-day stretch of time when I’ve introduced telescopes to my pupils where each night the sky was usable. Wednesday produced the best images with Saturn, Jupiter, and Mars also stealing the show. My students seemed to be the most impressed with the moon and its craters which were well placed and vividly displayed, especially along the southern terminator where the sun was rising and shadows were at their longest. Since last Thursday, the moon has blossomed, drenching rural landscapes in its eerie bluish glow as it heads toward its full phase on Thursday, October 1. This week’s full moon is called the Harvest Moon, the name given to the full phase that occurs closest to the autumnal equinox. Normally, two out of every three harvest moons happen in September, but this October is even stranger yet because it will also host a blue moon, two full moons occurring in the time span of a single month. More about that in a future StarWatch. The reason for this lunar phenomenon in late summer and early fall results from the moon’s orbit which is tilted at a very shallow angle to the horizon. The moon moves about 13 degrees per day as it circles the Earth, but its orbital motion does not greatly change its distance below the horizon from night to night around the period of the full moon. The moon’s orbital path at 40 degrees north latitude can be tilted by as little as 22 degrees to the horizon, thus allowing the Earth’s rotational motion (spinning) to bring Luna to a rising position at nearly the same time for several days after its full phase. For the Lehigh Valley, it is about a half hour difference. However, the effect is even more pronounced in Europe where the concept of the harvest moon originated perhaps as far back as medieval times. At higher latitudes, the moon’s orbital plane is even closer to being parallel to the horizon, and differences in moonrise on two successive nights can be as little as 10 minutes. Harvesting their crops before electric lights, farmers would be able to continue working uninterrupted as western twilight blended into the brilliance of a full moon rising in the east. If you think that the full or a nearly full moon is not bright enough to harvest crops, take a ride into the country this week during the early evening, and bring along a book. Find a location untainted by street lamps but under moonlight. Let your eyes adjust to the lower levels of light for about five minutes and see if you can read the printed word by the illumination of the silvery moon, something my grandfather was able to accomplish during WWI on the snow-packed Russian front. Like him, I’ll bet you’ll be able to meet with success and prove the relevance of the harvest moon to agriculture!