StarWatch: Moravian College Astronomy

StarWatch for the greater Lehigh Valley

JULY 2018

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

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1141 JULY 1, 2018: Aphelion This Week: I thought you might be interested in this little factoid. On July 6, at 1 p.m., EDT, Earth will have reached a milestone in its orbit around the sun. We will be at aphelion, formed from Greek, apo (away from) and helios, meaning sun. This is in opposition to our location on January 3 of this year when the Earth was at perihelion, or nearest to the sun. The exact dates change from year to year, but perihelion clusters around the beginning of January and aphelion at the start of July, and as you can see, these times have nothing to do with the seasons. It is an uphill battle that I have waged with my students for the past 45 years, this emblazoned misconception that the sun is like a giant fireplace and when you are near, it must be summer, and far from Sol, winter. This misconception became greatly publicized in the late 1980s and 90s when a video entitle A Private Universe received notoriety. Twenty-three graduates, faculty, and alumni of Harvard University were picked at random and asked to explain the basic reason for the seasons. Twenty-one of them failed, some with a smirky smile on their faces, reasoning that distance from the sun was the main player in why the seasons occurred. Please understand, I have nothing against Harvard University. My undergraduate astronomy professor, who was also best man at my wedding, is an alumnus of that school, and he taught me very well; but the seasons are a concept that need careful pedagogical practices to squash this pervasive misconception. Harvard believed that part of the problem was related to how we illustrated the seasons in books. We depict the Earth very close to its orbital plane and view it as a highly oval-shaped path rather than observing it from overhead where its path is virtually a circle. The ovalness or eccentricity of Earth’s orbit is about 1.67 percent. Take the average solar distance of Earth, 93.0 (92.96) million miles, and multiply it by the ovalness or eccentricity of Earth’s orbit. The answer is a little more than 1.5 million miles. Add and subtract that number from Earth average distance and you get 91.5 and 94.5 million miles which represents an excellent approximation of the Earth’s perihelion distance (January) and its aphelion distance (July). On July 6 we will be 94,499,000 miles from Sol. On January 3 we were 91,393,000 miles distant. Unlike Harvard, I personally believe that we take away from childhood the strong misconception of considering the sun like a roaring fire. We approach a fire to get warmer and step back to get cooler. It’s as simple as that. So how do we get more energy from a sun that is farther away from us in the summer and less energy from Sol when we are closer to it in the winter? It all depends upon the angle that the sunlight is striking our locale, and that is created by the 23-1/2 degree tilt of the Earth’s axis (from the perpendicular to its orbital plane). It is analogous to taking a flashlight (the sun) and shining it against a wall, first from a nearly vertical position (summer), and then, keeping the light at the same distance from the wall and shining the flashlight at a low angle (winter). You’ll notice how the beam spreads and dims during the winter, providing less energy per unit area on the wall’s surface. The same analogy applies if the Earth’s axis is tilted. During summer we lean towards the sun and receive more direct energy from a high sun. In winter the northern hemisphere leans back, and we receive less energy from Sol which is much lower in the sky. Because of Earth’s axial tilt, the sun is above the horizon for a briefer period of daylight in winter and an extended period of sunshine during the summer, sort of a double whammy which amplifies the effects of cooling and heating. If you are already fed up with summer’s heat, it will take at least two more months before more permanent relief can be expected.

1142 JULY 8, 2018: Planets and Moon Steal the Spotlight Again: It is another interesting week as the planets once again take center stage amidst the stars of the summer sky. Brilliant Venus and bright Jupiter highlight the early evening hours. Even though Venus is moving away from the sun towards its angle of greatest (eastern) elongation on August 17, it is getting lower in the sky. Just like the sun lowers its altitude as summer merges into autumn, Venus is already approaching the sun’s fall equinox location, but the main culprit is how the ecliptic is positioned to the horizon. The ecliptic is the projection of Earth’s orbit into space, the plane of our solar system, and as we move towards autumn, it is tilted at a low angle to the horizon making it difficult for objects moving away from the sun to gain much altitude. During July, Venus drops 14 degrees, so that by August 17, when it is farthest from the sun in what is traditionally its best position for observation, the Goddess of Beauty and Love will only be 15 degrees above the WSW horizon, 20 minutes after sundown. Venus will still be very bright and easily spotted, but a better western horizon will be necessary. Had this same event occurred in late March, when the ecliptic was tilted at its steepest inclination to the horizon, Venus would have been nearly halfway up in the sky at the same time after sundown. Still, don’t give up; Venus is still a beauty, the brightest of the ancient wanderers. This week holds yet another surprise. If you wait an additional 10 minutes or so to a half hour after sunset, you may notice another starlike object becoming visible below and to the right of Venus. That will be Mercury. My rule for the Messenger God is to bring along a pair of binoculars to make the initial sighting. On average, ordinary binoculars will collect 25 to 50 times the amount of light that the eye gathers, making the observation much less difficult. Then see if you can spot Mercury with the unaided eye. The beginning of the week is better to glimpse Mercury because it fades by about 30 percent by week’s end, unless you are planning to make Saturday, July 14, your observation date. Then the five percent, waxing crescent moon will shine just one-and-one-half degrees above Mercury, creating an easy way to spot the Messenger God. The following evening, July 15, Luna, now at 12 percent illumination, is situated only 2.5 degrees to the right of Venus, another splendid and more accessible sight because Venus will be much higher above the WSW horizon. Again, binoculars will make the view more memorable because they collect more light than the eyes. Look for earthshine coming from the unlit portions of the moon, the light from a nearly full Earth reflected back to us from the lunar surface still in darkness. Moving your eyes to the left, just west of south, you’ll see the king of all planets, Jupiter. Use binoculars on the evenings of July 14/15, when the crescent moon will be near to Mercury and Venus, and try spotting two faint “stars” just to the left of Jupiter. They will be Ganymede and Callisto (farthest). Both satellites are about the size of the planet Mercury, Ganymede a little bigger and Callisto a little smaller. If they were against a dark background and had no competition from the glare of Jupiter, they would be a “no brainer” to view. The two moons are best positioned for binocular observations on July 15 when the moon is in conjunction with Venus. Low in the east about 30 minutes after sundown, you’ll see Saturn which will be followed by Mars a little later. All in all, this week offers a wonderful opportunity to view all of the naked eye planets sweeping from west to south to east, outlining the plane of the solar system on which they nearly orbit. Keep looking up!

1143 JULY 15, 2018: Lunar Luminescence: This week offers a wonderful opportunity to watch the moon blossom from a thin waxing crescent into its bulbous gibbous phase. As you watch more and more of Luna’s surface waxing in the sunlight of a new day, you’ll also notice the moon growing in brightness. The changes from night to night, while the moon is in its thin crescent phase, are actually quite subtle because crater walls and mountainous terrain are casting long shadows across the moon’s surface. In addition, the lunar surface is not very reflective, only about 12 percent. Twelve percent represents the appearance of a worn asphalt roadway, but as everyone knows, the moon’s reflectivity is not smooth. The lava flooded basins which are the circular dark regions of the moon mirror the sun’s light about as well as a freshly macadamed road surface which is about four percent. The brightest regions of the moon return to space about 30 percent of the sunlight which falls upon them. As an example, sand reflects sunlight at around 40 percent and green grass at 25 percent. The low lunar reflectivity comes as a shock to most people because the moon appears very bright against the nighttime sky; so even a little luminescence against a black background goes a long way in exaggerating the effects of brightness. Astronomers use a system called apparent magnitude to quantify the brightness of astronomical objects. A variance of one magnitude represents a difference of 2.51 times the intensity between the two light sources. A difference of five magnitudes is a 100-fold intensity change. In the magnitude system, the more negative the number the brighter the object, a difficult concept for students to grasp because it is so counterintuitive. Leave it to astronomers to make what should be a simple concept more complex. By the time you view the moon on Monday, a relatively thin 22 percent sunlit crescent, its apparent magnitude will be about -8.6 or approximately 50 times brighter than Venus which will be nearby—to the right and below Luna. At first quarter on Thursday, July 19, when the moon appears half lit and half in it its own shadow, its magnitude will have grown to -10.3, not quite five times more intense (4.8) than it was on Monday. A first quarter or last quarter moon is still not objectionable when it comes to viewing other objects in the nighttime sky, but you will begin to notice ground objects casting shadows by the light of the brightening moon from suburban locales. Give the moon an additional three days of growing to July 22, when it will be in a waxing gibbous phase located between Jupiter and Saturn, and its light will have become obnoxious and detrimental to seeing faint sky objects. Its brightness will have blossomed to magnitude -11.3, two and one-half times brighter than the first quarter moon and nearly 12 times brighter than the slim crescent that was seen on Monday, July 16. At full moon on the 27th, when Luna will appear near to bright ruddy Mars, its magnitude will have grown to -12.7, yet another 3.6 times brighter than the luminescence of the moon on the 22nd. How much brighter is a first quarter moon in comparison to a full moon? The answer is about 10 times. If you use the data provided in this article, you will get an intensity difference slightly greater than nine. The difference is created by the fact that when we see the moon on July 19, it will actually be just beyond first quarter with 53 percent of its surface illuminated, thus reducing the difference in brightness between the quarter and full moons. Have fun watching the moon brighten during the next two weeks, and then start thinking about meteors. The Perseids are coming in less than a month, and this year they will be visible under a new moon, a moon not visible at all.

1144 JULY 22, 2018: Perverted: It was all about “Optics”: I was a third or fourth year educator in the Allentown School District and teaching in the planetarium at Dieruff High School when I had decided to instruct an evening in-service course on introductory astronomy to ASD professionals. I was the youngest member of the group. The 10-week seminar filled quickly; the first couple of lessons went well, and everybody seemed to be enjoying themselves. For Lesson Three, the syllabus called for discussing telescopes, and I introduced the unit with vocabulary which was pertinent to the subject. Telescopes bring light to a focus in many creative ways. Since the image is formed beyond the focus position of the lenses or mirrors collecting the light, they must cross each other before entering the eyepiece. If you are looking at a very distant tree, it will appear up-side down. That is called an inverted image. Likewise, the rays of light that form the left side of the image will now be viewed on the right side in the eyepiece. Left and right are also interchanged. There is a word for that in optics, and it is called a perverted image. The second that word slipped from my lips, a woman stood up in the back of the classroom. I only knew that she was a librarian from a different school. She literally shouted, “That is not the definition for PERVERTED! Where’s a dictionary?” I was stunned, but my audience remained strangely aloof. It turned out that I was teaching in an English as a Second Language (ESL) classroom, and there were dictionaries all over the place, little tiny paperbacks, highly-abridged, yellow-covered Webster’s dictionaries. She found one, paged through it to perverted and read the definition out loud. “See,” she touted in an assertive tone. Then she grabbed her purse and sweater and left the room. Another teacher also departed. I was mortified. A quick glance at the clock revealed that I still had an hour-and-a-half of instruction left. From that point onward, every word, every gesture, every movement seemed to extend itself in agonizingly slow motion. It was the longest 90 minutes of my teaching career. I went home that evening depressed, convinced that after that course I would never teach teachers again; but the Earth rotated, the sun rose the next morning, and I put on a happy face and went back to school. The day went well. Sixth period arrived, my prep, and I hurriedly walked down to the Dieruff Library to confront the mother of all American dictionaries, the brown, worn-looking Webster’s unabridged, the one that weighed 75 pounds and had its own highly varnished wooden table placed in a corner of the main library room. There were over 30 definitions for perverted, and so I began reading. By definition 25, I was in a panic—nothing. The very last entry had optics in italics and spoke of the interchanging of left and right in an image. “Yes!” I thought feeling vindicated. I knew I had been correct. Hoisting that Webster’s (with permission), I lugged it to the only copier in the building and produced 25 duplicates of the page with the multitude of definitions for perverted. By 5 p.m. every teacher in my class had an intradistrict envelope addressed to them with a copy of the page which defined perverted and the optics definition highlighted in yellow. I was much more confident the next time I met my class the following Tuesday. The lesson went well, but the two people who had abandoned the seminar the week before never returned. In a way I was relieved. I never wanted to have that kind of public confrontation again. After class a teacher pulled me aside to explain what had really happened. “It was nothing that you did,” he explained in almost a whisper. “They were having an affair and your class and that moment merely provided them with their escape.” I went on to instruct many more astronomy in-service classes to professionals throughout my 38-year public-school teaching career, but the memory of that night still remains vividly etched in my mind. Perverted: The interchanging of left and right in an optical image among a host of more scandalous entries.

1145 JULY 29, 2018: Shooting Stars on the Increase: It was the very first time that I had observed meteors seriously and stayed up all night from dusk to dawn. It was the greatest number of meteors that I had ever seen in a single evening, 219. It was the first time I had ever photographed a meteor and, in fact, multiple meteors on the same photographic frame. It was the summer of ’69, and the Perseid meteor shower was in full swing. With an all-consuming interest in astronomy, but not having enough money to buy a telescope, I turned to observing meteors as a means to get my observational fixes. My grandfather, Arthur Becker, gave me his binoculars; my parents bought me an 8-inch mirror grinding kit to make my own telescope which had been completed the year before, but I never lost the enjoyment of laying out under a velvety, black firmament sprinkled with the familiar stars that I loved during those simpler times. With a weekly allowance of 25 cents in elementary school, 50 cents in junior high, and one dollar in high school, there wasn’t much money left over for astronomy, so visual stargazing which involved meteor observations became my passion. It sounded like tough love, but my parents were always very supportive of my astronomical pursuits which would become my teaching career for 45 years, and continues today at Moravian College. Now with the Perseids almost upon us, nearly 50 years after that first all-nighter at Pulpit Rock Astronomical Park near Hamburg, PA, I’m headed again to Guernsey State Park in eastern Wyoming with Pete Detterline and Jacob Wetzel to share this same excitement with park visitors who also want to participate in all-night Perseid watches. I saw the total solar eclipse at Guernsey last year and was hooked with the park’s dark skies and Guernsey’s administrators who were eager to promote astronomy. I have gotten some of my greatest enjoyment when I’m with my telescope out alone observing the heavens. I personally believe the night sky is the greatest “church” ever constructed, and for me it fulfills a spiritual vacuum that the secular world and organized religion simply cannot provide. The universe is just so much bigger than anything that we can imagine. However, when it comes to viewing meteors, the desire to participate in a group event wins all of the time. To hear the excited exclamations of old and new friends sometimes spontaneously yelling in exuberance as a blast of unexpected light from a bright fireball singes the upper atmosphere to incandescence, far outweighs the solemnity of self-contemplation. It’s just more fun to observe meteors in a group. I believe another draw of meteor astronomy is the serendipity of the events. Sometimes you will wait five minutes for an unimpressive meteor to glimmer swiftly among the stars to be followed by another five minutes of nothingness. As a counterpoint, a bright meteor will suddenly sputter to its death or waver from its straight-line path or leave a trail of glowing ionized air in its wake which stays visible for several seconds after the initial explosion of light. Sometimes a meteor will end with a terminal burst which strobes the dilatated eyes. I’ve seen it all, including one bolide that was brighter than the full moon. With the Perseids there can also be flurries of activity, two or three meteors witnessed in just a minute’s time. My personal record was set in 2016 in Montana, six Perseids in 20 seconds, all within a limited area of the sky, none of them exceptionally bright, but probably related to a single larger fragment that had broken up hundreds of years earlier, its smaller pieces drifting slowly apart. The final incentive to viewing meteors is that expensive equipment is not necessary. A sleeping bag or bedroll, a tarp if dew is a problem, a relatively dark sky, some snacks, a caffeinated drink, a dash of insomnia, and a desire for the unexpected can create a very satisfying experience. There will be more specifics about meteor observing next week or read ahead at http://astronomy.org/StarWatch/August/index-8-18.html#8-5-18.

[July Star Map]

[July Moon Phase Calendar]