StarWatch for the greater Lehigh Valley

JUNE  2024


[Moon Phases]


Current Solar X-rays:    

Current Geomagnetic Field:    


1450    JUNE 2, 2024:   Auroras: Dancing Curtains of Light
On the evening and morning of May 10/11, North America and Europe experienced one of the most spectacular aurora displays in recent decades. The northern lights were seen as far south as the Florida Keys and Mexico. The Lehigh Valley was cloudy for most of the evening, but the weather cleared briefly around dawn, about an hour after I went to bed. My friend, Adam Jones, happened to be traveling near Gillette, Wyoming, on May 10 and witnessed the display from Devils Tower National Monument. There was no phenomenon associated with auroras that he did not observe, including arcs, bands, curtains, rapid auroral motion, rayed arcs, and coronas in all the rainbow colors. Adam's four-minute video taken in actual time with an iPhone 13 gives credence to the magnificence of the display. The spectacle lasted from dusk to dawn and was bright enough for him to navigate without artificial lighting. The three-day-old moon set around 10:30 p.m. See Adam's impressive video here. * We are in the auroral high season, which can last for several years, sparked by particularly intense geomagnetic storms associated with an active sun, now approaching sunspot maximum. I am headed to Iceland this fall to see the northern lights (hopefully), but other solar outbursts that might bring the auroras to the Lehigh Valley are certainly possible during the next several years. Here is a condensed primer on their origins. * Auroras are formed by the solar wind interacting with our atmosphere. Charged particles, primarily electrons and protons, escape the sun's corona, its outermost atmospheric layer, at the location of coronal holes where the sun's magnetic field snaps (does not loop back into the sun) transporting them wayward. Primarily coronal mass ejections (CMEs) and flares are the events that allow large quantities of plasma to escape and spiral outward from the sun. Some solar events are directed toward the Earth along the sun's tangled and twisted magnetic web. The solar wind travels at an average speed of one million miles per hour, but much higher velocities occur in major outbursts. Charged particles from the sun surrender their solar influences to the Earth's magnetic field (magnetosphere), created by the synchronized motion of electrons in the Earth's liquid outer core. * The magnetosphere wiggles like Jell-O, sculpted by the pressure waves created by the solar wind. It dragged into an elongated, teardrop-shaped structure called the magnetotail. In this area, magnetic lines of opposite polarity can occasionally connect, heating, accelerating, and trapping solar plasma within the magnetosphere of Earth. Once ensnared, the plasma works its way back along magnetic field lines generated by the Earth into the inner and outer Van Allen Radiation Belts. In these two densely packed magnetosphere regions, particles can spiral back and forth from geomagnetic pole to pole at near-light speed. When the Van Allen Belts become overwhelmed, the plasma will detach and spill deep into the Earth's atmosphere around a circular region called the auroral oval that hovers near the Arctic Circle. In the Northern Hemisphere the oval centers itself southwest of the town of Alert on Canada's Ellesmere Island. As the plasma descends, primarily electrons and some protons sideswipe air molecules, transferring their momentum (energy) to the outer electrons of oxygen atoms and nitrogen molecules. This causes them to jump to specific higher energy levels. When these excited electrons transition back to their lowest energy states, they descend in a ladder-like pattern in a variety of steps or jumps. Many of these transitions produce ultraviolet and infrared radiation, invisible to the human eye; however, a small number of these downward energy transitions generate the flickering, dancing, quivering light that we see as auroras. Let's hope another vibrant display occurs soon. Ad Astra!

1451    JUNE 9, 2024:   Judging Auroral Activity
One of my favorite local aurora stories dates back 30 years to an evening program in the Allentown School District Planetarium, now the Learning Dome. The presentation had ended; my audience had departed when the phone rang. It was a reporter from the Morning Call newspaper. He was getting accounts from people around the Lehigh Valley about strange lights in the sky, and indeed, he thought there might be an outbreak of the northern lights. I had been talking about the aurora borealis in my presentation, and when I ventured out of doors in the northwest, I was able to confirm a little patch of slowly moving pink light. I never saw the aurora again from Allentown or anywhere in the local region, but I serendipitously saw a display from Pulpit Rock Astronomical Park in Berks County on November 7, 1971. My best observation of the northern lights was from unlikely New Mexico on November 9, 1991. Go here and scroll down to see my three digitized emulsion images. * Southeastern PA is also an improbable location for viewing auroras. So what are the indicators that best describe when we might see a display? Start with a wonderful application, spaceweatherlive, and its web sidekick, which devotes all of its attention to the auroral phenomena. The Kp index will be seen first. It is a combined measurement over the past three hours determined by widely spaced magnetic observatories. Its range is between 0 and 9. Click on the Kp index forecast to see predicted activity for the next three days. If you are under the auroral oval where downward moving solar plasma focuses, indices of 3 and 4 can produce good displays. In SE PA, the Kp index should most likely be bumped up to at least 7 or 8. During the May 10-11 mega-storm, the Kp index was 9. Indices from 5 through 9, equal G1-5 storms. Therefore a G2 storm translates to a Kp 6 event. Another consideration is the speed of the solar plasma passing Earth. Numbers above 400 km/sec, along with particle densities in the 10-20 plasma particles per cubic centimeter, begin to pique aurora chasers like Marybeth Kiczenski who wrote an inspiring article published in the May 2024 issue of Sky and Telescope magazine and answered questions that I posed to her in several emails. She lives in the upper peninsula of Michigan, which is considerably closer to the auroral oval than the Lehigh Valley. Hopefully, her advice will help enthusiasts make better independent decisions to see auroras from our local area. * There are many other markers needed for a good observation. The interplanetary magnetic field (Bz) measured in nanoteslas provides another valuable resource because it assesses the magnetic polarity of the incoming plasma. Solar plasma will not be swept into the magnetosphere of the Earth unless the polarity is opposite to the polarity of the Northern Hemisphere, so the Bz numbers must be negative to be an influential aurora producer. In the world of magnetism like charges repel and opposite charges attract. Marybeth's instincts begin to become piqued when the Bz numbers are -5 or greater. For the Kp 9 storm of May 10/11, the Bz was measured at -40.42 nT. Another interplanetary magnetic field indicator, the Bt number, also measured in nanoteslas is a potential sign of auroras. Here the higher the number, the better the chances of seeing a display. During the May storm, the Bt was measured at 47.21 nT. When I wrote this portion of my blog on May 27, just after midnight, conditions were calm, with a Bz of -0.2 and a Bt of 3.8. The Kp index was 1.4; the solar wind was moving at 414 km/s with a 5.03 particles/cubic centimeter density. However, even these ho-hum numbers will produce a thin undulating strip of green under the auroral oval from places like northern Alaska and Canada, Iceland, and northern Norway. * Please note that all times in spaceweatherlive are given in Coordinated Universal Time (UTC). Subtract 4 hours for EDT and 5 hours for EST. Another good site is the Aurora Forecast, which predicts what the auroral oval will look like about an hour into the future. Find the link here. Auroras are most active around magnetic midnight, which is 1 a.m. during periods of daylight saving time and midnight at other standard times of the year. The northern lights also favor the spring and fall months for unknown reasons. Now, go outside and find yourself an aurora to view. Ad Astra!

1452    JUNE 16, 2024:   A Number to Remember, 23.5
Another summer is almost upon us with the solstice to be reached on Thursday, June 20, at 4.51 p.m. EDT. It is the highest noontime sun (1 p.m. EDT) of the year and the longest day to boot. The sun will reach its northernmost limit shining directly upon the Tropic of Cancer, and then begin its slow descent past the equator on Sunday, September 22, at 8:42 a.m. (Autumnal Equinox) to reach its southernmost limit, the Tropic of Capricorn on our winter solstice, which occurs on Saturday, December 21 at 5:19 a.m. * As an elementary school pupil, I remember being introduced to these particular tropic limits somewhere in the third or fourth grade and thinking very seriously of some worker with a very wide paintbrush inscribing these precise circles across the landmasses of the Earth. One day, when I was older and could drive, I would witness the lines for myself. I also remember receiving no specific reason why these circles were present, including the Arctic and Antarctic circles, until perhaps high school. The explanation is not complicated. * The seasons are a condition that results from Earth's axial tilt, which is 23.5 degrees from the perpendicular to its orbit. Put another way, the Earth's equator, the great circle equidistant from Earth's two poles, is tilted 23.5 degrees to our planet's orbital plane, called the ecliptic. As our inclined planet travels around the sun, Sol must shine north of the equator when the northern hemisphere leans towards the sun and below the equator when we lean back from Sol. This leaning toward and back from the sun has nothing to do with distance. Earth is actually farthest from the sun in early July, but Sol's energy is more direct, heating the northern hemisphere more effectively. * The maximum deviation from the equator that the sun can reach must be the same as the Earth's axial tilt, so the Tropic of Cancer is located 23.5 degrees north of the equator, while Capricorn is positioned 23.5 degrees south of the equator. If Earth's axial tilt were 40.5 degrees, the Tropic of Cancer would pass through the Lehigh Valley, and maybe I'd be in charge of "maintaining that portion of the path." * There is also the consideration of the tropic names, Cancer and Capricorn. The northern constellation of Cancer, the supposed location of the sun at summer solstice in the 21st century, is incorrect. That position has shifted to the Gemini-Taurus border. The answer lies with precession, the conical wobbling of Earth's axis over 26,000 years. This movement over time causes the solstice positions to migrate westward against the constellations. Thousands of years ago when the zodiac (ecliptic) constellations that Sol passes in a year's time was first understood, the summer solstice sun was in Cancer, and the winter solstice occurred when Sol was in Capricorn. * However, what about the Arctic and Antarctic circles, the first locations, 66.5 degrees north and south latitudes, where the sun remains visible for 24 hours at their respective summer solstices? These smaller circles girding Earth are located 23.5 degrees south of the North Pole (Arctic Circle) and 23.5 degrees north of the South Pole (Antarctic Circle). Multiply 23.5 degrees by two, and you get 47 degrees, the complete altitude change in the sun's position during one year. Every location on Earth experiences this change. In the Lehigh Valley, the sun's height above the horizon at noon can swoop to a low of 26 degrees at the winter solstice and a maximum altitude of 73 degrees at the summer solstice. * Have you had enough of the number 23.5? So have I. Ad Astra!

1453    JUNE 23, 2024:   Spring and Summer's Extended Twilight
I get a lot of astronomy questions from Bonnie Brooks, a dedicated StarWatch aficionado, lifelong observer of the heavens, and a Professor of Mathematics and Physics. Bonnie's question went like this. As the days are lengthening, why does twilight last longer? I honestly never thought about that for a mid-latitude location like ours. * Moravian's latitude is approximately 40.5 degrees north of the equator. Subtracting 90 degrees from your latitude position gives the rising and setting angles of the sun. They are always the same all year long because they are only a function of Earth's rotation and latitude position. The arithmetic is simple: 90 degrees – your latitude position = the sun's rise and set angles: 90 deg. – 40.5 deg. = 49.5 degrees. The sun will always rise and set at that angle from the Sky Deck at Moravian University or when an observer's latitude, north or south of the equator, matches that of Moravian's location. However, I also realized that the sun's angular distance below the horizon at midnight (1 a.m. EDT) varies greatly throughout the year, and this would have to change the length of twilight, increasing its duration as the sun's nadir (lowest) position became shallower during the spring and summer months. Although the angle of the rising and setting sun remains constant for any given location, the slope of the sun's path below the horizon does not. It changes (decreases) more rapidly when the sun's depth is shallow, like it is in the spring and summer. This has the effect of lengthening the time of twilight. * The nine-hour trek of Sol after sunset at summer solstice brings our daystar to a depth of only 26 degrees below the horizon at local midnight, 4-1/2 hours after sundown where it is traveling parallel to the horizon before beginning its upward climb towards sunrise. * Mathematically, the greatest angular distance (depth) the sun can go beneath the horizon is equivalent to 90 deg. - (your latitude +/- the solar declination). The solar declination is simply the angle that the sun is north (+) or south (-) of the equator. The sun shines directly over the Tropic of Cancer at the summer solstice, 23.5 deg. north of the equator or at a declination of +23.5 deg. So, 90 – (40.5 + 23.5) = 90 – 64 = 26 degrees. The sun will only reach a position of 26 degrees below the horizon at 40.5 degrees north latitude on the summer solstice. However, at winter solstice we have 15 hours of sundown. Sol is over the Tropic of Capricorn, 23.5 deg. south of the equator (-23.5 deg.). In winter, the sun gets much farther below the horizon at local midnight. Because Sol has a longer distance (and time) to travel before its arc turns upward, 7-1/2 hours, the slope of the sun's path will change less rapidly after sundown, causing the sun to gain a greater depth below the horizon in a shorter time interval. Twilight will have a shorter duration. Therefore, 90 deg. – [your latitude + (-23.5)] would be how the formula would be structured at the winter solstice. So, 90 – (40.5 - 23.5) = 90 – 17 = 73 degrees below the horizon for Moravian's latitude. Note that the number of degrees the sun is below the horizon at the winter solstice is equivalent to the noontime altitude of the sun above the horizon at the summer solstice. That makes sense since the seasons are opposite to each other. * For Moravian University, summer solstice twilight lasts 123 minutes while at winter solstice, that duration shrinks to 98 minutes according to my calculations in The Astronomical Almanac, a product of the US Naval Observatory. Twilight ends when the sun is 18 degrees below the horizon. Technically, twilight lasts all night at summer solstice, northward of 48.5 degrees north latitude and never changes at the equator, making this concept nonlinear. Hopefully, Bonnie, that makes a little sense. Thanks for your interest, questions, and for reading StarWatch. See below for an image explaining extended twilight. Ad Astra!

[Changing Length of Twilight]

1454    JUNE 30, 2024:   Earth's Equinoxes, Solstices, Perihelion, Aphelion Change

[Table of Changes]


[June Star Map]

[June Moon Phase Calendar]