StarWatch: Moravian University Astronomy

SEPTEMBER 2024

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

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lunar phases

1463 SEPTEMBER 1, 2024: The Other Perseid Meteor Shower: Not quite three weeks have passed since the astronomy community worldwide was treated to the Perseid Meteor Shower. Combining reasonable hourly rates with pleasant summer weather makes this the ideal experience for meteor enthusiasts, a community of like-minded individuals. However, did you know that there is another event where shooting stars emanate from the constellation of Perseus the Hero? The shower is called the September Epsilon Perseids or September Perseids for short. * Back in the day (1970), when money was tight and free time was more abundant, I found myself with friends at Pulpit Rock Astronomical Park near Hamburg, PA, the 1535-foot elevation, dark sky, observing location of the Lehigh Valley Amateur Astronomical Society, Inc. The location is seven miles south of Hawk Mountain along the first escarpment of the Blue Mountains. The date was probably September 5, 1970, and I was psyched for an all-nighter of observing shooting stars. Around 1 a.m., as Perseus began to rise above the nearby trees, I noticed that many of the meteors that I was observing seemed to emanate near the star, Epsilon Persei, a star in the eastern leg of the constellation. I did not see scores of meteors like I had with the Perseids, but rather a few shooting stars short of 20, coming from a distinct radiant over perhaps three hours of time. Thinking I might be the sole proprietor of this discovery, upon returning home, I quickly found that the September Perseids were documented by meteor enthusiasts as an authentic shower. Having said all that, I was proud that I had enough observing experience to realize something interesting was happening in the heavens. * The September Perseids are still with us and will reach maximum activity on Monday, September 9, around 2 a.m., an ideal time for East Coast observers to see the greatest activity from this event. The moon will set on the eighth at about 9:40 p.m. * Very little is known about this meteor stream, as the International Meteor Organization (IMO) has reported. All meteor events have a progenitor spilling dross into its orbital path when near to the sun. Most showers originate from a comet's dusty tail, but at least one is a comet turned asteroid. No precursor is known for the September Perseids. Rates are low, about five meteors per hour, perhaps due to its unknown parent object having made only one near Earth passage, according to IMO information. Occasionally, outbursts have occurred, as observers witnessed in 2008 and 2013. The meteors are relatively bright and fast. If you observe this event, meteor counts in 10-minute intervals would be helpful. The shooting stars that I observed from this shower were witnessed with the abovementioned characteristics. A map showing the location of the radiant can be found here. Much success! Ad Astra!

1464

SEPTEMBER 8, 2024: Partial Lunar Eclipse, Neptune Occultation

I'm always up for an eclipse, and one is about to happen on Tuesday, September 17. It is a shallow partial lunar eclipse that reaches maximum coverage at 10:45 p.m., with about 8.5 percent of the moon intruding into the Earth's primary shadow. * Eclipses are named for the object to be hidden, so solar eclipses involve the moon hiding the sun, and lunar eclipses have the Earth's shadow covering the moon. Because the sun, the Earth, and the moon are not point sources, eclipses do not require the Earth, sun, and moon to be in a perfect alignment with each other. * Whether it is a lunar or solar eclipse, there are always two shadows, the umbra and the penumbra, which are involved. Both shadows stem from the Latin, with the umbra defined as shade. It is the primary shadow of the Earth or the moon. If the moon is entirely within the Earth's umbra, the lunar eclipse is said to be total because from the moon's vantage point, no part of the sun's light-emitting disk can be viewed. Likewise, if Luna is partly in the umbra and partly in the penumbra, the lunar eclipse is said to be partial. An observer in the penumbra (light shade) of the Earth's shadow will see part of the sun hidden by part of the Earth, hence a lessening of the sun's light intensity and a dusky appearing moon as seen from the Earth. * The eclipse begins at 8:41 p.m. EDT, as the moon enters the penumbra of Earth's shadow. As the penumbral phase of the eclipse begins, there is so little shading of the sun's light on the moon's surface that the eye cannot distinguish any light diminution. Give the moon about a half hour (9:10 p.m.) to get deep enough into the penumbra for it to have a slightly subdued appearance and another hour (10:13 p.m.) before the moon's limb (edge) touches the umbra. The one hour, 32 minute foray through the Earth's penumbra before the central shadow is encountered is a consequence of the moon's near tangential motion to the Earth's umbra. The time will be 10:13 p.m. Mid-eclipse occurs at 10:45 p.m. By 11:17 p.m., the moon has passed through the thin slice of umbra and returned to the penumbra to begin another one hour, 30 minute sojourn, bringing Luna tangent and outside of the penumbra once again. The time will be 12:47 a.m. on Wednesday, September 18. These tangential eclipses are long penumbral events (4 hours, 10 minutes) when only a tiny fraction of the moon encounters the Earth's umbra. * However, don't put your telescope away just yet. You may as well go inside, grab something to eat, and take a short nap for the next major event of the night. The occultation of the moon by Neptune will be just a few hours away. Saturn was occulted by the moon earlier on the 17th, but that event was not visible from the East Coast. According to my computer, the Lehigh Valley is positioned near the southern limit of this event which occurs at approximately 4:12 a.m. The moon will be 30 degrees above the WSW horizon. The occultation will end about 20 minutes later. Because the full moon is so bright and Neptune is relatively faint, try watching the moon pass 5.5 magnitude, 20 Piscium (Pisces the Fish), at 12:12 a.m. on September 18. This will happen while Luna is still within the Earth's penumbral shadow. If 20 Piscium is challenging to view, then Neptune, 8.3 times fainter, will be impossible, and you can rest easy and get to bed at a more reasonable time. For the Neptune occultation, you will need a telescope. Ad Astra!

1465 SEPTEMBER 15, 2024: Sliding into Fall: That simple word always brings a feeling of sadness to my soul—a respite from summer's heat, a prelude to winter's shiver. Spring and summer are my favorite times of the year; however, my wife, Susan, is just the opposite, loving the fall through the New Year as her time and tolerating the summer months in an air conditioned environment. So why does the sun do its downward slide to cold and darkness? * The Earth's axial tilt causes the equator to be angled at 23.5 degrees to the plane of its orbit, the ecliptic, where the sun is always positioned. The sun glides along this path, a reflection of Earth's orbital motion reaching its high point above the equator in June, producing those endless days of summer, as well as Sol's low, shadowed, short days of winter. After the summer solstice, the sun slowly begins its downward trek, accelerating until the autumnal equinox, where its daily descending spiral reaches its most significant change. The transition from summer to fall happens when the sun crosses the equator, moving from favoring the Northern Hemisphere to shining more directly over the Southern Hemisphere. That occurs on Sunday, September 22, at 8:42 a.m. EDT, when the sun reaches its midpoint, the autumnal equinox, the mean between the extremes of the seasons. From the Latin, equinox means "equal nights," but conditions are not exactly the same. Day and night would be equal if the Earth had no atmosphere, but since it does, the Earth's ocean of air refracts the sun's disk upward by approximately 35 minutes of arc (35/60th of a degree) when the center of the sun should be on the horizon. This is called astronomical refraction. The effect of an artificially higher sun causes Sol to set just over three minutes later for us than it would if our planet were airless, like the moon. Astronomical refraction also causes the sun to rise about three minutes earlier than if the Earth had no atmosphere. * Despite my gloominess over the colder weather that awaits, 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 fall begins, 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, like Moravian University's location. The effects are more pronounced northward and less noticeable southward. Reykjavik, Iceland at 64 degrees north latitude will lose about six minutes of sunlight each day surrounding the fall equinox, one hour, 24 minutes during the same two week interval. * 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, causing the land and oceans to heat. Temperatures along the East Coast reach their yearly maximums around the third week in July, which causes the effects of warmer conditions to spill over well into the autumn months. The big chill does not commence until sometime in November. Halloween can still be balmy, but by Thanksgiving, normally 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, and my wife is in a celebratory mood. Winter people, rejoice. Ad Astra!

1466 SEPTEMBER 22, 2024: Collision: A Cosmic Dance in the Making: Looking up into the night sky, it feels like a peaceful, quiescent, static montage of stars, but our universe is far more dynamic than it appears on the surface. One of the most expectant cosmic events is the impending collision between our Milky Way Galaxy and its neighbor, the Andromeda Galaxy. "Cannibalism" is how many galaxies evolve. This grand cosmic, head-on crash, expected to occur in about 3.5 billion years, will be a spectacular event, reshaping both structures in ways that astronomers are only beginning to understand. * The Milky Way and Andromeda are part of the Local Group, a collection of at least 80 much smaller galaxies bound together by gravity. Andromeda is the largest, with about 600 billion stars, compared to the Milky Way's estimated 400 billion luminaries. Despite their massive sizes, these island universes are relatively close celestial neighbors, separated by only 2.5 million light years. A light year is the distance that light travels in one year, almost six trillion miles. * Observations using the Hubble Space Telescope and other instrumentation have shown that Andromeda is approaching the Milky Way at approximately 80 miles per second. This might sound alarmingly fast, but it is only at a snail's pace on the cosmic scale. When the collision commences, the galaxies will take hundreds of millions of years to merge in an event spanning over a billion years. * As the Milky Way and Andromeda approach, they will start to distort each other's shapes, creating long tidal streamers of stars and hot gasses. The first close encounter will happen in about 3.45 billion years when our night skies will be transformed by an increasing number of bright stars in open clusters, regions where star generation transpires. Despite the trillion stars and stellar systems involved, the vast distances among individual objects mean that there will be very few direct collisions. Instead, the primary effect will be on the interstellar gas and dust. This material will be churned and compressed, triggering star bursting, the rapid creation of new massive, short-lived stars, as well as supernovae, the explosive deaths of these same stars, that will also add to the cosmic turmoil, creating lower mass stars in their wake. * Eventually, the Milky Way and Andromeda will settle into a single, larger galaxy, most likely elliptical, surrounded by a diffuse halo of stars from both parent structures. This new entity, formed from the merger of the Milky Way and Andromeda, has been tentatively named Milkomeda, an obvious combination of Milky Way and Andromeda. Let's hope that galactic nationalism will be dead by that point. * While the merging process will dramatically alter the interstellar landscape, the aging sun and its planets are expected to survive the amalgamation. The future collision of the Milky Way and Andromeda galaxies is a testament to the dynamic nature of our evolving universe. Galaxies are not static objects, but systems engaged in an ongoing dance of creation and destruction. * For now, as we look into the heavens, the distant future will be ablaze with the light from the creation of billions of new stars, a testament to the grandeur of this future galactic merger. Ad Astra!

1467 SEPTEMBER 29, 2024: Down Low Moon: On September 18, we celebrated the Harvest Moon, a time of the year when Luna rises sequentially over several nights after its full phase with a minimum time difference. This moon is special because its orbital path has its shallowest intersection to the horizon. So although the moon moves an average of 13 degrees per day along its orbital path, it loses very little angular distance below the horizon. This situation causes the Earth's rotation to make Luna rise with very little difference in time. Nineteen-century agrarians who did not have the luxury of electric lighting on their farming equipment could continue harvesting crops after sundown, substituting the bright moon for the vanished sun. This past Harvest Moon for Moravian University's latitude saw Luna rise at 7:29 p.m. (full moon), 7:54 p.m. (+ one day), 8:23 p.m. (+ two days), and 8:57 p.m. (+ three days). The difference of only one hour and three minutes in moon risings from full moon to day three does not go on forever, but it is quite demonstrative of why this moon has been called the Harvest Moon. The effect is even more pronounced at higher latitudes, where in the London, England area, farmers saw the moon rise at 7:19 p.m. on the night of the full moon and 8:07 p.m. three days later, a difference of only 48 minutes. * What is interesting about the Harvest Moon is that it also creates a similar sequence of close moonsets around the time of the new moon, keeping the moon low in the west and virtually invisible for three or four days. The moon is new on Wednesday, October 2. Sol's orbital path (called the ecliptic) will be inclined to the horizon by 27 degrees. The moon's orbit is tilted five degrees to the plane of the ecliptic and will pass through its descending node, moving from above to below the ecliptic on the day the moon is new. This condition will position Luna near its greatest angular distance below the ecliptic about the time of its first quarter phase. At first quarter, Luna will only be 21 degrees above the southern horizon, its highest point for the day. That is about as low as the moon can go. Enjoy the down low moon this week and early next week if you can find it. Ad Astra!