September 6: In the predawn eastern sky, the waning crescent moon (26.0 d, 15%) is on a virtual line that connects the Gemini Twins, Castor (α Gem, m = 1.6) and Pollux (β Gem, m = 1.2). The moon is 9.2° from Pollux. Thirty minutes before sunrise, with binoculars, look in the east-northeast for Mercury (m = −1.1) 1.2° to the left of Regulus. Mercury is 6.5° up in the sky.
In the evening sky, Saturn’s retrograde ends. It is 5° to the upper right of Kaus Borealis (λ Sag, m = 2.8), the star at the top of the lid of the Teapot of Sagittarius, and 45° from Jupiter. Mars and Jupiter have the same visual brightness (m = −1.9). Because they have distinctly different colors, do they appear to be the same brightness to you? While they are over 70° apart, it is not appropriate in formal astronomy to compare respective objects’ brightness, but it is a fun activity. Look near the end of twilight because they have nearly the same altitude early in the evening.
The North America Nebula Credit & Copyright: Jason Ware
September 4: As the moon approaches its new phase, look for the Milky Way arching across the evening sky. Cygnus, with its bright star Deneb (α Cyg, m = 1.2), appears to be flying southward along the soft glow of our galaxy. It ranks near the bottom of the 20 (visually) brightest stars list. Along with Rigel (β Ori, m = 0.2), Deneb is one of the brightest stars in our sky. Both stars are approximately 100,000 times brighter than our sun. Deneb seems dimmer to us because it is likely twice the distance of Rigel, and it is surrounded by fewer bright stars. It seems less notable than those stars that gleam from the Orion region of the sky. The Cygnus neighborhood is crowded with dim features. Walter Scott Houston, the expert chronicler of deep sky observing, noted this in his columns in Sky & Telescope magazine. Houston explained that certain sections of the sky had many objects that were not identified on charts, especially those in the densely packed Milky Way. His articles were compiled into a book, Deep-Sky Wonders, by Stephen James O’Meara. In the chapter for September, Houston provided accounts of viewing the North American Nebula. Houston’s description: “One of the most controversial objects among amateur astronomers, the North American Nebula, is familiar from photographs as a large diffuse glow about 3° east and 1° south of Deneb. There has been remarkable diversity of opinion on how small a telescope can show this object visually” (p. 202). While I have never seen it naked eye, Houston stated that some observers reported seeing it without optical assistance. (If you’ve seen the nebula without a telescope or binoculars, describe your observations for me. Enter your description on this document: https://tinyurl.com/north-american-nebula.)
Opposing Crescents (October 8-October 9): The morning of October 8 and the evening of October 9 present an opportunity to see very thin crescent moons, a waning crescent followed by a waxing crescent – opposing crescents. The two windows to see the two crescents are very narrow, weather dependent, and location critical. The western wild fires and dust from the fall harvest could influence whether the crescents are visible as well. Observing the crescents may require you to change locations. The shortest reported interval for viewing opposing crescents is 34.6 hours by Robert C. Victor, former staff astronomer at Abrams Planetarium, Michigan State University. This 2018 interval is about 36.3 hours. Not a record, but certainly a test of observing skills. Shortest intervals occur when the New moon occurs near perigee, hence moving its fastest between the two visible crescents. Because the moon’s crescent is less than 1% illuminated, knowing the moon’s altitude and azimuth are essential. When I asked Mr. Victor about the observing prospects, he made this analysis for my location:
The October pair is worth a try, because the timing of the New Moon for U.S. is good, and the Moon passes well north of the Sun at New. Where will you be on Oct. 8 and 9? If at your hometown, then at morning civil twilight on Oct. 8 at 06:28 CDT, the Moon will be at azimuth 91.1 deg, alt. 3.2 deg (corrected for refraction), and it will have a topocentric elongation of 9.1 degrees and be 16 hrs 19 min before New. You can preview the Moon’s location of that time by observing the Sun from the same site on Sept. 20 at 6:57 a.m. You can start looking for the Sun earlier that morning, and where/whenever you see the Sun just above the eastern horizon relative to your horizon landscape on Sept. 20, the Moon on Oct. 8 will appear in very nearly the same place (azimuth and altitude) 29 minutes earlier. For example, if you see the Sun at 6:57 a.m. CDT on Sept. 20, then the Moon will appear in almost exactly the same place at 6:28 a.m. on Oct. 8. I used Xephem software to calculate all that.
From your location, the Moon at evening civil twilight on Tuesday, October 9, at 6:49 p.m. CDT, will be at azimuth 259.2 deg., altitude 2.5 deg., with a topocentric elongation of 11.2 degrees and an age of 20 hr 02 min. Just by an unusual coincidence, the Moon’s declination at the time it can be viewed that evening is almost precisely the same as the Sun’s. So you can preview the Moon’s location simply by noting the Sun’s location 45 minutes earlier. For example, note the Sun’s location at 6:04 p.m. (17 minutes before sunset), and the Moon will be there 45 minutes later, at 6:49 p.m.
So even for a very experienced sky watcher, some planning is necessary.
Using a planetarium program like Stellarium (stellarium.org) to calculate the crescents’ positions is helpful. Using Google Sky during your observing may help. Use the USNO’s online calculator (http://aa.usno.navy.mil/data/docs/RS_OneDay.php), use Form B, to calculate the moon’s rising and setting times, and the moon’s rising and setting azimuths for your specific latitude. On October 8, look east about 45 minutes before sunrise. The moon is 5° up. On the evening of the next day, about 20 minutes after sunset, look west. Mercury is 5.5° to the lower left of the very thin moon. To prepare for this, follow the moon in the morning sky as outlined in our daily notes that follow. As the moon approaches Regulus, this should give you some indication whether you have a clear horizon for the morning observation. On the evenings of October 7 and October 8, look for Mercury about 20 minutes after sunset. This, again, indicates whether you have a clear horizon for the evening crescent. For the morning observation, note the last time you see the crescent. For the second observation record the time you first see the emerging moon. If you view either crescent, please enter your observations here: https://tinyurl.com/opposing-crescents-2018
September 1: Mercury (m = −9) is 10° up in the east-northeast, 30 minutes before sunrise. It is approaching a conjunction with Regulus (α Leo, m = 1.3). This morning Regulus is 7.8° below Mercury, and very difficult to locate, even with optical assistance and a perfect horizon.
In the evening sky, for most of the month, Venus (m = −4.7) and Spica (m = 1.0) set at nearly the same time, 85 minutes after sunset this evening. As they separate, Venus moves farther south. (Recall that the farther north an object the longer it stays in the sky.) As they slide into twilight the largest time gap in their setting times is 15 minutes. Jupiter (m = −1.9) is 24° to the upper left of Venus. (If the Martian dust storms subside,) At 10 p.m. CDT, when Mars is near the meridian about 22° up, it may provide excellent telescopic views. Mars moved into the boundaries and in front of the sidereal backdrop of Capricornus.