EYES TO THE SKY: Full Corn Moon, Sirius the Dog Star harbinger of autumn

September 4 – 17, 2017

Mount Washington — A nearly full moon rises in the east-southeast this evening at 6:41 p.m. Tomorrow’s moonrise at 7:15 p.m. is synchronized with sunset at 7:20 p.m. on the opposite horizon. The Full Corn Moon reaches full phase about eight hours later, at 3:04 a.m. on the 6^th. That makes for viewing on the 5^th the closest to full.

For those of us who, two weeks ago, witnessed the total solar eclipse in faraway locations and those of us who observed the partial eclipse locally, this full moon is especially charged. Then, our eyes were riveted on the dark moon, new moon, orbiting between Earth and Sun, the sun’s light at the moon’s back. In the coming days, we receive the reflected light of the sun streaming from a fully lit moon that is positioned opposite the sun with Earth in-between–the position of the moon at full phase. Contrast the photograph of full moon with the image of new moon eclipsing the sun. We are privileged to publish the solar eclipse photograph of Berkshire County’s Craig Boynton. He explains, “There was a slight cloud cover at my Missouri location; I think that was the cause of the magenta color [in the corona.]”

Totality: Solar Eclipse photographed in Nelson, Mo., by Craig Boynton of Hinsdale, Mass., on Aug. 21, 2017.

While absorbed in writing about preparations for the solar eclipse, I postponed celebrating the heliacal* rising of Sirius the Dog Star that occurred around Aug. 17. At that time the beautiful harbinger-of-autumn star had just emerged from the sun’s glare and still posed a challenge for pre-dawn stargazers with obstructed views to the southeast horizon. By now, Sirius is 20 degrees above that horizon and may be spotted along with brilliant planet Venus shining in the east: look for Sirius to the right of Venus until about 5:50 a.m. on the 5^th and 6:00 a.m. on the 17^th. Sunrise tomorrow is at 6:23 a.m. and, on the 17^th, 6:35 a.m.

Courtesy EarthSky.org, published with permission

Follow the moon in the coming weeks as it wanes to a crescent in the early morning sky. Familiar constellations of winter nights rise in the darkness and are still visible about an hour before sunrise. On the 17^th and 18^th, looking east, the crescent moon guides us to a three-planet line-up. Timing is crucial if we are to see the planets: Mercury rises at 5:09 a.m. on the 17^th and 5:13 a.m. on the 18^th. It could be 30 minutes later over a hilly skyline. Bring binoculars.

As seen from North America, the waning crescent moon will be above Venus and Regulus on Sept. 17 and below these two celestial luminaries on Sept. 18. Depending on where you live worldwide, Regulus and Venus will come closest together on the sky’s dome on the morning of Sept. 19 or 20.

*Heliacal risings occur after a star has been behind the Sun for a season and it is just returning to visibility. There is one morning, just before dawn, when the star suddenly reappears after its absence. On that day it “blinks” on for a moment just before the sunrise and just before it is then obliterated by the Sun’s presence. That one special morning is called the star’s heliacal rising. Each day that passes after the heliacal rising, the star will appear to rise earlier and remain in the sky longer (that is, not blink) before its soft glow is obliterated by the rising sun. From https://solar-center.stanford.edu/AO/dawn-rising.html

This map shows the sky (as seen when facing east) on July 15, 3000 B.C.E. from the ancient city of Memphis (near Cairo) in Egypt. Sirius stands 3° high 32 minutes before sunrise around the time of its heliacal rising. Stellarium via SkyandTelescope.com

Resources

https://www.skyandtelescope.com/observing/a-real-scorcher-sirius-at-heliacal-rising/
https://solar-center.stanford.edu/AO/Sirius.mov
https://solar-center.stanford.edu/AO/dawn-rising.html