Evan Hazard: A fuzzy swath in the nighttime 'firmament'
The greatest wonder for naked eye stargazing was, for ages, also its greatest mystery.
Away from light pollution on a clear moonless late evening this week, you will see lots of stars and one planet, Saturn low in the WSW. But, stretching from behind Perseus, which is now rising in the NNE, then Cassiopeia, Cepheus, Cygnus, and Aquila, to Sagittarius and Scorpius above the southern horizon, is a "milky" swath, broader here and narrower there, with dark "holes" in it. Ancient Greeks called it galaksias kyklos or "milky circle."
Galileo resolved nearby parts of the Milky Way into individual stars but it was less than a century ago that Edwin Hubble showed it was not the only galaxy, but one of many. We have also learned that the "holes" it are really vast clouds of gas and dust that obscure the jillions of stars behind them.
The nearest full-size galaxy, M31 in Andromeda, is now visible low in the northeast. The photons from it that you see left it 2.5 million years ago. We now know there are billions of galaxies. The Wikipedia web listing for Milky Way is a good intro to what we now know about the structure and behavior of our home galaxy. It's worth reading all the way through.
Leo has left us in the WNW and Virgo, with bright Spica, is low in the west. Get as good a look as you can now at Sagittarius and Scorpius. These two southern constellations are easy objects for us northerners in summer. Both have many interesting nebulae and clusters for those with binocs. Fall constellations are rising: Andromeda, Pegasus, Pisces, Aquarius, and Capricornus, from northeast to southeast.
Venus remains much the same low distance above the western horizon all month, though the stellar background moves rapidly behind it. Saturn is the only other naked eye evening planet. Jupiter and Mars rise in the wee hours. They were less than a degree apart July 22, but are 5 degrees apart by Thursday. By Labor Day weekend, they will rise earlier and Jupiter will be 18 degrees above and right of Mars. Mercury is still visible in the pre-dawn sky, but is heading sunward. It will be at greatest brilliancy Aug. 10, but against brightening morning twilight.
The annual Perseid meteor shower, at its height after midnight Aug. 12-13, will be free of moonlight. Algol, our easiest eclipsing variable, is in Perseus. Next month we'll begin listing evening to midnight minima of Algol.
Evan Hazard also writes "Threescore and Ten" for The Pioneer's "Prime Time.