Venus blazes before dawn
Although Mars has been resisting being swallowed up by evening twilight, its luck runs out this month. You can still spot it low in the west as twilight fades in early September. Look for the magnitude 1.7 planet some 20° below 1st-magnitude Spica. Mars’ ruddy hue contrasts nicely with the blue-white star. Unfortunately, Mars sinks lower with each passing day, and it disappears from view by month’s end. We’ll have to wait until 2024 for it to return in the predawn hours.
It’s a completely different story with Saturn. The ringed planet stands out in the eastern sky at midevening and climbs highest in the north around midnight local time. Saturn reached opposition in late August, and it remains near peak visibility throughout September. It shines at magnitude 0.4 against the backdrop of Aquarius the Water-bearer.
A telescope paints a stunning portrait of Saturn. The planet’s disk measures 19" across the equator while the ring system spans 43" and tips 10° to our line of sight. The disk appears more prominent now than in recent years because the rings don’t tilt as much. Also look for Saturn’s biggest and brightest moon, 8th-magnitude Titan, and three or four 10thmagnitude satellites.
In early September, Jupiter rises shortly after 11 p.m. local time, and it comes up about 30 minutes earlier with each passing week. The giant planet gleams at magnitude –2.7 among the dimmer background stars of Aries the Ram.
Jupiter looks magnificent when viewed through a telescope. Spend some time observing its dynamic atmosphere and four bright Galilean moons. The planet’s flattened shape proves to be more subtle, though it’s obvious once you know to look for it. In midSeptember, its equatorial diameter measures 46" while its polar diameter appears 3" less. The difference arises because Jupiter is gaseous and spins rapidly, completing a rotation in less than 10 hours.
As morning twilight approaches, brilliant Venus pokes above the eastern horizon. The inner planet passed between the Sun and Earth in August and spends this month climbing higher before dawn. You won’t mistake Venus for any other object. The world shines at magnitude –4.6 in early September and brightens to magnitude –4.8 by greatest brilliancy on the 19th.
It’s worth tracking Venus through a telescope all month because its appearance changes so rapidly. On the 1st, the planet spans 50" and the Sun illuminates just 11 percent of its disk. By the 30th, its size shrinks to 32" across while its phase waxes to 36 percent lit.
Mercury proves much harder to see. The innermost planet reaches inferior conjunction September 6 and then climbs slowly into view. At greatest elongation on the
22nd, an observer at 30° south latitude will see Mercury just 3° high in the east a half hour before sunrise. Although those closer to the equator will find the planet a little higher, it still won’t be easy to observe.
The starry sky
Few observational astronomers come close to the contributions of American scientist Edwin Hubble. He made two fundamental discoveries in the 1920s that helped shape our understanding of the universe: proving that the Milky Way is not the only galaxy in the universe and showing that we live in an expanding cosmos.
This month I want to explore the first of these discoveries. When most people think of Hubble’s definitive work on galaxies, they turn to his observations of the Andromeda Galaxy (M31). In the early 1920s, astronomers called it the Andromeda Nebula because it was just one of the many unresolved spiral-shaped objects that dotted the sky.
When Hubble observed M31 with the 100-inch Hooker Telescope in California, however, he discovered many Cepheid variable stars. Henrietta Leavitt’s groundbreaking studies previously had shown that Cepheids could be used as distance indicators. In a seminal paper published in 1929, Hubble deduced that M31 lies about 900,000 light-years from Earth — far outside the Milky Way and proof that it was a separate galaxy. (At the time, astronomers considered the two Magellanic Clouds to be appendages to our galaxy.) Although his calibration was off — M31 actually lies 2.5 million light-years away — the implication stood.
Few people know that Hubble had published details about another external stellar system in 1925. In that system, now known as Barnard’s
Galaxy (NGC 6822), Hubble identified several Cepheids in the object and calculated a distance of 700,000 light-years. (Its true distance is 1.6 million light-years.) Hubble commented that NGC 6822 was
“the first object definitely assigned to a region outside the galactic system.”
Take some time to track down NGC 6822. It lies in northern Sagittarius the Archer and climbs highest in the north early on September evenings. It lies 9° due west of Beta (β) Capricorni and about 1.5° north-northeast of 55 Sagittarii. You’ll need to use averted vision under a dark sky and a wide-field, low-magnification eyepiece. (Some sharp-eyed observers claim to have seen it through 10x50 binoculars.) Even Hubble made this point when he wrote that NGC 6822 was “fairly conspicuous in a short 4-inch finder with a lowpower eyepiece, but is barely discernible at the primary focus of the 100-inch.”