FRIDAY, SEPTEMBER 23
■ Marking the summer-to-fall transition as it always does, Vega gives way to Deneb as the bright star nearest the zenith after nightfall (for skywatchers at mid-northern latitudes).
■ During Saturday's dawn, can you spot the very old waning crescent Moon low in the east? The Moon is 37 hours from new if you're on the East Coast; 34 hours from new when seen from the West Coast.
Look about 30 or 45 minutes before sunrise, and you'll have an easier time than usual with such a thin Moon. Why? Because at this time of year, the thin waning crescent is nearly an upright cup (for mid-northern latitudes). As shown below.
Why does that make a difference? Because it means the line from the Moon to the Sun is nearly vertical before sunrise. And that means we see the Moon as high above the horizon as possible when it's near the Sun and therefore thin.
At the opposite time of the year, around March and April, the same thing happens in the evening sky with the cupped waxing crescent.
■ Action at Jupiter: With a telescope, watch for Io's tiny black shadow to edge onto Jupiter's eastern limb at 9:54 p.m. EDT, followed very closely by Io itself five minutes later. They depart Jupiter's western limb around 12:10 a.m. EDT.
Meanwhile, Jupiter's Great Red Spot should cross Jupiter's central meridian around 11:15 p.m. EDT.
■ Globular clusters are most abundant on summer evenings. In this last week of September, with moonlight not yet washing the evening sky, set up your telescope to look in on four of them that Josh Urban calls The Last Wildflowers: Globular Clusters Greet Autumn. There's "Sagittifolia," the Arrowhead Flower M71 next to Sagitta; "Queen Anne's Lace," M15 sneezed from the nose of Pegasus; "Watercress by the Stream," M2 in Aquarius a little farther south; and "Seaflower of the Deep," M30 deep in southern Capricornus.
SATURDAY, SEPTEMBER 24
■ Arcturus shines ever lower in the west-northwest after dark. The narrow kite shape of its constellation, Bootes, extends two fists at arm's length to Arcturus's upper right. Arcturus is where the kite's downward-hanging tail is tied on.
To the right of Bootes, the Big Dipper is turning more level.
And this is the time of year when, during the evening, the dim Little Dipper high above "dumps water" into the bowl of the Big Dipper. The Big Dipper dumps it back in the evenings of spring.
■ With moonlight not yet back in the evening sky, explore the telescopic sights of the West Wing of Cygnus overhead using Ken Hewitt-White's Suburban Stargazer article and chart in the September Sky & Telescope, page 54. My favorite here is the Blinking Planetary.
For a deeper, darker-sky challenge, work through the big North America and Pelican nebulae near Deneb in Cygnus using Alan's Whitman's Going Deep guide on page 58.
SUNDAY, SEPTEMBER 25
■ Arcturus shines in the west these evenings after twilight fades out. Pretty soon after nightfall is complete, brighter Jupiter shines in the east at the same height as Arcturus. How well can you time their exact balance?
When that happens, barely rising in the north-northeast is Capella (depending on your latitude; the farther north you are the higher Capella will be.)
Later in the evening, it's the turn of Arcturus and Capella to shine at the same height. Both are magnitude 0. Again, the time of this will depend on your latitude and longitude.
When it happens, turn and look low in the south-southeast. There will be 1st-magnitude Fomalhaut at about the same height too — exactly so if you're at latitude 43° north (Boston, Buffalo, Milwaukee, Boise, Eugene). Seen from south of that latitude, Fomalhaut will appear higher than Capella and Arcturus are. Seen from north of there, it will be lower.
■ New Moon (exact at 5:55 p.m. EDT).
MONDAY, SEPTEMBER 26
■ This is the time of year when the rich Cygnus Milky Way crosses the zenith soon after nightfall is complete (for skywatchers at mid-northern latitudes). The Milky Way extends straight up from the southwest horizon between Sagittarius and the tail of Scorpius, passes overhead, and runs straight down to the northeast through Cassiopeia and Perseus.
TUESDAY, SEPTEMBER 27
■ House-shaped Cepheus, in the edge of the Milky Way, is high in the northeast to north these evenings, as shown on the center star charts of the September and October Sky & Telescope issues.
Cepheus is home to the prototype Cepheid variable star, Delta Cephei. Throughout its cycle Delta Cep remains naked-eye; it pulses from magnitude 4.4 to 3.5 and back every 5 days 9 hours. Make a point of looking up from time to time and comparing Delta with nearby Epsilon and Zeta Cephei, mags 4.2 and 3.3 respectively. Those two comparison stars nearly frame Delta's brightness range. Use the chart for this trio in Matt Wedel's Binocular Highlight column with the September S&T 's center chart.
In a telescope Delta Cep is also a lovely double star. Its companion, magnitude 6.3, is 41 arcseconds away.
More about Cepheus is in the same issue, page 45.
WEDNESDAY, SEPTEMBER 28
■ The starry W of Cassiopeia stands high in the northeast after dark. The right-hand side of the W (the brightest side) is tilted way up.
Look at the second segment of the W counting down from the top. Notice the dim naked-eye stars along that segment (not counting its two ends). The brightest of these, on the right, is Eta Cassiopeiae, magnitude 3.4. It's a remarkably Sun-like star just 19 light-years away, but it has an orange-dwarf companion, magnitude 7.3, separation 13 arcseconds — a lovely binary in a telescope.
Left of Eta, and fainter, is a naked-eye pair in a dark sky: Upsilon1 and Upsilon2 Cassiopeiae, 0.3° apart. They're yellow-orange giants unrelated to each other, 200 and 400 light-years distant from us. Upsilon2 is slightly the brighter of the pair. It's also the closer one.
THURSDAY, SEPTEMBER 29
■ As twilight fades, look southwest for the waxing crescent Moon — now with its horns fairly near vertical. Orange Antares is off to its left by about 12°. Between them is white Delta Scorpii, second in brightness to Antares.
FRIDAY, SEPTEMBER 30
■ At dusk the crescent Moon shines only about 2° from Antares for North American viewers, as indicated above. As always, the Moon's configuration with stars nearby will differ a bit depending on your location and time.
SATURDAY, OCTOBER 1
■ The waxing Moon hangs in the southwest as twilight fades to darkness. Use binoculars to look 10° below the Moon and perhaps a bit left for what will probably be your last sighting this year of the Cat's Eyes, the pair of stars in the tail of Scorpius. At this late date the Cat's Eyes are tilted even more than usual, with the fainter one lower right of the brighter one. They're 0.6° apart.
This Week's Planet Roundup
Mercury barely emerges very low in bright dawn by the end of this week, just above the horizon due east. It's on its way to a good dawn showing in the second week or so of October.
Venus is getting almost unobservably low in very bright dawn. Mercury is on its way up to replacing it there.
Mars, magnitude –0.6 in Taurus, clears the east-northeast horizon around 10 or 11 p.m. and gains altitude for the rest of the night. Mars is four times as bright as Mars-colored Aldebaran, magnitude +0.9, sparkling to its right. Look for the Pleiades above Aldebaran.
Mars is 11½ arcseconds in diameter and growing. The best time to observe it telescopically is before dawn, when it's highest. Mars will be 17.2 arcseconds across when closest to Earth on December 1st.
Jupiter comes to opposition this week on the 26th. As twilight fades, spot it glaring low due east. After dark Jupiter dominates the east, blazing at magnitude –2.9 in dim Pisces. It climbs higher across the southeast and stands at its highest in the south around midnight or 1 a.m.
In a telescope Jupiter is a remarkable 50 arcseconds wide across its equator, its maximum possible; Jupiter this season is very close to the perihelion of its 12-year orbit around the Sun. See Jupiter's Exceptionally Close Opposition by Bob King. "A sentinel in the autumnal sky, Jupiter marks its closest opposition since 1963."
Saturn, magnitude +0.5 in dim Capricornus, crosses the southern sky these evenings. Spot it 44° west of Jupiter. That's about four fist-widths at arm's length. It's upper right of Jupiter early in the evening and directly right by around 9 or 10, when Saturn is about at its highest.
Uranus, magnitude 5.7 in Aries, is up in the east in good binocular or telescope view by late evening. See the Uranus finder charts in the November Sky & Telescope, page 49.
Neptune, magnitude 7.8 at the Aquarius-Pisces border, is 10° west of Jupiter. See the Neptune finder charts in the September Sky & Telescope, page 49. And use the charts there too for the asteroid 3 Juno nearby, not quite the same magnitude.
All descriptions that relate to your horizon — including the words up, down, right, and left — are written for the world's mid-northern latitudes. Descriptions and graphics that also depend on longitude (mainly Moon positions) are for North America.
Eastern Daylight Time, EDT, is Universal Time minus 4 hours. (Universal Time is also called UT, UTC, GMT or Z time.)
Want to become a better astronomer? Learn your way around the constellations. They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope.
This is an outdoor nature hobby. For an easy-to-use constellation guide covering the whole evening sky, use the big monthly map in the center of each issue of Sky & Telescope, the essential magazine of astronomy.
Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of charts). The basic standard is the Pocket Sky Atlas (in either the original or Jumbo Edition), which shows stars to magnitude 7.6.
Next up is the larger and deeper Sky Atlas 2000.0, plotting stars to magnitude 8.5; nearly three times as many. The next up, once you know your way around, are the even larger Interstellarum atlas (stars to magnitude 9.5) or Uranometria 2000.0 (stars to magnitude 9.75). And be sure to read How to Use a Star Chart with a Telescope. It applies just as much to charts on your phone or tablet as to charts on paper.
You'll also want a good deep-sky guidebook. A beloved old classic is the three-volume Burnham's Celestial Handbook. An impressive more modern one is the big Night Sky Observer's Guide set (2+ volumes) by Kepple and Sanner.
Can a computerized telescope replace charts? Not for beginners, I don't think, and not on mounts and tripods that are less than top-quality mechanically, meaning heavy and expensive. And as Terence Dickinson and Alan Dyer say in their Backyard Astronomer's Guide, "A full appreciation of the universe cannot come without developing the skills to find things in the sky and understanding how the sky works. This knowledge comes only by spending time under the stars with star maps in hand."
Audio sky tour. Out under the evening sky with your
earbuds in place, listen to Kelly Beatty's monthly
podcast tour of the heavens above. It's free.
"The dangers of not thinking clearly are much greater now than ever before. It's not that there's something new in our way of thinking, it's that credulous and confused thinking can be much more lethal in ways it was never before."
— Carl Sagan, 1996
"Facts are stubborn things."
— John Adams, 1770