A Beautiful Evening Moon, Some Deep Sky Delights, and Binocular Comet Giacobini-Zinner Peaks!
Binocular Comet Update
This fall, astronomers are keeping an eye on two periodic comets — the type that return predictably every few years. One is already bright enough to see with binoculars. A second one should be bright enough to see with your unaided eyes by Christmas time. I’m going to continue updating you about them, even though, like cats, comets have tails and are extremely prone to doing their own thing, despite what we might want them to do.
Comet 21P/Giacobini-Zinner returns to our vicinity every 6.5 years — hence the “P” for “periodic” in its designation. It has been gradually brightening because it will be closest to Earth and the sun tomorrow (Monday), then it will slowly begin to fade. This week will be an excellent time to see it because the moon will be out of the late evening sky. You should be able to see the faint fuzzy greenish object in binoculars or a small telescope, if you can escape city lights. A faint tail might be noted extending to the upper right of the comet (although your telescope will flip that direction around).
The comet is located in the northeastern sky on a track that carries it lower every night — so we’ll have to stay up after midnight, or get up before dawn, to see it. Tonight (Sunday), the comet will be positioned just below the imaginary line that joins the modest star Mahasim and the bright star Elnath — the lowest two stars in Auriga’s (the Charioteer) rough circlet. The comet will be left of their mid-point.
At two different times on Monday, the comet will reach its closest point to the sun (perihelion) and to Earth (perigee). Around then we expect it to peak in visual brightness — about magnitude 7.0.
In the early hours of Monday and Tuesday mornings, the comet will glide past the open star cluster designated Messier 37, passing only 10 arc-minutes (or one-third of the full moon’s diameter) from the cluster at 19:00 GMT on September 10. Observers in Asia and Oceania will see the objects close together. Most of the Western Hemisphere will have to settle for seeing the comet 1 finger’s width from the cluster on each night (above on Monday, then below on Tuesday). This meet-up will look best when viewed in a telescope.
On Saturday morning at about 6 am in the Eastern time zone (10:00 GMT), the comet will glide directly through the open star cluster designated Messier 35 — a great sight in a telescope, and a photo opportunity! Finally, between midnight and dawn sky on Sunday, September 16, the comet will approach the bright star Propus (Eta Geminorum) in Gemini (the Twins), passing within a full moon’s diameter to the upper right of the star just before dawn. The dim Jelly Fish Nebula (IC443) will be within a low magnification telescope’s field of view of the comet, too. Closest approach to Propus will occur at around 15:00 GMT, an event only visible for skywatchers in Asia and Oceania.
The second comet, named 46P/Wirtanen, is predicted to become much brighter in late fall. Right now, it is tickling the belly of the whale (Cetus), but it’s far too dim to look for yet.
During moonless periods in September and October annually, the steep morning ecliptic favors the appearance of the zodiacal light in the eastern sky for about half an hour before dawn. This is reflected sunlight from interplanetary particles concentrated in the plane of the solar system. During the two weeks starting just before the new moon on September 9, look east below the stars of Cancer (the Crab) for a broad wedge of faint light rising from the horizon and centered on the ecliptic. Don’t confuse it with the Milky Way, which is sitting further to the southeast. I posted a picture here.
The Moon and Planets
The moon reached its new phase, when it is near the sun and hidden from our view, this afternoon. By sunset tonight, it will have moved several finger widths away from the sun and might be glimpsed right after sunset as an extremely thin crescent sitting just above the western horizon. On Monday evening, it will be much easier to spot — until it sets at about 8:30 pm local time.
For the rest of this week, the moon will wax fuller and climb steadily eastward, passing two bright planets as it goes. On Wednesday evening, look for the crescent moon sitting less than a fist’s diameter above bright Venus. On Thursday, it will sit a palm’s width to the upper right of bright Jupiter. On Saturday night, look for the waxing crescent moon sitting a palm’s width above Mars’ imposter — the bright reddish star Antares, the heart of the Scorpion in Scorpius.
During this week, and the early part of next week, pull out your binoculars or small telescope and aim at the moon. As the moon waxes fuller, the sun is slowly rising over the hemisphere that faces Earth. The “dawn” sunlight hits the moon at a very shallow angle, casting deep black shadows to the west (our left) of crater walls and mountain peaks. It also reveals subtle topographic features, especially on the floors of seemingly flat craters and dark maria (the Latin word for “seas”). The shadowed zone shifts west every night, highlighting new lunar geology. First Quarter phase, when the half-illuminated moon will be sitting at a 90° angle from the sun, will occur worldwide next Sunday evening.
People who live in the GTA and mid-northern latitudes around the world have a few more days to easily see Mercury in the eastern pre-dawn sky. Early this week, Mercury will rise in the east about an hour before the sun. You’ll be able to see it between about 6 and 6:30 am local time. Try to pick a spot with a low and uncluttered eastern horizon.
Venus’ intensity is allowing us to continue to see it while it sinks into the western evening twilight every night. Tonight, it will set just before 9 pm local time. A week from now, that will occur at 8:30 pm. In a small telescope, Venus’ disk will resemble a waning crescent moon, lit only on the sunward side (although your telescope might flip the view). The planet is growing larger in apparent diameter because it is travelling towards the Earth right now.
We are in the closing chapter for observing Jupiter this year. After it exits “stage west” in a few weeks, it won’t return to the evening sky until late next April. The earlier autumn sunsets will buy us some extra evening observing time, but its position low in the sky will add a great deal of extra blurring atmosphere between it and our telescopes. This week, the very bright planet will emerge from the southwestern twilight shortly after dusk, and then it will set in the west-southwest at about 10 pm local time. Jupiter will continue to pull away from the nearby bright star Zubenelgenubi, the brightest star in Libra (the Scales). In binoculars, you’ll plainly see that Zubenelgenubi is a pair of stars. While you have the binoculars handy, see if you can see Jupiter’s four Galilean moons (Io, Europa, Callisto, and Ganymede) flanking the planet.
Medium-bright and yellow-tinted Saturn will pop out of the darkening southern sky shortly after dusk this week, about 2 fist diameters above the horizon. The planet will set in the west by about 12:30 am local time. This summer, the ringed planet has been 5 finger widths to the upper right of the “lid” star of the Teapot in Sagittarius (the Archer). As the sky darkens, even a small telescope should be able to show you some of Saturn’s larger moons, especially Titan. Using a clock’s dial analogy, Titan will move counter-clockwise this week from a position at 3 o’clock (right of the planet) to 10 o’clock (above left of the planet). (Remember that your telescope might flip and/or invert the view. Use the moon to find out how your telescope changes things.)
Mars will still be bright this week. Visually, it will appear pink or orange. Mars will appear above the south-southeastern horizon after dusk and then climb higher until 10:15 pm local time, when it will reach an elevation of about 20° (or two outstretched fist diameters) above the southern horizon. (That will be the best hour to view the planet in a telescope because it will then be shining through the least amount of Earth’s distorting atmosphere.) Note that 20° is lower than many trees and buildings, so a clear southern vista is essential. It will set in the west before 3 am. The Red Planet will set at about 2:30 am local time.
At visual magnitude 5.8, blue-green coloured Uranus is visible from mid- evening until dawn. You can see it without optical aid under very dark skies, or in binoculars and telescopes under moderately light-polluted skies. The ice giant planet is located in the eastern sky, about 4.5 finger widths to the left of the modestly bright star Torcular (Omega Piscium). That star sits a generous palm’s width above the “V” where the two starry cords of Pisces (the Fishes) meet.
Distant Neptune recently reached opposition, when it is visible all night and closest and brightest for this year. Using a decent quality telescope you can see the very blue planet among the dim stars of Aquarius (the Water-bearer). Look for the magnitude 7.8 planet sitting 2 finger widths to the right of the modestly bright star Phi (φ) Aquarii and 3 finger widths to the lower left of the brighter star Hydor (Lambda Aquarii). It will highest in the sky (best viewing conditions) at about 1 am local time.
September Dark Sky Treats
The constellation of Cygnus (the Swan) is nearly overhead in the eastern sky after dusk and nears the zenith at about 10:30 pm local time. The swan is oriented so that it seems to be flying south along the Milky Way! Dark nights around the new moon are ideal for seeing the many spectacular deep sky objects in that constellation. At the other end of the constellation from it brightest star, Deneb is the blue and gold colored double star Albireo, which marks the swan’s beak. Particularly good open star clusters include bright and large Messier 39 and smaller Cooling Tower Cluster (Messier 29). Planetary nebulae, a term coined because they visually resemble a small planet’s disk, are the corpses of low mass stars like our sun. Viewed in a telescope, the Blinking Planetary (also designated NGC 6826) exhibits a central star enveloped in a small blue disk that grows in size when averted vision is used — hence the name.
With the moon mostly absent from the night sky, now is the perfect time to explore the countless knots and clumps of stars distributed along the Milky Way. Many of them were included in Charles Messier’s list, and other modern lists, of the sky’s best deep sky objects. Start by finding the objects with binoculars, and then follow up with a telescope at low magnification. Particularly good clusters include NGC 7243 in Lacerta (the Lizard), the Wild Duck Cluster (Messier 11) and Messier 26, both located in Scutum (the Shield), and the Sagittarius Star Cloud (Messier 24). In the northern Milky Way, look at Caroline’s Rose (NGC7789) in Cassiopeia (the Queen) and the Double Cluster (NGC 884 and 869) in Perseus (the Hero).
Astronomy Skylights for this week (from September 9th, 2018) by Chris Vaughan.
Keep looking up to enjoy the sky!