The Bright Corn Moon Prunes Perseid Meteors, but Saturn Shines while Jupiter Dances and Sports Spots!
The Perseids Meteor Shower Peaks!
The prolific Perseids Meteor Shower peaks from Tuesday night until Wednesday morning before dawn. Within a couple of nights before and after the peak date, the quantity of meteors will be reduced somewhat, but still well worth looking up for. Unfortunately, the moon will be extremely full and bright during the peak this year, so the dimmer meteors will be hidden by the moon-lit sky. Thankfully, Perseids are often very long and bright!
Meteor showers are annual events that occur when the Earth’s orbit passes through zones of debris left by multiple passes of periodic comets. (The analogy would be the material tossed out of a dump truck as it rattles along. The roadway gets pretty dirty if the truck drives the route a number of times!) Over centuries, or longer, the dust-sized and sand-sized (or larger) particles accumulate and spread out a bit. When the Earth encounters them, the particles are caught by our gravity and burn up as they fall through our atmosphere at speeds on the order of 200,000 km/hr. The grains moving that fast through the air generate heat that ionizes the air — producing the long glowing trails we see. The duration of a meteor shower depends on the width of the zone, and the intensity depends on whether we pass through the densest portion, or merely skirt the edges.
The nickname for meteors is “shooting stars” or “falling stars”, but they bear no physical connection to the distant stars, and all your favourite constellations will look the same as ever at the end of the shower!
The source of the Perseids material is thought to be 133-year-period Comet 109P/Swift-Tuttle. The active period for this shower is July 13 through August 26, so keep an eye out for them beyond this week. This shower is known for producing 60–80 meteors per hour at the peak — many manifesting as bright, sputtering fireballs!
While visible anywhere in the night sky, meteors will appear to radiate from a location in the sky (called the radiant) between the constellations of Camelopardalis (the Giraffe) and Perseus (the Hero), which gives this shower its name. The radiant is low in the northeastern sky during mid-August evenings — and nearly overhead by dawn. Meteor showers are best observed in the dark skies before dawn, because that’s the time when the sky overhead is plowing directly into the oncoming debris field, like bugs splatting on a moving car’s windshield. When the radiant constellation is overhead, the entire sky down to the horizon is available for meteors.
The highest Perseid meteor rates this year are expected to occur on from Tuesday night into Wednesday morning August 12–13, when the Earth will be closest to the orbit of comet 109P/Swift-Tuttle and densest part of its debris trail. If you begin to watch after dark on Tuesday evening, you might catch very long meteors that are skimming the Earth’s upper atmosphere. These are fewer, but spectacular. As the night rolls on, the radiant of the meteors will rise higher in the sky, revealing more meteors because they are no longer hidden by the bulk of Earth. The absolute best time to view is around 4 am local time when the radiant will be almost overhead.
For best results, try to find a safe viewing location with as much open sky as possible. If you can hide the moon behind a building or tree, that will help. You can start watching as soon as it is dark — to catch very long meteors produced by particles skimming the Earth’s upper atmosphere. These are rarer, but leave very long streaks. Don’t worry about watching the radiant. Meteors from that position will be heading directly towards you and have very short trails.
Bring a blanket for warmth and a chaise to avoid neck strain, plus snacks and drinks. Try to keep watching the sky even when chatting with friends or family — they’ll understand. Call out when you see one; a bit of friendly competition is fun!
Don’t look at your phone or tablet — the bright screen will spoil your dark adaptation. If you can, minimize the brightness or cover the screen with red film. Disabling app notifications will reduce the chances of unexpected bright light, too. And remember that binoculars and telescopes will not help you see meteors because they have fields of view that are too narrow. Good hunting!
The Moon and Planets
This week, the moon will reach its full phase and illuminate the night sky worldwide — to the disappointment of Perseid Meteor Shower viewers. Then the moon will commence its two-week swing back towards the sun. In the meantime, the moon will visit Saturn, and Jupiter will sport spots on Monday and Saturday. Here are your Skylights for this week!
In the southeastern sky after dusk tonight (Sunday), look for the waxing gibbous moon positioned just four finger widths to the right (celestial west) of the bright, yellowish planet Saturn. The pretty duo will cross the sky together for most of the night and will easily appear together within the field of binoculars. If you watch the pair over several hours, starting at dusk, you will see the moon’s orbit carry it closer to the planet while the rotation of the sky lifts Saturn higher than the moon. Observers in eastern Indonesia, most of Australia, northern New Zealand, Melanesia, and Polynesia (except Hawaii) will see the moon occult Saturn on August 12.
The August full moon, known as the “Sturgeon Moon”, “Red Moon”, “Green Corn Moon”, and “Grain Moon”, always shines among or near the stars of Aquarius (the Water-Bearer) or Capricornus (the Sea-Goat). Full moons always rise at sunset and set at sunrise. Since this full moon phase will occur in the morning daylight hours of Thursday, the moon will appear to be full on both Wednesday and Thursday evening in the Americas. When full, sunlight is hitting the moon vertically and casting no shadows. All of the variations in brightness we see are generated by differences in the reflectivity, or albedo, of the lunar surface rocks.
On nights around the full moon phase, bright ray features may be seen radiating from the younger craters on the lunar near side. A particularly interesting example of this is the ray system for the crater Proclus. The 28 km wide crater and its ray system are visible in binoculars. They are located at the lower left edge of Mare Crisium, the round, grey basin near the moon’s upper right edge (northeast on the moon). The Proclus rays, about 600 km in length, only appear on the eastern, right-hand side of the crater, and within Mare Crisium, suggesting that the impactor that made them arrived at a shallow angle from the southwest. (Note that east and west are reversed on the moon).
The still-very-bright moon will end the week below the stars of western Pisces (the Fishes), but it won’t rise until late evening and then linger into the morning daytime sky.
Aside from the moon, Jupiter will be the brightest object in the night-time sky this week. As the sky begins to darken, look for the giant planet sitting less than halfway up the southwestern sky. As the evening passes, Jupiter will sink lower, setting in the west just before 1:30 am local time. On Sunday, August 11, Jupiter will end a westerly retrograde loop that began in April, and resume its regular eastward motion with respect to the stars of southern Ophiuchus (the Serpent-Bearer).
The difference in orbital speed between a given planet and Earth generates these predictable, temporary reversals in motion that astronomers call retrograde loops. During Jupiter’s retrograde period, Earth was passing Jupiter “on the inside track” of the Solar System’s “racetrack” around the sun. The stars, which are far beyond the planets, are fixed in place, allowing us to see the planets move among them. The word planet comes from a Greek word for “wanderer”. Take note of Jupiter’s position with respect to the bright, reddish star Antares, which is sitting about a palm’s width to Jupiter’s lower right this summer. If you check back every week or two, Jupiter’s orbital motion will be apparent.
On a typical night, even a backyard telescope will show you Jupiter’s two main equatorial stripes and its four Galilean moons — Io, Europa, Callisto, and Ganymede looking like small white dots arranged in a rough line flanking the planet. If you see fewer than four dots, then some of them are in front of Jupiter, or hidden behind it. Good binoculars will show the moons, too!
From time to time, the small, round, black shadows cast onto Jupiter’s surface by those four Galilean moons become visible in amateur telescopes as they cross (or transit) Jupiter’s disk. On Monday evening from 9:07 to 11:20 pm EDT, observers in the Americas can watch Io’s small shadow transit Jupiter. On Saturday evening from 8:53 to 11:25 pm EDT, observers in the Americas can watch Europa’s shadow transit the northern hemisphere of Jupiter.
Due to Jupiter’s rapid 10-hour rotation period, the Great Red Spot (or GRS) is only observable from Earth every 2nd or 3rd night, and only during a predictable three-hour window. The GRS will be easiest to see using a medium-sized, or larger, aperture telescope on an evening of good seeing (steady air). If you’d like to see the Great Red Spot in your telescope, it will be crossing the planet tonight (Sunday evening) from 9:30 pm to 12:30 am EDT, on Tuesday night after 11:30 pm EDT, after dusk on Wednesday and Friday, and after 10:30 pm EDT next Sunday.
Yellow-tinted Saturn is prominent this summer, too — but its less bright than Jupiter. The ringed planet will be visible from dusk until about 3:30 am local time. Saturn’s position in the sky is just to the upper left (celestial east) of the stars that form the teapot-shaped constellation of Sagittarius (the Archer). To find Saturn, look about 3 fist diameters to the left (east) of Jupiter.
Dust off your telescope! Once the sky is dark, even a small telescope will show Saturn’s rings and several of its brighter moons, especially Titan! Because Saturn’s axis of rotation is tipped about 27° from vertical (a bit more than Earth’s axis), we can see the top surface of its rings, and its moons can arrange themselves above, below, or to either side of the planet. During this week, Titan will migrate counter-clockwise around Saturn, moving from the right of Saturn tonight (Sunday) to the left of the planet next Sunday. (Remember that your telescope will flip the view around.)
Tiny, blue Neptune is low in the southeastern sky in late evening, among the stars of Aquarius (the Water-Bearer). The planet will be rising shortly before 9:30 pm local time this week. You’ll find the magnitude 7.8 Neptune sitting half a finger’s width to the left (east) of a medium-bright star named Phi (φ) Aquarii, so both objects will appear together in the field of view of a telescope. The planet is actually moving slowly toward that star and will “kiss” it in early September.
Blue-green Uranus will be rising just after 11 pm local time this week. It is sitting below the stars of Aries (the Ram) and is just a palm’s width above the head of Cetus (the Whale). At magnitude 5.8, Uranus is bright enough to see in binoculars under dark skies, but not this week.
This week, Mercury is in the northeastern pre-dawn sky — below the stars of Gemini (the Twins). After swinging widely away from the sun last week, it will now be descending again. Your best opportunity to see it will land between 5:15 and 5:45 am local time.
Venus and Mars are lost in the sun’s glare for the next while.
Astronomy Skylights for the week of August 11th, 2019 by Chris Vaughan.
Keep looking up, and enjoy the sky when you do. I love questions and requests — so, send me some!