A Hovering Harvest Moon, Perceiving the Plane of the Planets, and Touring Tiny Constellations!
The Moon and Planets
The full moon, traditionally known as the “Corn Moon” and “Barley Moon”, will occur worldwide on Monday evening at 10:52 pm EDT. The September full moon always shines in or near the stars of Aquarius (the Water-bearer) and Pisces (the Fishes) — but the modest stars of those two constellations will be all but hidden by the bright moonlight. Full moons always rise around sunset and set around sunrise. They are fully illuminated because, when we turn and face a full moon, the sun is shining on it from behind our head — like the projector in a cinema.
This full moon occurs closest to the equinox this year, which occurred yesterday (Saturday), so this full moon is also the Harvest Moon. On the evenings around its full phase, the moon usually rises 50 minutes later each night. But the shallowly sloping evening ecliptic on the dates around the equinox causes Harvest Moons to rise at almost the same time each night — only delayed by as little as 10 minutes per night, depending on your latitude. This allowed farmers to work into the evening under bright moonlight. It also means that if you drive home from work, or walk the dog, at the same time every evening, you might notice the full moon for several days in a row.
After Monday, the moon will wane and rise later every night. It will continue to move eastward in its orbit, passing through Cetus (the Whale) on Thursday and then crossing Taurus (the Bull) through next weekend. When the moon rises in the eastern sky at about 10 pm local time on Saturday, it will be sitting less than 2 finger widths to the upper right of Aldebaran, the brightest star in Taurus. This will be their second close encounter this month. The moon and star will cross the sky together while the moon’s eastward orbital motion will slowly carry it just above the star. Their minimum separation of 0.5 degrees (a moon’s diameter) will occur around 2 am EDT. At that time, both objects will fit into the field of view of a backyard telescope.
When the nearly full moon rises in the west after dusk, bright Venus will be setting in the west. Those two objects will anchor a chain of bright solar system objects that allows us to roughly define the plane of our solar system arcing across the evening sky. From Venus, jump east to white Jupiter, then yellowish Saturn, reddish Mars, and the moon. The ecliptic, which represents the sun’s path through the stars due to Earth’s orbital motion, also passes through the objects. But the orbital inclinations of Venus, Mars, and the moon have caused those three objects to sit 4–6 finger widths below the mean solar system plane. The sun is on the same plane. During the summer, the very high elevation of the daytime sun results in low night-time planet (and moon) positions. Let’s run down your planet-viewing opportunities for this week…
Only Venus’ brilliance is allowing us to continue to see it while it sinks into the western evening twilight every night. Tonight, it will set at 8 pm, less than an hour after the sun. In a small telescope, Venus’ disk will resemble a 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.
Jupiter is being carried towards Venus by the westerly motion of the sky, but Venus is outpacing him as she swings towards the sun — so they won’t meet. This week, the two brightest planets are only about 1.4 fist diameters apart.
We are nearly finished with decent telescope views of Jupiter for this year. By the time the sky gets dark at 8 pm local time, the mighty planet will sit only a fist’s diameter above the southwestern horizon. It will set in the west-southwest at about 9 pm local time. Using binoculars, try to see Jupiter’s four Galilean moons (Io, Europa, Callisto, and Ganymede) forming a line to either side of the planet.
Medium-bright and yellow-tinted Saturn will pop out of the darkening southern sky shortly after dusk this week. At that time, it will be about 2 fist diameters above the horizon. The planet will set in the west at around 11:30 pm 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 its largest satellite, Titan. Using a clock’s dial analogy, Titan will move counter-clockwise this week from a position at 4 o’clock (lower right of Saturn) to 11 o’clock (nearly above Saturn). (Remember that your telescope might flip and/or invert the view. Use the moon to find out how your telescope changes things and keep a note of it, since that will always be the case.)
Mars is the very bright, ruddy coloured, star-like object sitting over the south-southeastern horizon after dusk this week. It reaches a maximum height in the sky of two outstretched fist diameters at about 9:30 pm local time. Then it will set in the west a little before 2 am. It’s still well worth a look, even in a small telescope. Look for a small white oval near the top of its disk. That’s the southern polar cap!
Distant Neptune recently reached opposition, so it is visible all night and almost its 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 midway between the modestly bright star Phi (φ) Aquarii and the brighter star Hydor (Lambda Aquarii). It will highest in the sky (best viewing conditions) at about midnight local time, but the bright moon will make it tough to see this week.
Blue-green coloured Uranus is visible from mid-evening until dawn. You can see it without optical aid under very dark skies, but binoculars and telescopes work better, especially with this week’s bright moon. During late evenings, the ice giant planet is located in the eastern sky, about 4 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.
Touring Some Small Constellations
Four very small constellations sit high in the southern sky on late September evenings. The easiest one to recognize is Delphinus (the Dolphin). It is composed of four modest stars that form a small, elongated diamond. Two more stars beside the top edge of the diamond form a tail that bends to the lower right of the critter.
About a fist’s diameter to the lower left of Delphinus sits Equuleus (the Little Horse). The constellation only includes his head. It is composed of four stars arranged in an elongated box that is wider at the bottom (the horse’s ears) and narrower at the top (his muzzle). By area, there is only one constellation smaller than Equuleus; that’s Crux (the Southern Cross). But it’s only visible from latitudes south of 25° North.
Sitting about a fist’s diameter to Delphinus’ right is the next smallest constellation in the sky, Sagitta (the Arrow). Its straight line, a palm’s width long, is formed by a line of stars. The arrowhead star is to the upper left, and a close-together pair of stars forms the feathers at lower right.
Finally, look a palm’s width to the upper right of Sagitta for the bent-stick shape of Vulpecula (the Fox). Although most star charts show Vulpecula as a short constellation, its official boundary actually extends two fist diameters to the upper left, all along the eastern wing of Cygnus (the Swan). Vulpecula includes a famous deep sky object known as the Dumbbell Nebula. It resembles an apple core. And, just a fist’s diameter to the upper right of Sagitta, or three finger widths beyond Vulpecula, sits the dim, but visible coloured double star Albireo, the beak of Cygnus.
Except for the slightly larger fox, each of these small constellations will fit within the field of view of binoculars. Let me know if you spot them!
Astronomy Skylights for this week (from September 23rd, 2018) by Chris Vaughan. Keep looking up to enjoy the sky! I love getting questions so, if you have any, send me a note.