The Waxing Moon Passes Planets, Peering at Pegasus, and the Orionids Open Meteor Shower Season!
Orionids Meteor Shower
We’ve now entered meteor shower season! Over the next few months, we’ll experience a wave of several showers. The excellent Orionid Meteor Shower, which is derived from material dropped by repeated past passages of Comet Halley, is underway this week, and observable world-wide. It will peak in the hours after midnight (in your local time zone) on Sunday, October 21 (i.e., Monday morning). At that time, the sky overhead will be ploughing through the densest region of the particle field, generating up to 25 meteors per hour.
The meteors can appear anywhere in the sky, but true Orionids will be travelling in a direction away from a location (the radiant) a fist’s diameter to the upper left of the bright red star Betelgeuse in the constellation of Orion. Although not too numerous, Orionids are known for being bright and fast-moving. You can watch for meteors in the evening, too — but many of them will be hidden behind the Earth’s horizon.
This shower has a broad period of activity because the debris field is very spread out and the comet’s orbit does not cross Earth’s at a sharp angle. So, technically, the Orionids will linger until late November — weakening as time passes. Unfortunately, the moon will be close to full around the peak evenings this year, somewhat spoiling the shower.
To see the most meteors, find a wide-open dark location, preferably away from light polluted skies, and just look up with your unaided eyes. Binoculars and telescopes are not useful for meteors — their field of view are too narrow. If the peak night is cloudy, several nights on either side will be almost as good. Happy hunting!
The Moon and Planets
The moon will spend this week in the southern evening sky, well-placed for after-dinner observing. And, since its orbit is close to the ecliptic, it will pass by the evening planets.
On Sunday evening (tonight) the waxing crescent moon will land two finger widths to the right of yellowish Saturn. The moon will slide closer to Saturn by the time they set in the west at around 10:30 pm local time. In the southern sky on Wednesday evening, the slightly gibbous moon will be positioned about a palm’s width to the right (west) of Mars. From dusk, until they set at around 1 am local time, the moon’s eastward orbital motion will carry it towards the Red Planet. The following evening, the moon will appear a similar distance from Mars, but now on the left (east) side of the planet. Finally, next Saturday night, the very bright moon will pass a few finger widths below distant, dim Neptune.
On Tuesday evening, the moon will reach its First Quarter phase. That’s when it has completed the first quarter of its trip around Earth (as measured from the previous New Moon). First quarter moons are always half illuminated because the sun is shining on them from the side. At this phase the moon will rise around local noon and set at around midnight. At sunset, the moon will be due south. You can point to the sun with one arm and at the moon with the other and see that the objects make a 90° angle with Earth.
As we watch the lit portion of the waxing moon grow larger every evening, we are witnessing the sun slowly rise over its eastern horizon. The slanting sunlight will be illuminating topographic high points and casting deep black shadows from them, especially along the terminator — the imaginary line that runs pole-to-pole between the light and dark hemispheres of the moon. That’s the best area to look at with your binoculars or backyard telescopes.
Mercury will be visible this week, but it will be tough to see it because it will be barely above the west-southwestern horizon after sunset. Later in the week will offer better odds. The planet will set at about 7 pm local time — soon after the sun. Because Mercury is on the far side of the sun right now, its appearance in a telescope will be a nearly fully illuminated disk. (For eye safety, be sure to wait until the sun has vanished completely before using binoculars or a telescope on Mercury.)
While you are out, take a look for Jupiter. When the sky is getting dark, the mighty planet will sit about a palm’s width above the southwestern horizon. It will set in the west-southwest shortly before 8 pm local time. Binoculars will let you see Jupiter’s four Galilean moons (Io, Europa, Callisto, and Ganymede) forming a line to either side of the planet.
You can use the moon to positively identify brighter, reddish Mars and dimmer, yellow-tinted Saturn on the evenings I noted above. For the rest of the week, those two bright planets will remain in the southern sky all evening. As soon as the sky becomes dark enough to reveal them, both planets will be located about two fist diameters above the horizon, with Mars located 35° (three and a half fist diameters) east (to the left) of Saturn. Mars will set in the west before 1:30 am local time. It’s still well worth looking at, even in a small telescope. Try to see a small white oval near the top of its disk. That’s the southern polar cap (but inverted by your telescope’s optics)!
Saturn will set at about 10:15 pm local time. Once 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 over the course of this week — starting from a position at 2 o’clock (to the upper right of Saturn) and ending at 9 o’clock (to the left of 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.)
This autumn is an ideal time to peruse the ice giant planets Uranus and Neptune because they will be conveniently placed for evening observing and also bigger and brighter than normal because Earth is roughly between them and the sun right now, minimizing our distance from those planets.
Distant Neptune will be visible nearly all night. Using a decent quality telescope you can see the very blue, magnitude 7.8 planet among the dim stars of Aquarius (the Water-bearer) — sitting roughly midway between the modestly bright star Phi (φ) Aquarii and the brighter star Hydor (Lambda Aquarii). Neptune will highest in the sky (and best viewing conditions) at about 10:15 pm local time. The bright moonlight will hinder seeing the planet this week.
Blue-green coloured Uranus will be visible all night long. You can see it without optical aid under very dark skies, but binoculars and telescopes are better. After mid-evening, Uranus will be high enough in the eastern sky to see clearly. It is located 3 finger widths to the left (east) 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. The planet will climb higher until 1:30 am local time.
Peering at Pegasus
When the moon grows brighter in the evening sky, we can still enjoy objects that include bright stars — with unaided eyes, binoculars, and telescopes.
The Great Square of Pegasus (the Winged Horse) sits in the eastern evening sky every October. This giant square is composed of four equally bright stars that form edges about 1.6 fist diameters long and measures 2 fists, corner to corner. This asterism might remind you of a baseball diamond when you see it because it’s usually tilted with one corner downwards. For the Lakota people, the square represented the great shell of Keya, the Turtle. In Greek mythology, the square represents the wings of Perseus’ flying horse. The steed is upside-down. His forelegs extend to the upper right (northwest), his neck descends to the lower right (southwest), and his head bends upwards to the northwest, ending near the lovely globular star cluster designated as Messier 15. You can see that object as a small fuzzy patch in binoculars.
The star at the bottom-most (southeast) corner of the Great Square of Pegasus is Algenib, Arabic for “the Side”. This hot, blue-white star (magnitude 2.8) is located about 350 light-years away, and actually emits 4,000 times more light than our sun! The white star at the right-hand (southwestern) corner is Markab “the Saddle”. This magnitude 2.45 star appears slightly brighter than Algenib — it emits less light, but it is only 140 light-years away. The fairly bright magnitude 2.4 star at the upper right (northwest) corner is Scheat “the Foreleg”, the second brightest star in the constellation. It’s a cool, red giant star 200 light-years away. (Pegasus’ two forelegs start at Scheat and extend upwards to the right.) The star at the upper left (northeastern) corner is Alpheratz “The Horse’s Shoulder”. It’s another hot, blue-white supergiant star, but it’s located only 97 light-years away from us. The spectrum of this star’s light indicates that it is highly enriched in the metal Mercury. In actuality, Alpheratz does not belong to Pegasus — it’s the brightest star in Andromeda, and marks the princess’ head.
At first glance, the Great Square of Pegasus appears empty. You might be able to pick out one or two dim stars inside it — more if you are away from light-polluted skies. Draw an imaginary line joining Markab and Scheat and look 1.5 finger widths to the upper right (west) of the midpoint on that line for the magnitude 5.45 star designated 51 Pegasi, or Helvetios. In 1995, using the radial velocity technique, a large planet was discovered orbiting 51 Pegasi — the first exoplanet to be discovered around a sun-like star. The planet’s mass has been estimated to be half of Jupiter’s. With a “year” of 4.23 Earth-days, the planet orbits only 0.05 AU (that’s one-sixth the distance between the sun and Mercury!) from its star, making it the original “Hot Jupiter” type of exoplanet.
During moonless periods in September and October, the steep morning ecliptic favors the appearance of the zodiacal light in the eastern sky for about half an hour before dawn. The glow is sunlight reflected from interplanetary particles drifting in the plane of our solar system. During this week, look east, below the stars of Leo (the Lion), for a broad wedge of faint light rising from the horizon and centered on the ecliptic. (The ecliptic passes directly through the bright star Regulus in Leo.) Don’t confuse the zodiacal light with distant light pollution, or the Milky Way, which is sitting further to the southeast. I posted a picture here.
Keep looking up to enjoy the sky! I love getting questions so, if you have any, send me a note.
Astronomy Skylights for this week (from October 14th, 2018) by Chris Vaughan.