See Jupiter in the Daytime, the Evening Moon Meets Many Planets, and then Exhibits an X on Saturday!
The Moon and Planets, and a Lunar X
This is the week of the lunar month when our natural satellite is best positioned for after-dinner viewing AND at looking its most picturesque. The moon will visit the four brightest planets this week and sport the famous Lunar X. Keep your binoculars and telescopes handy — here are the Skylights!
During this week, the moon will wax in phase and climb higher as it shifts east away from the sun. On Monday and Tuesday evening it will pass in front of the stars of Libra (the Scales). By the way — don’t forget that the stars are incredibly far away and the planets are in the “foreground”. Even though stars are sprinkled within a three-dimensional volume of space that is entirely within our Milky Way galaxy, they appear be fixed in place — like the pattern on cosmic wallpaper. The moon and planets are much, much closer — and re-arrange themselves — as furniture in a large room can be. Even that analogy fails to convey the difference in the distances. For example, the stars around Jupiter are several million times farther away than the planet!
Once we get to mid-week the moon will be nice and high in a dark sky — perfect for telescope viewing. As the moon waxes fuller, the sun is rising over the moon’s eastern horizon. Locations along the terminator, which is the pole-to-pole line that separates the lit and dark hemispheres, are seeing the sun peek above the moon’s horizon. The near-horizontal sunlight arriving there casts long black shadows to the west and illuminates any elevated mountain, crater, ridge, or bump.
Every night, the terminator shifts west, casting new parts of the moon into breathtaking relief. So there’s something new to see every night.
On Thursday, the almost half-illuminated moon will be located a finger’s width from Jupiter. The moon and Jupiter will easily fit together within the field of view of binoculars or a backyard telescope at low magnification. This conjunction will offer an excellent opportunity to see Jupiter in the daytime, too — using the moon as a reference. In mid-afternoon, Jupiter will be directly below the moon. It will appear as a tiny bright point of light to your unaided eyes, a tiny disk in binoculars. Telescopes will work, too! Later, in the evening, the moon will have moved slightly farther away from Jupiter, and will sit to the upper left (or to the celestial northeast) of it.
But the moon has one more planet to visit before the week is done. On Saturday afternoon, the moon will complete one quarter of its orbit around Earth (counting from the last new moon) and will reach its first quarter phase. At first quarter, the moon always appears half-illuminated on its right-hand, eastern side. It rises around noon local time. Look for yellowish Saturn sitting less than two finger widths to the upper right (or to the celestial northwest) of the moon. The moon and Saturn will fit together into the field of view of binoculars and backyard telescopes at low magnification. Observers in southern Africa will get to see the moon occult Saturn. It does this fairly often.
Several times a year at the moon’s first quarter phase, a feature called the Lunar X becomes visible in strong binoculars and small telescopes. For a few hours centered on approximately 11 pm EDT on Saturday, October 5, the illuminated rims of the craters Purbach, la Caille, and Blanchinus will combine to form a small, but very obvious X-shape. Using binoculars or a backyard telescope, look for the X along the terminator, and about one third of the way up from the southern pole of the moon (at lunar coordinates 2° East, 24° South).
As I mentioned earlier, Mercury and Venus are near the setting sun. They are currently shifting farther from the sun, but the shallow ecliptic will prevent them from climbing above the glare of sunset for a while longer. This week, you might be able to see Mercury and Venus sitting very low in the west for about 20 minutes after the sun disappears. Tonight (Sunday), Mercury will be positioned about a palm’s width to the left (celestial east) of Venus. It will increase that separation all this week. Observers near the equator and in the Southern Hemisphere will see those two inner planets very easily during the next couple of weeks.
The earlier-arriving sunsets of September is allowing us to view spectacularly bright Jupiter well, even though the planet is steadily being carried west towards the sun. As the sky begins to darken this week, look for the giant planet sitting less than a quarter of the way up the southwestern sky. Hour by hour, Jupiter will sink lower — then set in the west by about 10 pm local time. Jupiter has spent this entire year below the stars of Ophiuchus (the Serpent-Bearer) and above Scorpius (the Scorpion).
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. Good binoculars will show the moons, too! If you see fewer than four dots, then the others are in front of Jupiter, or hidden behind it. Or, it might be that some moons are not being illuminated by sunlight because they are in eclipse!
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 on Tuesday evening from 7 to 10 pm EDT, and on Sunday, October 6 from dusk to 9:15 pm EDT — with the black shadow of Io.
Yellow-tinted Saturn is prominent in the southern evening sky, too — but it is much less bright than Jupiter. The ringed planet will be visible from dusk, when it will be about 2.5 fist diameters above the southern horizon, until about midnight local time. Saturn’s position in the stars is just to the upper left (or celestial east) of the stars that form the teapot-shaped constellation of Sagittarius (the Archer).
Saturn is well worth dusting off your old 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 upper right of Saturn tonight (Sunday) to the left of the planet next Sunday. (Remember that your telescope will flip the view around.)
Blue-green Uranus will be rising in the east just before 8 pm local time this week; and it will remain visible all night long. The planet is sitting below (or to the celestial south of) the stars of Aries (the Ram) and is just a palm’s width above the circlet of stars that form the head of Cetus (the Whale). At magnitude 5.7, Uranus is actually bright enough to see in binoculars and small telescopes, under dark skies. You can use the three modest stars that form the top of the head of the whale (or sea-monster in some tales) to locate Uranus for the next several months. That’s because the distant planet moves so slowly in its orbit.
Distant, blue Neptune is still more or less at its closest to Earth for the year. The dim, magnitude 7.8, blue planet is visible all night long among the stars of eastern Aquarius, less than a finger’s width to the right (or celestial west) of a medium-bright star named Phi (φ) Aquarii. Both blue Neptune and that golden-coloured star will appear together in the field of view of a backyard telescope at medium power. The distance between the star and the planet is steadily increasing due to Neptune’s westward retrograde orbital motion.
Mars is now pulling away from the sun’s glare in the eastern pre-dawn sky. It rises at about 6:20 am local time and will become more easily visible later this month. Unfortunately, the red planet is on the far side of the sun from us — so it will remain rather small and faint until early next year.
Zodiacal Light
During moonless mornings 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.
Astronomy Skylights for the week of September 29th, 2019 by Chris Vaughan.
Keep looking up, and enjoy the sky when you do.
