Lovely Luna in Evening, Saturn Shines at Maximum, and Moonlight-Friendly Sights!
The Moon and Planets
This is the week of the lunar month when skywatchers worldwide can enjoy the moon in the early evening sky — just ahead of next week’s 50th anniversary of Apollo 11. And, two more planets will reach opposition and maximum visibility this week. Here are the Skylights!
Tonight (Sunday) the moon will begin the week as a pretty waxing crescent in the western, early evening sky on the border between Virgo (the Maiden) and Leo (the Lion). On Tuesday morning, the Earth-orbiting moon will reach a 90 degree angle from the sun — setting up its First Quarter phase, when it appears half-illuminated from Earth. We call this phase “first quarter” because the moon has completed one-quarter of the trip around Earth.
First quarter moons rise around noon and set around midnight. The evenings surrounding this lunar phase are the best times to look at the moon under magnification. As the sun slowly rises over the moon’s eastern horizon (which takes a week!), its steeply slanted rays of light cast deep, black shadows to the west of every elevated feature — mountains, crater rims, boulders, and even fault lines. Every hour, and every night, the zone alongside the terminator, the pole-to-pole boundary dividing the lit and dark hemispheres, creeps west — revealing new breathtaking vistas.
On Tuesday night, the moon will take up a position above (celestial north of) the very bright, white star Spica in Virgo. Over the course of several hours, you can watch the moon’s orbital motion carry it eastwards of that star. That’s toward the left for Northern Hemisphere observers, and vice versa.
On Thursday and Friday night, the waxing gibbous (which means “more than half-illuminated”) moon will visit Libra (the Scales) and then land above the up-down row of three little white stars that mark the claws of Scorpius (the Scorpion).
In the southern sky on Saturday evening, the waxing gibbous moon will land about three finger widths to the left (east) of the bright planet Jupiter. If you watch the moon and Jupiter over several hours, starting at dusk, you will see the moon’s orbit carry it farther from the planet, while the rotation of the sky will lift the moon above Jupiter.
For about an hour after sunset on the Sunday evening, Mercury’s orbital motion downwards towards the sun will bring it less than 4 finger widths to the lower left (south) of slightly brighter Mars. The two dim planets will be very low in the north-northwestern twilit sky. Take care that the sun has completely disappeared below the horizon before attempting to search for them with binoculars or telescopes. The best time to look for Mercury falls between 9:30 and 9:45 pm local time. Mars sets nearly 30 minutes later, so you’ll have more time to look for it. Observers closer to the Equator will have a better chance to see the two planets — due to a darker sky after sunset. Mercury will drop too low to see after mid-week.
That incredibly bright object that you’ve been seeing in the southeastern evening sky recently is Jupiter! This week, Jupiter will be visible from dusk to almost 4 am local time. Even a backyard telescope can show Jupiter’s saucy equatorial stripes and the four Galilean moons named Io, Europa, Callisto, and Ganymede. They always appear in a rough line flanking the planet. If you see fewer than four, then some are either in front of Jupiter, or hidden behind it.
From time to time, the small, round black shadows cast by the Galilean moons become visible in amateur telescopes as they cross (or transit) Jupiter’s disk. Io’s shadow will transit Jupiter after midnight on Thursday (i.e., Friday morning) from 12:31 am to 2:43 am EDT.
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 starting late on Monday evening (from 10:15 pm until 2:15 am EDT). More GRS viewing opportunities will occur after dusk on Tuesday and Thursday, and on Saturday night from 10:15 pm to 1:30 am EDT.
On the afternoon of Tuesday, July 9, Earth’s orbit will carry us between Saturn and the sun. Sitting opposite the sun in the sky on that date, Saturn will rise at sunset and arrive at its minimum separation from Earth of 9.0 Astronomical Units (an AU is the average sun-Earth distance). That’s 1.351 billion km, or 75 light-minutes! Saturn will shine at a peak brightness for the year (magnitude +0.05) and exhibit an apparent disk diameter of 18.4 arc-seconds. The rings, which will be getting narrower every year until the spring of 2025, will subtend 42.86 arc-seconds. (For comparison, the full moon is 1,800 arc-seconds across.) Don’t fret if Tuesday is cloudy. Saturn will look nearly as good for several days.
Yellow-tinted Saturn will remain visible all night long. Its position in the sky is just to the left (east) of the stars that form the teapot-shaped constellation of Sagittarius (the Archer). Saturn is quite a bit dimmer than Jupiter. To find it, look about 3 fist diameters to the lower 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 is), we can see the top surface of its rings, and its moons can appear above, below, or to either side of the planet. During this week, Titan will migrate counter-clockwise around Saturn, moving from Saturn’s right tonight (Sunday) to the upper left of the planet next Sunday. (Remember that your telescope will flip the view around.)
For night owls, distant and dim, blue Neptune is in the southeastern pre-dawn sky, among the stars of Aquarius (the Water-Bearer). The planet will be rising shortly before midnight local time this week. You’ll find the magnitude 7.9 planet sitting a thumb’s width to the left (east) of a medium-bright star named Phi (φ) Aquarii.
Blue-green Uranus will be rising at about 1:30 am 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. At magnitude 5.8, Uranus is bright enough to see in binoculars.
Venus is bright enough to see within the pre-dawn twilight sky that surrounds it, but it is sitting very low in the northeast — sinking ever-closer to the rising sun. Venus will be rising at about 5 am local time all week.
But wait, there’s more! On Sunday, July 14, the dim and distant dwarf planet Pluto will also reach opposition. (After all, it IS sitting near Saturn this year.) On that date, Pluto will be the closest to Earth (4.91 billion km, or 273 light-minutes) and reach its greatest visual magnitude (+14.2) for 2019. Pluto will rise in the east at sunset and reach its highest elevation, over the southern horizon, at 1:20 am local time. While Pluto is far too dim to see in amateur-grade telescopes, an astronomy app can show you where it is compared to the brighter nearby stars. Even if you can’t see it directly, you will know that Pluto is there.
Some Moonlight-Friendly Sights
Stars shine with a colouration that is produced by their surface temperatures, and this is captured in their spectral classification. The three bright stars of the Summer Triangle, namely Deneb, Vega, and Altair, are all A-class stars that appear blue-white to the eye and have surface temperatures in the range of 7,500 to 10,000 K. High in the southwestern sky, orange Arcturus is a K-class giant star with a temperature of only 4,300 K. Finally, reddish Antares, the heart of Scorpius, is an old M-class star with a surface temperature of 3,500 K. By comparing these stars colors’ to other stars, you can estimate those stars’ temperatures. The classification letters, from hottest to coolest are: OBAFGKM. Can you think up a mnemonic phrase to remember the order? I have one.
The constellation of Lyra (the Harp) is positioned high overhead in late evening in early July. This constellation features a coffee and a donut! Keen eyes might reveal that the star Epsilon Lyrae, located just one finger’s width to the left (east) of the bright star Vega (Alpha Lyrae), is a double star. Binoculars or a small telescope will certainly show the pair. Examining Epsilon at high magnification will reveal that each of the stars is itself a double — hence its nick-name, “the double-double”. To see the donut, aim your telescope midway between the stars Sulafat and Sheliak, which form the southern end of Lyra’s parallelogram. Messier 57, also known as the Ring Nebula, will appear as a faint grey ring. Higher magnification works well on this planetary nebula — which is the corpse of a star that had a similar mass to our sun.
Mid-July evenings bring us one of the best asterisms in the sky, the Teapot in Sagittarius (the Archer). This informal star pattern features a flat bottom formed by the stars Ascella on the east and Kaus Australis on the west, a triangular pointed spout pointing west, marked by the star Alnasl, and a pointed lid marked by the star Kaus Borealis. The stars Nunki and Tau Sagittarii form its handle. The asterism reaches maximum height above the southern horizon around midnight, when it will look as if it’s serving its hot beverage — with the steam rising as the Milky Way!
Astronomy Skylights for the week of July 7th, 2019 by Chris Vaughan.
Keep looking up, and enjoy the sky when you do. I love questions and requests — so, send me some!
