Shooting Star Season Starts, Pallas Pauses, and the Librated Full Thunder Moon Greets Gas Giants!

On Friday, July 23, the moon will reach its full phase. On that evening, the libration of the moon will cause the moon to appear to tilt downwards and twist to the left a bit, allowing Earthlings to see features along the moon’s northeastern limb that are normally not visible (green labels). Major lunar features mentioned in the text are also highlighted.

Hello, Summer Stargazers!

Here are your Astronomy Skylights for the week of July 18th, 2021 by Chris Vaughan. Feel free to pass this along to your friends and send me your comments, questions, and suggested topics. You can also follow me on Twitter as @astrogeoguy! Unless otherwise noted, all times are expressed in Eastern Time. To subscribe to these emails please click this MailChimp link.

I can bring my Digital Starlab portable inflatable planetarium to your school or other daytime or evening event, or teach a session online. Contact me through AstroGeo.ca, and we’ll tour the Universe, or the Earth’s interior, together!

This week, the moon will shine brightly in the evening sky all around the world, and after reaching its full phase on Friday and tilting its NE edge toward Earth, it will spend the weekend with Jupiter and Saturn. Jupiter will host two shadow transits starting late Saturday night, two meteor showers slowly get underway, and dim Mars pulls away from brilliant Venus after sunset. Read on for your Skylights!

Southern Delta Aquariids Meteor Shower

The Southern Delta Aquariids meteor shower runs annually from July 21 to August 23 – so you can begin to keep an eye out for shooting stars this week. That shower is caused when the Earth passes through a cloud of tiny particles dropped by a periodic comet – likely Comet 96P/Machholtz. The shower will peak before dawn on Thursday, July 29, but is quite active for a week surrounding that date. More information next week.

The Moon

The moon will spend this week shining brightly in the evening sky as it waxes towards full on the coming weekend. Although full moons aren’t as thrilling to see in a telescope, most of this week will offer fantastic views of Selene – both with your unaided eyes and with optical aids. The terminator is the boundary line that separates the eastern lit hemisphere of the moon from its unlit western hemisphere. From night to night the terminator migrates from lunar east to west (that’s right to left, if viewed from the Northern Hemisphere of Earth).

The zone alongside of the terminator is the best place to aim your binoculars or telescope. The nearly-horizontal sunlight arriving there brightly illuminates every crater rim, mountain peak, ridge, and bump – and casts deep, black shadows toward lunar west. With no air to scatter light on the moon, the effect is breathtaking. And, new vistas are revealed each night as the terminator slides west. Watch the same crater over several nights, and see it gradually fill with light! Let’s review some of the lunar highlights this week.

Everyone on Earth sees the same phase of the moon. Tonight (Sunday), the gibbous, 69%-illuminated moon will shine inside Libra (the Scales). For the first half of the week you can see the pale moon shining in a blue sky starting in late afternoon. At about 11:15 pm EDT (or 03:15 Greenwich Mean Time) tonight some parts of the world will see the moon skim closely past (or graze), or completely pass in front of (or occult) the medium-bright star Zuben Hakrabi, also known as v Librae.

On Monday night, the moon will slip between Acrab and Dschubba, the northern two claw stars of Scorpius (the Scorpion). On Tuesday it will pass through the “lost Zodiac constellation” of Ophiuchus (the Serpent-Bearer), and on Wednesday-Thursday it will hop across the Teapot asterism in Sagittarius (the Archer).

Tuesday also marks the 52nd Anniversary of humankind’s first steps upon another world! The six crewed Apollo Missions were sent to different regions of the moon in order to carry out experiments and to bring back rock samples that help us determine the age and composition of the lunar surface. For safety reasons, Apollo 11 was sent to the flat and relatively featureless terrain of Mare Tranquillitatis “Sea of Tranquility”. They were also investigating why that mare is enriched in titanium, making it look bluer than the others. Later missions landed in more rugged regions with complex geology. When the moon approaches the full phase, all of the regions where the astronauts explored are illuminated by sunlight, including the most westerly site, Apollo 12 in Oceanus Procellarum.

On Friday night the moon will enter Capricornus (the Sea-Goat), beginning its monthly visit with the gas giants Saturn and Jupiter. On Friday, the moon will sit to the lower right (southwest) of Saturn. On Saturday it will be positioned below and between the two planets. And on Sunday night, it will take up a position below (or celestial south of) Jupiter. Take a series of wide-field photos, and tag me @astrogeoguy on social media if you post them!

The moon will reach its full phase on Friday at 10:37 pm EDT (or 02:37 GMT on Saturday). The July full moon, commonly called the Buck Moon, Thunder Moon, or Hay Moon, always shines in or near the stars of Sagittarius or Capricornus. The indigenous Ojibwe people of the Great Lakes region call this moon Abitaa-niibini Giizis, the Halfway Summer Moon, or Mskomini Giizis, the Raspberry Moon. The Cherokees call it Guyegwoni, the Corn in Tassel Moon. The Cree Nation of central Canada calls the June full moon Opaskowipisim, the Feather Moulting Moon (referring to wild water-fowl habits), and the Mohawks call it Ohiarihkó:wa, the Fruits are Ripened Moon. All of these names describe their natural surroundings in July.

Because the moon is full when it is opposite the sun in the sky, full moons always rise in the east as the sun is setting, and set in the west at sunrise. Since sunlight is striking the moon vertically at that time, no shadows are cast; all of the variations in brightness you see arise from differences in the reflectivity, or albedo, of the lunar surface rocks.

Due to its orbit’s 5° inclination and ellipticity, the moon tilts up-and-down and sways left-to-right by a small amount while keeping the same hemisphere pointed towards Earth at all times. Over the course of many months, this lunar libration effect lets us see 56% of the moon’s total surface from Earth. The motions can be detected by noting the positions of major features near the limb of the moon. One of the best of these is dark and very round Mare Crisium, the Sea of Crises. The 556 km-wide basin is easy to see using your unaided eyes or binoculars – and in telescopes. It always sits in the northeastern quadrant of the moon – that’s the upper right for Northern Hemisphere observers (lunar east is the opposite of sky east). Keep an eye on Mare Crisium over the next few months and note how it shifts towards and away from the moon’s edge while also riding higher and lower.

On Friday night, the moon’s libration will allow us to see features along its northeastern limb that are normally not visible. These include Mare Humboldtianum, Hubble crater (named for Edwin Hubble), and Beals crater (named for Canadian astronomer Carlyle Beals). A large telescope will not be needed.

Near every full moon phase, bright streaks called rays can be seen extending from the younger craters on the lunar near side. The impact that created the bright crater Tycho, which is located in the south-central area of the moon, threw out streams of bright material that extend thousands of km in all directions across the moon’s near side. Another particularly interesting ray system surrounds the crater Proclus. The 28 km-wide crater and its ray system are visible in binoculars and telescopes, at the lower left edge of Mare Crisium. The Proclus rays, about 600 km in length, only exist on the eastern, right-hand side of the crater, and also within Mare Crisium, suggesting that the object that produced them arrived at a very shallow angle from the southwest.

The Planets

Venus and Mars will continue to occupy the low, western post-sunset sky this week. But hard-to-see Mars will be increasing its distance from brilliant, easy-peasy Venus, and is also shifting closer to the sun each night – so we won’t be able to use Venus as our guide to spotting Mars for much longer.

Venus, the Greco-Roman goddess of love is called Ikwe Anang “the Women’s Star” by indigenous Ojibwe skywatchers. As the sky begins to darken (after about 9:15 pm local time at mid-northern latitudes) this week, Venus will appear amidst the western twilight glow as a bright point of light shining about a fist’s diameter above the western horizon. It will get easier to see as the minutes pass and the sky darkens, but Earth’s inexorable rotation will carry the planet lower – and it will set at about 10:20 pm local time.

When viewed in a backyard telescope, 85%-illuminated Venus will exhibit a smallish disk and a slightly squashed shape. Aim your telescope at Venus as soon as you can spot the planet in the sky (but ensure that the sun has completely disappeared first). That way, Venus will be higher and shining through less distorting atmosphere – giving you a clearer view.

Currently, magnitude 1.84 Mars is 190 times fainter than magnitude -3.87 Venus. Tonight (Sunday) Mars will be positioned about two finger widths to the lower right (or 3° to the celestial west) of Venus, with the bright star Regulus in Leo (the Lion) located a similar distance to Venus’ left. The trio will be binoculars-close all week. On Wednesday night, Venus will pass just a finger’s width above (to the celestial north of) Regulus, and Mars will sit several finger widths to their lower right. Finally, next Sunday night, July 25, Mars will be parked a generous palm’s width to the lower right of Venus, with Regulus between the two planets.

Yesterday, July 17, the dim and distant dwarf planet designated (134340) Pluto reached opposition, the day of the year when Earth is positioned between Pluto and the sun, minimizing our distance from that outer world. For the next week or so, Pluto will be located 4.98 billion km or 277 light-minutes from Earth – and it will shine with an extremely faint visual magnitude of +14.3. That’s far too dim for visual observing through backyard telescopes.

Pluto will be located in the sky about midway between Saturn and the bright star Nunki in Sagittarius’ Teapot asterism, and it will climb highest in the southern sky at about 1 am local time. Telescope-owners can focus on a magnitude 7.8 star named HIP97602, which will be sitting about 11 arc-minutes (or one-third of the moon’s diameter) to the lower left of Pluto this week. Even if you can’t see Pluto directly, you will know that it is there! (Remember that your telescope will flip and/or mirror-image the sky view. You can check what your telescope does by viewing the moon with and without the telescope. It’ll have the same effect on anything you view through it – so take note.)

This week, the yellow-tinted dot that is Saturn will rise within the faint stars of central Capricornus (the Sea-Goat) shortly before 9:30 pm local time and then it will cross the overnight sky until dawn swallows it up in the southwestern sky. Saturn is being pursued across the sky by 16-times brighter Jupiter, which sits two fist diameters to Saturn’s left (celestial east). Watch for the Milky Way and the teapot-shaped stars of Sagittarius (the Archer) sitting several fist diameters off to Saturn’s right (celestial west). The very bright moon will make its monthly trip past the gas giant planets during the coming weekend – setting up some nice wide-field photo opportunities.

Even a small telescope will show Saturn’s rings and several of its brighter moons – especially its largest moon, Titan! Because Saturn’s axis of rotation is tipped about 27° from its orbital plane (a bit more than Earth’s tilt), 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 left (celestial north) of Saturn tonight to below (celestial south) the planet next Sunday. (Remember that your telescope will probably flip the view around.) Saturn’s rings will be narrowing every year until the spring of 2025. This year, they are already narrow enough to reveal a bit of Saturn’s southern polar region extending past their edge.

This week the very bright, white, magnitude -2.7 planet Jupiter will rise soon after 10 pm local time, and then follow Saturn across the southern sky until sunrise hides them both. Jupiter is now moving slowly westward across the distant stars of central Aquarius (the Water-Bearer), a parallax effect produced because Earth, on a faster orbit, is passing Jupiter on the “inside track” around the sun.

For Eastern Time Zone observers the Great Red Spot (or GRS) will be visible crossing Jupiter starting late on Sunday (July 18), Wednesday and Friday night, also during the wee hours of Wednesday and Saturday, and before dawn on Friday and Sunday morning. From time to time, the small round black shadows cast by Jupiter’s four Galilean moons become visible in amateur telescopes when they cross (or transit) the planet’s disk. On Saturday night, July 24, Ganymede’s big shadow will cross Jupiter from 10:40 pm until 2:15 am EDT. Ganymede itself will start crossing the planet on Sunday morning at 1:15 am, and then Europa’s tiny shadow will join the scene starting at 3:34 am EDT – with the GRS, too!

Today (Sunday), the main belt asteroid designated (2) Pallas is ceasing its regular eastward motion in front of the distant stars. After this temporary pause, it will begin a retrograde loop that will last until early November. Pallas’ visual magnitude of 9.7 allows it to be seen in amateur telescopes starting in late evening. This week Pallas will be positioned in the eastern sky, less than half a degree to the right (or celestial southeast) of a medium-bright star named HIP116417 and a fainter star HIP116431, which sit near the ring of stars that forms the western fish in Pisces. The asteroid and those stars will appear together in the eyepiece of your telescope.

Dim, blue Neptune is located near the border between Aquarius and Pisces (the Fishes) – about 2 fist diameters to the lower left (or 22° to the celestial east) of Jupiter. It rises at about 11 pm local time and reaches peak visibility halfway up the southern sky before dawn. Magnitude 5.8 Uranus is bright enough to see in binoculars and backyard telescopes, even near cities. This week it will rise at about 1 am local time. Hamal and Sheratan, the two brightest stars in Aries (the Ram), will sit above it all summer.

Last up, literally, is Mercury, which will spend all of July in the eastern pre-dawn sky. Its position below (or to the celestial south of) the morning ecliptic will prevent Mercury from rising very long before the sun – making this appearance a poor one for Northern Hemisphere observers, but a good one for people viewing the planet from south of the equator. Mercury is now descending toward the sun – but the bright, magnitude -1.2 planet will be visible starting at about 5:15 am local time – when it will sit very low over the east-northeastern horizon.

Some Moonlight-Friendly Sights

If you missed last week’s post with some celestial sights to see in July I posted it with sky charts here.

Public Astro-Themed Events

Every Monday evening, York University’s Allan I. Carswell Observatory runs an online star party – broadcasting views from four telescopes/cameras, answering viewer questions, and taking requests! Details are here. Their in-person Wednesday night viewing has been converted to online via the observatory YouTube channel.

Don’t forget to take advantage of the astronomy-themed YouTube videos posted by RASC Toronto Centre and RASC Canada.

My free, family-friendly Insider’s Guide to the Galaxy webcasts with Jenna Hinds of RASC National will return this Tuesday, July 20 with a Summer Stargazing Spectacular, including how to see the Perseids Meteor shower! You can find more details, and the schedule of future sessions, here.

On Wednesday, July 21 at 7 pm EDT, SkyNews Magazine will continue their weekly online sessions Games in Space – Out of Space featuring editor Allendria Brunjes, Jenna Hinds and guests. Details are here and the registration link is here! Sessions are also live-streamed to YouTube here and can be watched at any time.

Our public sessions at the David Dunlap Observatory may not be running at the moment, but we are pleased to offer some virtual experiences instead, in partnership with Richmond Hill. On Friday, July 23 from 9:30 to 11 pm EDT, join RASC Toronto Centre astronomers for DDO Astronomy Speakers Night. Vidya Ganesh Rangarajan will present Mars – an Active World? All about Mars volcanism. Deadline to register for this program is Friday, July 23, 2021 at 3 pm. Prior to the start of the program Richmond Hill will email you information on the virtual program links and any specific information relating to your program. The registration link is here.

On Sunday afternoon, July 25 from 1 to 2 pm EDT, join me for Ask the Astronomer. We’ll talk about what’s up in the sky lately and answer your space-related questions! Depending on the weather, you may even be able to see views of the sun through a solar scope! Deadline to register for this program is Friday, July 23, 2021 at 3 pm. Prior to the start of the program Richmond Hill will email you information on the virtual program links and any specific information relating to your program. The registration link is here.

Keep looking up, and enjoy the sky when you do. I love questions and requests. Send me some!