Northern Summer Starts, Evening Moon Has a Gang of Four, Pre-dawn Jupiter, and Stars to Wish Upon!
A Close-up view of Mare Crisium when the terminator will be directly to its west on Wednesday, June 21, 2023 at 10 pm EDT. (Starry Night Pro)
Hello, Summer Stargazers!
Here are your Astronomy Skylights for the week of June 18th, 2023 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.
If you’d like me to bring my Digital Starlab portable inflatable planetarium to your school or other daytime or evening event, or deliver a session online, contact me through AstroGeo.ca, and we’ll tour the Universe, or the Earth’s interior, together! My terrific book with John A. Read entitled 110 Things to See With a Telescope is a guide to viewing the deep sky objects in the Messier List – for both beginners and seasoned astronomers. DM me to order a signed copy!
The moon will wax and visit some planets in the western sky after sunset this week. Brilliant Venus will dominate, but the wishing stars will soon appear. And Jupiter and Saturn will be chased by Neptune and Uranus before sunrise. Read on for your Skylights!
The June Solstice starts the Northern Summer
The beginning of summer for the Northern Hemisphere, known as the June Solstice or Summer Solstice, will occur on Wednesday, June 21 at 10:58 am Eastern Daylight Time, or 7:58 am PDT and 14:58 Greenwich Mean Time. At that time, the northern end of Earth’s axis of rotation will be tilted by its maximum amount of 23.44° towards the sun. Astronomically, the sun will reach its greatest northern declination and its greatest angle from the celestial equator.
The observation that the sun was ceasing to vary in declination (its north-south component of motion compared to the fixed stars) on that date is reflected in the name solstice, which arises from the Latin expression sol sistere, ”the sun is standing still”.
For Northern Hemisphere dwellers, the sun will climb to its highest mid-day position in the sky for this year. At local noon, your shadow will be shorter than at any other time of the year. The place on Earth where the sun will be shining directly overhead at local noon is called the subsolar point. As the Earth completes its full rotation in 24 hours, the subsolar points form a ring at latitude 23.44° known as the Tropic of Cancer. (The December solstice places the sun above the Tropic of Capricorn.) In antiquity, when the phrase was coined, the sun resided in those constellations on the two solstices. The slow wobble (or precession) of the Earth’s axis of rotation shifts the celestial positions of the sun at the solstices and equinoxes, and varies the precise latitude of the two tropics.
The sun will take the longest to cross the daytime sky – maximizing the amount of daytime hours (15 hours and 26.5 minutes for Toronto) and minimizing the night-time hours, making stargazers a bit sad. (At the December solstice, Toronto will have only 8 hours and 55.7 minutes of daytime.)
The points on the horizon where the sun rises and sets will also reach their most northerly azimuths on Wednesday. At the latitude of Toronto, the sunset position at the June and December solstices varies by 67 degrees! That’s why the different windows in your house favourably catch the sunset at different times of the year.
Just as the beam of a flashlight is strongest when pointed directly towards a wall, and weaker when held obliquely, the higher sun’s light is more intense (more watts per square metre, to be technical). More hours of more concentrated sunlight translates to more solar energy delivered and warmer days! It is NOT the case, as some people think, that we are warmer in summertime because we are closer to the sun. That event, called perihelion, actually happens in early January every year! Moreover, the Earth is now only two weeks away from reaching its greatest distance from the sun, or aphelion, for this year, which will occur on July 6.
For our friends in the Southern Hemisphere, this solstice signals the sun’s lowest noon-time height for the year, and marks the start of their winter. The June solstice is eagerly awaited by Northern Hemisphere astronomers because, after Wednesday, the nights will start to lengthen!
Noctilucent Clouds
The period from late May to mid-August, and especially around the June Solstice, is the best time for mid-Northern latitude observers to watch for noctilucent clouds (NLC) in the hours before sunrise and after twilight, especially when there is no bright moonlight in the first half of this week. Those are very high clouds that form in the mesosphere layer of Earth’s atmosphere, 80-82 km above the ground. That’s high enough to catch and reflect sunlight while conditions are dark at ground level. Another name for them is polar mesospheric clouds (PMC).
The clouds form when very cold water vapour that develops in the stratosphere diffuses upwards and freezes around meteor dust particles that are descending from space. The mesosphere is coldest around the solstice. Visually, the clouds will appear as bright, cirrus-like ripples across the lower third of the northern sky, often tinted a silver-blue. If you see dark wisps of clouds overhead, those are regular cirrus clouds.
According to this terrific article from RASC, the first cloud sightings in North America were reported in 1933. The phenomenon may be increasing in frequency as climate change alters the upper atmosphere. More information, and a current polar map of the NLC coverage, can be viewed down the left-hand column of www.spaceweather.com. If you see them, or photograph them, tag me! I posted a photo of them here.
The Moon
This is the week of the lunar month when our natural satellite will be shining in the evening sky worldwide. As it ventures farther from the sun each evening, it will set later and wax in illuminated phase. It won’t really interfere with our views of the summertime Milky Way until Friday.
After the sun sets tonight (Sunday evening) in the Americas, you might be able to glimpse the thin crescent of the young crescent moon positioned just above the northwestern horizon – in the same direction where the sun set. The moon will set an hour after the sun. Seeing a crescent moon less than 24 hours after new is a bucket list challenge for astronomers! Don’t hunt for it with binoculars until the sun has fully set, though.
The moon will be far easier to see, again as a thin crescent, on Monday after sunset. By the time it sets towards 11 pm, the brightest stars of Gemini (the Twins), Pollux and Castor, will appear a few finger widths above it. The triangle they form will be tight enough to fit in the field of view of your binoculars. Watch for the Earthshine phenomenon. Also known as the Ashen Glow, or the Old Moon in the New Moon’s Arms, it is visible within a day or two of new moon. It is produced by sunlight reflected off Earth and back toward the moon, slightly brightening the unlit portion of the moon’s Earth-facing hemisphere.
On Tuesday evening, the crescent moon will shine among the faint stars of Cancer (the Crab), between Venus and Gemini’s bright twins. Earthshine should still be present. The group will make a lovely photo when composed with some interesting foreground landscape. Consider capturing multiple photos as the sky darkens (and keep the camera handy for Wednesday night, too).
This will also be a great night to turn your binoculars or a backyard telescope to the moon. The terrain alongside the curved pole-to-pole terminator will be spectacular, especially the Gang of Four, the big craters named Langrenus, Vendelinus, Petavius, and Furnerius. Their chain will be strung along the lower (southern) portion of the terminator. All four craters are about 130 km in diameter- a mere 90 minutes drive in your solar-powered electric moon buggy.
To kick off summer in the northern hemisphere, look in the lower part of the western sky on Wednesday night to see the gorgeous waxing crescent moon forming a triangle to the right (or celestial north) of extremely bright, white Venus and the much fainter, reddish planet Mars. In the Americas they’ll be close enough to share the view in binoculars. In more westerly time zones, the moon will appear even closer to the two planets. While Venus and the moon will be easy to spot after sunset, the sky will need to darken somewhat before the speck of Mars appears. The trio will set at about midnight local time. That will be a great night to zoom in on the rugged terrain ringing the left-hand (western) edge of Mare Crisium’s dark oval.
On Thursday night the moon will shine to the upper left of the two planets and several finger widths to the right of the Lion’s brightest star, Regulus. The brightening moon will spend Friday and Saturday among Leo’s stars. Each night will highlight new strips of the lunar terrain, so it’s worth inspecting it each evening. At this phase the moon will be shining in the daytime afternoon sky. Keep an eye out for it.
Our natural night-light will finish the week among the stars of Virgo on Sunday night. By then the moon will be approaching its first quarter phase – about 90° away from the sun. Use your telescope on Sunday to view the magnificent Montes Alpes, Montes Caucasus, and Montes Apenninus, the mountain ranges that border the eastern rim of the giant Imbrium Basin. When the terminator is that close to them, the Apennines resemble a row of jagged fangs that climb in altitude to the south.
The Planets
We’re down to the final two weeks of the Venus and Mars show in the evening sky. The brilliant dot of Venus will attract your attention while it shines in the lower part of the darkening western sky following sunset. Before long, the brightest stars of Gemini, golden Pollux and whitish Castor, will appear about two fist widths to Venus’ lower right (or celestial northwest). Regulus, the brightest star in Leo (the Lion), will twinkle a similar distance to Venus’ upper left.
The faint, reddish speck of Mars will shine weakly between Venus and Regulus. As Mars and the background stars are carried sunward (by four minutes’ worth) each evening, Venus will seem to climb “higher” compared to them. Venus has been approaching Mars, and they’ll get even closer together next week – but both planets will be dropping a little lower in the sky when viewed at the same time each evening. Venus and Mars (plus the crescent moon on Wednesday only) will set around midnight this week.
Viewed in a telescope this week, Venus will show a nearly-half full crescent that is waning and growing slightly larger each day as it swings into the space between Earth and the sun. To see the planet’s shape most clearly, wait until the sun has fully set and then aim your telescope at Venus while the sky is still bright. That way Venus will be higher, shining through less intervening air, and her glare will be lessened. As Venus looms larger, good binoculars can hint at Venus’ non-round shape. Like Venus, Mars will look best in a telescope while it is highest, right after dusk, but its ruddy disk will look tiny under any magnification.
The bright gas giant planets will continue to dominate the pre-dawn sky for weeks. Bright, yellowish Saturn will clear the trees towards the southeast after about 2 am local time. It will rise higher and shine among the moderately bright stars of Aquarius (the Water-Bearer) until we lose sight of the stars and the ringed planet in the southeastern sky by 5 am. Next week Saturn will start rising before midnight. It’ll be available for our evening viewing pleasure through opposition in late August and then through to mid-winter!
The planet Neptune, which is located in western Pisces (the fishes), is positioned two fist diameters to Saturn’s lower left, or 19° to its celestial east. The distant blue planet, 600 times fainter than Saturn, will be available to see in backyard telescopes from August onwards.
The brilliant white dot of Jupiter, about 16 times brighter than Saturn, will rise around 2:45 am local time. It will be clearing the eastern rooftops just as the sky starts to brighten before 4:30 am, but you should be able to see it easily until almost sunrise. In a few weeks, though, Jupiter will rise early enough to shine in a dark sky amid the stars of its host constellation, Aries (the Ram). Jupiter will start to rise before midnight by the second week of August, and we’ll be able to view it from then until March, 2024.
Binoculars will show Jupiter’s four Galilean moons named Io, Europa, Ganymede, and Callisto lined up beside the planet. Those moons complete orbits of the planet every 1.7, 3.6, 7.2, and 16.7 days, respectively. If you see fewer than four moons, then one or more of them is crossing in front of or behind Jupiter, or hiding in Jupiter’s dark shadow – or two of the moons are very close together or occulting one another. All four of them will huddle to the west of Jupiter on Thursday morning. Even a small, but decent quality telescope can show you Jupiter’s dark belts and light zones, which are aligned parallel to its equator. With a better grade of optics, Jupiter’s Great Red Spot, a cyclonic storm that has raged for hundreds of years, becomes visible for several hours when it crosses the planet every 2nd or 3rd night. For observers in the Americas, that GRS will cross Jupiter’s disk on Wednesday and Friday morning. If you have any coloured filters or nebula filters for your telescope, try enhancing the spot with them.
Like Saturn, Jupiter will be followed across the sky by an ice giant planet. Uranus will be positioned 1.4 fist diameters to the lower left of Jupiter this week. The blue-green planet will become much easier to see in the coming weeks.
Stars to Wish Upon
Venus’ brilliance makes it the first “star” to come out at night nowadays, but if you want to wish upon the first genuine star, then you’ve got two to choose from!
As the post-sunset sky darkens, cast your gaze high in the southern sky to spot yellow-orange Arcturus also known as Alpha Boötis. You can’t miss it! At magnitude -0.15, it’s not only the brightest star in Boötes (the Herdsman), but also the fourth brightest star in the entire night sky worldwide. Only our sun and Sirius are brighter for mid-Northern latitude skywatchers. Arcturus is a “neighbour” of ours. It’s only 37 light-years away from the sun.
Arcturus has that colour because it is just passing middle-age for a star, starting on its way towards the red supergiant stage. In Chinese, Arcturus is known as Dà Jiǎo xīng 大角星, “Great Horn Star”, part of the huge Azure Dragon of the East, which stretches from Boötes and Virgo on the west to Scorpius and Sagittarius on the east. The star Antares marks his heart.
All the stars within our Milky Way galaxy are in motion, jostling about as they orbit the galactic centre every quarter of a billion years or so. Some stars move faster, or are located closer to us – so they exhibit a greater apparent motion compared with the surrounding stars. Astronomers call this phenomenon proper motion. That’s why star charts need to be updated from time to time. Arcturus occupies the belt-buckle of the herdsman. It has a very high proper motion southward. In a few thousand years, the herdsman’s legs will be bent upwards with Arcturus below his knees!
You will probably spot Arcturus first (after Venus) because it will be so high in the sky at dusk. But if you happen to be facing east, you might see the equally bright star Vega, in Lyra (the Harp) first. Vega is the next brightest star in the sky after Arcturus. Astronomers use something called the magnitude scale to indicate the brightness of objects. The star Vega anchors the scale at magnitude 0.0. Each integer higher reduces the brightness by 2.5 times, and each integer lower increases the brightness by the same factor. Polaris, the North Star, shines at magnitude 1.95, which is 6.3 times fainter than Vega. A magnitude step of 0.5 changes the brightness by about 50%.
Before long, you’ll catch sight of another bright (magnitude 1.25) star named Deneb, the tail of Cygnus (the Swan) sparkling to the lower left of Vega. Vega and Deneb are the first corners of the Summer Triangle asterism to appear. The third corner star Altair, will be just clearing the eastern treetops. Although brighter than Deneb, Altair’s brilliance will be suppressed by the extra atmosphere we see it through this early in the evening during June.
Arcturus means “Guardian of the Bear” in Greek, because it rises after Ursa Major (the Big Bear), which sits to its upper right (celestial west). The darkening sky will reveal the seven prominent stars of Ursa Major’s Big Dipper asterism overhead. Meanwhile, reddish Antares “the Rival of Mars”, and the heart of Scorpius, will be twinkling low in the south, too. But, by then, you should already have made your wish!
Eyeing Ophiuchus
If you missed last week’s tour of the best sights to see in the lesser-known constellation of Ophiuchus (the Serpent-Bearer) I posted it here
Public Astronomy-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. They host in-person viewing on the first clear Wednesday night each month. Other Wednesdays they stream views online via the observatory YouTube channel. Details are here.
Eastern GTA sky watchers are invited to join the RASC Toronto Centre and Durham Skies for solar observing and stargazing at the edge of Lake Ontario in Millennium Square in Pickering on Friday evening, June 23, from 7 pm to midnight. Details are here. Before heading out, check the RASCTC home page for a Go/No-Go call – in case it’s too cloudy to observe.
On Saturday, June 24 from 10 to midnight EDT, the in-person DDO Astronomy Speakers Night program will feature Dr. Junwu Huang, a faculty member at the Perimeter Institute. He will speak on A Camera for the Dark Universe, about searching for dark matter in our Universe.After the presentation, participants will tour the observatory and see a demonstration of the 74” telescope pointed to an interesting celestial object for the visitors to view (weather permitting). More information is here and the registration link is here.
My free, family-friendly Insider’s Guide to the Galaxy webcast with RASC National returns on Tuesday, June 20 at 3:30 pm EST. We’ll focus on how to use star charts and which ones are best. Then we’ll highlight our next batch of RASC Finest NGC objects. You can find more details and the schedule of future sessions here.
Keep looking up, and enjoy the sky when you do. I love questions and requests. Send me some!