Maximum Mercury, Dark Nights, Bright Jupiter, and the Best of Böotes!
The Moon and Planets
The moon will be out of the evening sky this week as it slides towards its meeting with the pre-dawn sun next week. Meanwhile, it’s a perfect week to explore the night sky — now that the Northern Hemisphere daylight period is slowly shortening and our nights lengthening! Here are the Skylights!
Monday morning will find the late-rising moon among the modest stars of Aquarius (the Water-Bearer) in the southeastern pre-dawn sky; and then our natural satellite will linger in view in the southwestern sky until mid-day. For the rest of the week, the waning moon will pass through Cetus (the Whale) and dip into Pisces (the Fishes). On Tuesday morning at 5:46 am Eastern Time, the moon will officially reach its last quarter phase — appearing half-illuminated on the left-hand (its western) side.
On Saturday morning, the old, slim, crescent moon will land in the western end of Taurus (the Bull). On Sunday morning, just before sunrise, see if you can spot the delicate moon sitting a few finger widths above Taurus’ brightest star, Aldebaran.
While we’re talking pre-dawn, remember that 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 after 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 just after 3 am local time this week. It is sitting among the stars of Aries (the Ram) and is just a palm’s width above the head of Cetus. The large main-belt asteroid Vesta is nearby — a palm’s width below Uranus. Look for Vesta sitting only half a finger’s width below the modest star Xi (ξ) Ceti (and also named Al Kaff al Jidhmah). They will appear together in the field of view of a backyard telescope.
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 4:40 am local time all week.
The blazingly-bright object that you’ve been seeing in the southeastern evening sky recently is mighty Jupiter. We are still near our closest approach to the planet for 2019, increasing its apparent brightness and the size of its banded disk and moons in binoculars and telescopes. This week Jupiter will be rising at about 7:30 am local time, before sunset.
From time to time, the small, round black shadows cast by Jupiter’s four Galilean moons become visible in amateur telescopes as they cross (or transit) Jupiter’s disk. Starting at about 2 am EDT on Wednesday morning, Io and its shadow will cross Jupiter for more than two hours. Io’s shadow will also be completing a transit (with the Great Red Spot) just before 11 pm EDT on Thursday evening, so you can start to look for that as soon as the sky darkens. On Saturday morning, Europa and its shadow will cross Jupiter for two hours (with the Great Red Spot), starting at about 2:40 am EDT. Jupiter will set while they are crossing.
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 Monday evening from 10 pm until 12:30 am EDT. More GRS viewing opportunities will occur on Wednesday from 11 pm to 2 am EDT, Thursday evening before 11:30 pm EDT, and on Saturday night in the hours before midnight.
Yellowish Saturn will be rising in the east-southeast just before 10 pm local time this week. 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 the upper right tonight, to the left of the planet next Sunday. (Remember that your telescope will flip the view around.)
Dim, reddish Mars and brighter Mercury are still hanging out together just above the northwestern horizon after sunset this week, but not as close together as last week. Mercury will become easier to spot every night while it climbs away from the sun and brightens. The best time to look for Mercury falls between 9:45 and 10:15 pm local time. Tonight (Sunday) Mercury will peak in visibility when it reaches its widest separation, 25 degrees east of the Sun, for the current apparition. Look for Mars sitting a few finger widths to the upper left of Mercury. (Take care that the sun has set before attempting to view them using binoculars or a telescope.)
The Treats of Boötes
The absent moon for the next two weeks and the lovely early summer nights offer a fine opportunity to explore the realm of Boötes (“Bow-Oh-tees”), the Herdsman or Plowman.
After it gets dark, face southwest and look about halfway up the sky for a very bright, orange-tinted star named Arcturus. The fourth brightest star in the entire night sky, Arcturus means “Guardian of the Bear” in Greek, because it rises after Ursa Major (the Big Bear), which sits to its upper right (west). Arcturus is 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 大角, “Great Horn”.
Arcturus marks the bottom tip of the large, kite-shaped constellation Böotes, traditionally represented as a Herdsman or Plowman. The rest of the stars in the kite are medium-bright and visible under party light-polluted skies. About a fist’s diameter above and slightly left (east) of Arcturus are two close-together stars. The higher one named Izar, meaning “Loin Cloth”, is moderately bright. The lower one is much fainter. In a telescope, Izar splits into a gorgeous double star — one is golden and the other is white or greenish.
Moving another fist’s width upwards along the same line brings us to the Herdsman’s eastern shoulder, a modest star designated Delta Boötis, or Thiba. (The word Boötis is latin for “belonging to Boötes.) Thiba is a sun-like star, about ten times the mass of our sun, sitting 117 light-years away. Looking up and to the right about 7.5°, we find a star named Nekkar “Ox Driver”, marking the constellation’s head (or tip of the kite). This is an elderly blue star passing through a phase that is causing it temporarily resemble a large version of our sun — on its way to becoming a brighter red giant.
Sitting about three finger widths towards the upper left from the line connecting Delta Bootes and Nekkar (shoulder and head) is a triple star named Alkalurops, a name derived from “Shepherd’s Staff”. Two of Alkalurops’ stars can be discerned with good eyes or binoculars, and one of these becomes two stars when viewed in a telescope. All three stars are orbiting in a dance that takes at least 125,000 years for a turn.
As we head from Nekkar back down the crooked western side of the kite, we first stop at a medium bright star named Seginus, which marks the western shoulder. Seginus is also evolving — presently a white giant star that is on the way towards becoming a red giant one day. This 85 light-years distant star is spinning about 70 times faster than our sun!
Moving less than halfway down the constellation towards Arcturus, we find a modest star sitting about four finger widths to the right of Izar. This star, designated Rho (ρ) Boötis, marks the gentleman’s western hip. There’s a noticeable small star just to its left named Sigma Boötis.
Descending from Arcturus to the east and west are some minor stars that mark the legs and feet. The eastern foot, less than a fist’s diameter to the lower left of Arcturus, is designated Zeta Boötis. In a telescope it is revealed to be a nice matched pair of close together white stars. Moving about four finger widths down the western leg brings us to the bright star Muphrid. It’s also classed like our sun, and is actually the same distance from us as Arcturus. But its inherent brightness is less, so it looks much dimmer. Dropping down slightly and moving farther right is the star Upsilon Boötis, a very distant red giant star.
All the stars in our galaxy are in motion, jostling as they orbit the galactic centre every quarter of a billion years or so. Some stars’ move faster, or are closer to us — so they exhibit a greater apparent motion across the sky. Astronomers call this Proper Motion. That’s why star charts need to be updated regularly. Arcturus 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!
Boötes extends way up to the tip of the Big Dipper’s handle. Just before you get there, look for a tight grouping of three stars that represent the herdsman’s upraised hand. The star names are Asellus Primus, Asellus Secundus, and Asellus Tertius “First, second, and Third Donkey”. Two of them are telescopic double stars, but the area around them is a lovely rich field for viewing. The famous Pinwheel Galaxy (Messier 101) is located only a few finger widths above them!
Astronomy Skylights for the week of June 23rd, 2019 by Chris Vaughan.
Keep looking up, and enjoy the sky when you do. I love questions and requests — so, send me some!