The Moon Steps on the Goat’s Tail, Saturn Shines Brightly, Brilliant Mars, and Some Random July Delights!
Earth at Aphelion
On Friday, July 6, Earth will reach aphelion, its maximum distance from the sun for this year. The aphelion distance of 152.1 million km is 1.67% farther from the sun than the average Earth-sun separation of 149.6 million km, which scientists call one Astronomical Unit (AU). Earth’s perihelion, our minimum distance from the sun, will occur next January 3. At that time, the sun will be 147.1 million km from Earth. As you can see, our seasonal temperatures are not produced by our distance from the sun — but by the amount of sunshine we receive in a day. And that is controlled by the amount that Earth’s axis points toward (or away from) the sun.
The Moon and Planets
The moon will start this week by vacating the evening sky as it swings towards Last Quarter on Friday morning. Tonight (Sunday) the moon will rise shortly before midnight local time as a waning gibbous orb (86% illuminated) sitting 1.4 fist diameters to the left of bright reddish Mars.
Meanwhile, low in the southeastern sky, you can watch the moon pass in front of (occult) a medium-bright star named Deneb Algedi “Tail of the Goat” in Capricornus (the Sea-Goat). For the GTA, the lit leading edge of the moon will cover the star at about 12:01 am EDT. Your timing will vary a little depending on your location, so grab your binoculars or small telescope and start watching a few minutes beforehand. The star will be closer to the bottom (south pole) of the moon. (Note that your telescope will probably invert and/or mirror image the view.) The star will re-appear suddenly from behind the dark, opposite side of the moon at about 12:51 am EDT. The star’s return will be instantaneous because the star is a pinpoint light source and the moon has no atmosphere to “smear” the light.
On the following nights, the moon will pass through the rest of the zodiac water constellations: Aquarius (the Water-Bearer) and Pisces (the Fishes). Friday morning’s Last Quarter will place the moon at a 90° angle from the sun, half illuminated (on the left-hand side) and lingering past noon in the daytime sky.
All week, elusive Mercury can be spotted sitting low over the northwestern horizon for a brief period after sunset. It is still about as bright as the brightest summertime star Vega, but binoculars will be needed to pull it out of the evening twilight. The best time to look is between 9:45 and 10:15 pm local time. You’ll need a low open horizon because the planet will be only a few finger widths above the horizon (or less). On Tuesday evening, Mercury’s orbital motion will carry it through the southern edge of the Beehive Cluster (also known as Messier 44) in Cancer (the Crab). Binoculars or a small telescope should pull some of the cluster’s brighter stars out of the darkening twilight sky. The following evening, Mercury’s position will be east (to the left) of the cluster.
Venus continues to blaze away in the western evening sky this week while it continues to swing away from the sun — and it will still get brighter! Tonight (Sunday) Venus will be sitting just to the lower right (west) of Leo (the Lion), but we’ll see it rapidly approach that constellation’s brightest star Regulus all week, and closely passing it next Monday. The planet will be setting at about 11:15 pm local time all week because it is travelling east while the entire sky is shifting west, holding it in place. Venus will continue to grow larger as it moves towards Earth. In a small telescope, the planet’s disk will not look round. Instead, it will exhibit a waning gibbous (67% illuminated) phase.
Jupiter is the object you will see shining very brightly in the southern sky after dusk this week. Around that time, it will be at its highest elevation (about three fist diameters) above the southern horizon. Over the following four hours, it will move west and descend — setting in the west-southwest about 2:30 am local time. Once the sky is dark enough, look for a bright star sitting just to the lower left of Jupiter. That’s Zubenelgenubi, the brightest star in Libra (the Scales). In binoculars or a small telescope, it splits into a closely separated pair of stars.
On Tuesday morning, July 3, starting at 12:58 am, the little, round, black shadow of Jupiter’s moon Ganymede will cross (or transit) near the north pole of Jupiter’s disk. The planet will set before the crossing has ended in the GTA, but observers in central and western North America will see the entire event. On Sunday, July 8, between 9:37 pm (in twilight) and 11:46 pm, the shadow of Jupiter’s moon Io will transit the centre of the planet. A reasonable backyard telescope will show the black shadows, but a very good telescope is needed to see the moons themselves. More shadow transits are available in other time zones around the world.
The Great Red Spot (or GRS, for short) takes about three hours to cross Jupiter’s disk. But the planet’s 10-hour rotation period (i.e., its day) means that the spot is only observable from Earth every 2–3 nights. If you’d like to see the GRS, use a medium-sized telescope (or larger). You’ll have your best luck on evenings with steady air — when the stars are not twinkling too much. The best times to try this week are: Monday, July 2 at 9:57 pm, Wednesday, July 4 at 11:36 pm, and Saturday, July 7 at 9:06 pm. All times are given in Eastern Daylight Time (EDT), so adjust for your local time zone. Try to look within an hour before or after the times I’ve given.
Last week, the Earth’s orbit carried us between Saturn and the sun, so the ringed planet is now visible all night long. And it’s close to its brightest and largest appearance for the year. Saturn will appear as a medium-bright yellowish object shining over the southeastern horizon as the sky darkens this week. The ringed planet is spending the summer of 2018 just east (to the left of) the Milky Way, and just above the Teapot-shaped stars that form Sagittarius (the Archer). Saturn will reach highest point over the southern horizon at about 12:45 am local time. It will remain visible until it sets in the southwest after 5 am local time.
Mars is looking very bright now! The Red Planet will be rising in the east before 11 pm local time this week. Mars will continually brighten and increase in apparent size (when viewed through a telescope) as the Earth’s faster orbit brings us closer to the red planet until July 31. A small telescope should already be able to reveal some variations in brightness on its disk. Mars will reach its highest position, over the southern horizon, around 3:30 am local time, and then remain visible until just after 5 am.
Blue-green coloured Uranus is visible in binoculars, if you know where to look. It is in the eastern pre-dawn sky, located about four finger widths to the left of the modestly bright star Torcular (Omega Piscium), which is above the “V” where the two cords of Pisces (the Fishes) meet. The major asteroid Juno is in the same region of sky.
Distant blue Neptune, among the modest stars of Aquarius (the Water-bearer), is observable in telescopes in the eastern sky after it rises around midnight local time. This week, look for the magnitude 7.9 planet sitting one finger width to the right of the naked eye star Phi (φ) Aquarii and about 4.5 finger widths to the left of brighter Hydor (Lambda Aquarii).
Some Random July Delights
Here are some things to look at while you are out under the stars in early July.
Shining less than 20° (or two fist diameters) over the southern horizon after dusk, the bright, reddish star Antares (or Alpha Scorpii) is located mid-way between the bright planets Jupiter and Saturn in early July. At visual magnitude 1.06, it is the 16th brightest star in the heavens, despite its 550 light-year distance from the sun. This cool, M2-class supergiant star’s name means “Rival of Mars” for its visual similarity to the red planet — although Mars is presently far brighter!
For contrast with Antares, look at the two hot, white stars, both named Al Niyat, that flank the red supergiant. Magnitude 3.1 Al Niyat I, located 2 finger widths west (to the upper right) of Antares, is an O9.5-class star with a surface temperature of 30,000 K. Al Niyat II, located 2.25 degrees to the southeast (lower left) of Antares, is a B0-class star with a surface temperature of 30,700 K. At 734 light-years from the sun, Al Niyat I is nearly twice as far away as Al Niyat II.
The constellation of Hercules (the Hero) is situated nearly overhead during evenings in early July, ideal for exploring its treasures through a minimum of distorting atmosphere. The constellation’s two most famous deep sky objects are the globular clusters Messier 13 and Messier 92. Globular clusters are spherical balls of densely packed old stars (hundreds of thousands of them!) that orbit just outside our galaxy’s central bulge. Messier 13, one of the easiest to see summer messier objects, is visible in binoculars as a dim fuzzy patch two-thirds of the way up the western side of Hercules’ keystone-shaped torso. A telescope will reveal some of its myriad of densely packed stars — appearing like salt spilled on a black cloth. A large-aperture telescope might reveal the spiral galaxy NGC 6207 sitting just 0.5 degrees north of the cluster. Messier 92 is a slightly smaller and dimmer globular cluster located between the hero’s knees. NGC6229 is a third, dimmer globular cluster located in northern Hercules.
The constellation of Lyra (the Harp) is positioned high overhead in the 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 degree to the east of the very bright star Vega (Alpha Lyrae), is a double star. Binoculars or a small telescope certainly will. Examining Epsilon at high magnification will show that each of the stars is itself a double — hence its nickname, “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 gray ring. Higher magnification works well on this planetary nebula — the fading corpse of a star with similar mass to our sun.
With the moon out of the evening sky, explore the countless knots and clumps of stars distributed along the Milky Way. Many of them are included in Charles Messier’s list, and the modern Caldwell List, of the sky’s best deep sky objects. Scanning with binoculars will reveal them, and then follow up with a small telescope at low magnification. Particularly good clusters include Messier 39 and Messier 29 in Cygnus (the Swan), Caldwell 16 in Lacerta (the Lizard), the Wild Duck cluster (aka Messier 11) and Messier 26 in Scutum (the Shield), and Messier 24, the Sagittarius Star Cloud.
Astronomy Skylights for this week (from June 24th, 2018) by Chris Vaughan. Keep looking up to enjoy the sky! I love getting questions so, if you have any, send me a note.