(Above: A double shadow transit caused by Io and Europa will occur on Jupiter on Thursday, August 23, as shown here at 10:40 pm EDT. Only observers west of the Eastern time zone will be able to see both shadows.)

A Binocular Comet

Comet 21P/Giacobini-Zinner has been gradually brightening for some time because it is approaching Earth’s orbit. This week, you should be able to see the faint fuzzy greenish object in binoculars or a small telescope, if you can escape city lights — and especially after midnight when the bright moon has set. The comet is in the north-northeastern sky and heading downwards every night on a track that lies about a fist’s diameter (10°) to the left of the bright star Mirfak in Perseus (the Hero).

(Above: The path of Comet 21P/Giacobini-Zinner in the northeastern sky this week, shown here at 11 pm local time.)

The Moon and Planets

This week ends with next Sunday’s August full moon, known as the “Sturgeon Moon”, “Red Moon”, “Green Corn Moon”, and “Grain Moon”. This one always shines in or near the stars of Aquarius (the Water-Bearer) or Capricornus (the Sea-Goat), but you’ll be hard-pressed to see the dim stars in those modest constellations due to so much bright moonlight.

Full moons occur when the moon is at opposition, with Earth positioned between our natural satellite and the sun. Because of this, full moons always rise around sunset and set around sunrise. They are illuminated by vertically arriving sunlight, so nothing on the moon can cast a shadow when viewed from Earth. Moon phases can occur at any time of the day or night. This one occurs at 7:56 am Eastern Daylight time on Sunday morning, so the moon will appear a hair less than full on Saturday evening, and a hair less than full on Sunday evening, too. Binoculars or a telescope will reveal a narrow strip of shadowed terrain along the moon’s left (its western) edge on Saturday. That strip will shift over to the moon’s right (its eastern) edge on Sunday evening. To see the moon completely free of shadows, i.e., precisely full, you will need to look at it at the time I mentioned above.

Meanwhile, the moon will make a pretty sight in our evening sky all week long as it waxes fuller and slides eastward along its orbit. Tonight, it will be perched above the distinctive constellation of Scorpius (the Scorpion), about a fist’s diameter above that deadly creature’s heart, the bright reddish star Antares “Rival of Mars”.

In the southern sky after dusk on Monday, the waxing gibbous moon will sit 4 finger widths to the upper right of bright, yellowish Saturn. The two objects will easily fit within the field of binoculars. Over the course of the evening, the moon’s separation from the ringed planet will have noticeably decreased due to the moon’s eastward motion.

(Above: On Tuesday evening, August 21, as shown here at 11 pm local time, the moon will sit above the easy-to-see Teapot-shaped asterism of Sagittarius. Once the moon leaves the sky next week, look for the Milky Way’s “steam” rising from the spout. Night Sky Chart made via Star Walk 2 iOS and Star Chart for Android.)

On Tuesday evening, the moon will shift to sit above the Teapot-shaped asterism of stars that form Sagittarius (the Archer). This informal star pattern features a flat bottom formed by the stars Ascella “Armpit” on the east and Kaus Australis “Southern Bow” on the west, a triangular pointed spout pointing west, marked by the star Alnasl “Arrowhead”, and a pointed lid marked by the star Kaus Borealis “Northern Bow”. The stars Nunki and Tau Sagittarii form its handle. The asterism reaches maximum height above the southern horizon around 10 pm local time, when it will look as if it’s serving its hot beverage –the Milky Way appearing to be the steam rising as the teapot pours its celestial brew. (To see the Milky Way’s “steam”, look next week when the moon has moved away.) I’ll post a sky chart here.

On Wednesday and Thursday evening respectively, the moon will hop from bright reddish Mars’ upper right to its upper left.

(Above: The early evening sky, shown here Sunday evening at 9 pm local time, features all the naked-eye planets — Venus, Jupiter, Saturn, and Mars. Night Sky Chart made via Star Walk 2 iOS and Star Chart for Android.)

Extremely bright Venus is descending the western early evening sky a little by little each day as its orbit begins to carry it back towards the sun. Tonight it will set at about 9:45 pm local time and a week from now that will move up to 8:30 pm. Meanwhile, the bright planet will appear to be approaching the bright star Spica in Virgo (the Maiden). The effect is caused by Earth’s motion carrying the entire sky westward faster than Venus is moving. They’ll “kiss” next week! In a small telescope, Venus’ disk will resemble a first quarter moon, half-lit on the sunward side (although your telescope might flip the view). The planet will also be growing larger in apparent diameter because it is travelling towards the Earth right now.

We only have a few more good weeks to enjoy Jupiter this year. This week, the very bright planet will appear in the southwestern sky soon after dusk, and then set in the west-southwest at about 11:15 pm local time. Tonight, Jupiter, which has been slowly shifting eastwards, will pass close above nearby bright star Zubenelgenubi, the brightest star in Libra (the Scales). From here out it will draw farther away every night. In binoculars, you’ll plainly see that Zubenelgenubi is a pair of stars. While you have the binoculars handy, see if you can see Jupiter’s four Galilean moons (Io, Europa, Callisto, and Ganymede) flanking the planet.

From time to time, the small round black shadows cast by Jupiter’s four Galilean moons become visible in backyard telescopes as they cross (or transit) the planet’s disk. On Thursday, August 23, Io’s shadow will begin to transit at 10:02 pm EDT. Europa’s shadow will join Io’s at 10:58, just as Jupiter is setting in the Eastern time zone, but observers in the west can watch the event. 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, including some double shadow ones.

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. Try to look within an hour before or after the following times: Sunday, August 19 at 9:46 pm and Friday, August 24 at 8:56 pm. All times are given in Eastern Daylight Time (EDT), so adjust for your local time zone.

Around 8:40 pm local time, when the first bright stars appear overhead, medium-bright Saturn will appear not too high up the darkening southern sky. The yellow-tinted planet will reach its highest elevation of about 2 fist diameters above the southern horizon at around 9:30 pm, and then descend to set in the west at about 2 am local time. This summer, the ringed planet has been 4 finger widths to the upper right of the “lid” star of the Teapot in Sagittarius (the Archer). As the sky darkens, even a small telescope should be able to show you some of Saturn’s larger moons, especially Titan. Using a clock’s dial analogy, Titan will move counter-clockwise this week from a position at 7 o’clock (to the lower left of it) to 2 o’clock (left from the planet). (Remember that your telescope might flip and/or invert the view. Use the moon to find out how your telescope changes things.)

Mars will still be very bright this week. Visually, it will appear pink or orangey due to the global dust storm it has experienced recently. Mars will rise over the southeastern horizon at around 7:30 pm local time (give or take, depending on your latitude) and then climb higher until 11:30 pm local time, when it will reach an elevation of about 20° (or two outstretched fist diameters) above the southern horizon. (That will be the best hour to view the planet in a telescope because it will then be shining through the least amount of Earth’s distorting atmosphere.) Note that 20° is lower than many trees and buildings, so a clear southern vista is essential.

(Above: The ice giant planets Uranus and Neptune rise late and remain in view all night to the east of Mars, as shown here at midnight this week. Night Sky Chart made via Star Walk 2 iOS and Star Chart for Android.)

At visual magnitude 5.8, blue-green coloured Uranus is visible from late evening until dawn. You can see it without optical aid under very dark skies, or in binoculars and telescopes under moderately light-polluted skies. The ice giant planet is located in the eastern sky, about 4.5 finger widths to the left of the modestly bright star Torcular (Omega Piscium), which is above the “V” where the two starry cords of Pisces (the Fishes) meet.

Using a decent quality telescope you can also see the distant and very blue planet Neptune among the dim stars of Aquarius (the Water-bearer). It will rise in the east shortly before 9 pm local time. Look for the magnitude 7.8 planet sitting 1.75 finger widths to the right of the modestly bright star Phi (φ) Aquarii and 4 finger widths to the left of the brighter star Hydor (Lambda Aquarii).

Mercury will be observable in the eastern pre-dawn sky this week. Look for it low above the east-northeastern horizon at around 5:45 am local time. Next Sunday, Mercury will reach an angle of 18 degrees west of the Sun, its widest separation for this appearance. That means it will rise well before the sun, in a somewhat darker sky. You’ll be able to see it between about 5:15 and 6 am local time.

(Above: On Sunday, August 26, Mercury will reach its largest angle from the sun, and maximum visibility for this morning appearance, as shown here at 5:45 am local time. Night Sky Chart made via Star Walk 2 iOS and Star Chart for Android.)

Bright Stars Roundup

The first stars to appear in late August evenings are the bright, white stars of the Summer Triangle asterism — Vega, Deneb, and Altair. At dusk, they are high in the eastern sky and pass the zenith at about 11 pm local time. This annual feature of the summer sky will remain visible until the end of December! At magnitude 0.03, Vega is the brightest star in the summer sky, mainly due to its relative proximity to the sun of only 25 light-years. Altair is only 17 light-years from the sun, but Deneb is a staggering 2,600 light-years away; so bright because of its far greater inherent luminosity.

Stars shine with a colouration that is produced by their surface temperatures, and this is captured in their spectral classification. Our sun is a yellowish G-class star with a surface temperature of 5,800 K. The three bright stars of the Summer Triangle are A-class stars that appear blue-white to the eye and have high surface temperatures in the range of 7,500 to 10,000 K. Look in the western sky for orange Arcturus, a K-class giant star with a temperature of only 4,300 K. Sitting low in the southwest, reddish Antares, the heart of Scorpius, is an old M-class star with a surface temperature of 3,500 K. By comparing these stars colours’ to other stars, you can estimate those stars’ temperatures.

Astronomy Skylights for this week (from August 19th, 2018) by Chris Vaughan.

Keep looking up to enjoy the sky! I love getting questions so, if you have any, send me a note.