Science

This Week’s Sky at a Glance, July 31 – August 8 – Sky & Telescope

Comet NEOWISE was down to magnitude 5.0 or 5.5 by July 30th and fading by about a half-magnitude a day as expected. Finder chart through 0:00 August 5th UT, for the serious.

FRIDAY, JULY 31

■ This evening the waxing gibbous Moon is lined up with Jupiter and Saturn to its left, as shown below. The Moon is just over the Sagittarius Teapot. Covering the Moon with your finger will make the Teapot stars easier to see, assuming your evening sky isn’t too bright with summer haze.

On August 1st, the almost-full Moon shines with just-past-full Jupiter and Saturn. The Moon and planets appear full when they’re at opposition: opposite the Sun as seen from Earth. But planets move westward with respect to your landscape as the year revolves, while the Moon moves east from night to night. So, when the Moon and an outer planet appear near each other in the sky, one of them is on its way to its opposition and the other is past its opposition.

Another way to put this: Planets near a waxing Moon are always past opposition. Planets near a waning Moon are always on their way to opposition.

SATURDAY, AUGUST 1

■ The nearly full Moon this evening forms a triangle with Jupiter and Saturn, more or less as shown above. These sky scenes are always drawn exact for a viewer at latitude 40° north, longitude 90° west, near the middle of North America. They’re close enough for the rest of the continent, but don’t always expect a perfect match if you’re in Boston or Florida or California.

■ Today is Lammas Day or Lughnasadh, one of the four traditional “cross-quarter” days midway between the solstices and the equinoxes. Sort of. Over the centuries since this tradition took root in Europe, the calendar drifted with respect to Earth’s position in its orbit. So in 2020, the midpoint between the June solstice and the September equinox actually falls on August 5th: at 1:08 p.m. Eastern Daylight Time (17:08 UT).

That minute is the exact center-balance of astronomical summer: the very top of the circle of the year (as defined by the astronomical seasons, and for the Northern Hemisphere.)

SUNDAY, AUGUST 2

■ The bright Moon at dusk forms a gently curving line with Saturn and Jupiter to its upper right, as shown above.

Later in the evening the line shines higher and turns more level. It’s exactly level around 12:30 a.m. daylight saving time, depending on how far east or west you are in your time zone.

MONDAY, AUGUST 3

■ Full Moon (exact at 11:59 a.m. EDT). This evening the Moon rises about a half hour after sunset for most of North America. By nightfall it’s shining brightly low in the southeast, in dim Capricornus, to the lower left of Saturn and Jupiter. High above the Moon is Altair.

■ Mars is at perihelion, its closest to the Sun in its 1.88-year orbit. Its closer-than-average solar distance this summer contributes a little to Mars’s brightness.

TUESDAY, AUGUST 4

■ The Big Dipper hangs diagonally in the northwest after dark. From its midpoint, look to the right by about three fists at arm’s length to find Polaris (not very bright at 2nd magnitude) glimmering due north in the same place it always does.

Polaris is the handle-end of the Little Dipper. The only other parts of the Little Dipper that are even modestly bright are the two stars forming the outer end of its bowl: 2nd-magnitude Kochab and 3rd-magnitude Pherkad. On August evenings you’ll find them to Polaris’s upper left (by about a fist and a half). They’re called the Guardians of the Pole, since they ceaselessly circle around Polaris throughout the night and throughout the year.

WEDNESDAY, AUGUST 5

■ The exact midpoint of summer comes today at 1:08 p.m. Eastern Daylight Time (17:08 UT). This is the halfway point between this year’s June solstice and September equinox.

THURSDAY, AUGUST 6

■ Low in the northwest or north at the end of these long summer twilights, would you recognize noctilucent clouds if you saw them? They’re the most astronomical of all cloud types, what with their extreme altitude and their formation on meteoric dust particles. And they’re fairly rare — though becoming more common in recent years as the Earth’s atmosphere changes. See Bob King’s Nights of Noctilucent Clouds.

FRIDAY, AUGUST 7

■ Bright Vega passes closest to overhead around 10 or 11 p.m., depending on how far east or west you are in your time zone. How closely it misses your zenith depends on how far north or south you are. It passes right through your zenith if you’re at latitude 39° north (Washington DC, Cincinnati, Kansas City, Lake Tahoe). How closely can you judge this just by looking?

Deneb crosses its closest to the zenith almost exactly two hours after Vega. To see Deneb exactly straight up, you’d need to be at latitude 45° north: Portland, Minneapolis, Montreal, southern France, northern Italy.

SATURDAY, AUGUST 8

■ Mars and the waning gibbous Moon rise together around 11 p.m. or so daylight saving time, separated by only 2° or 3° for North America. By midnight they’re quite the spectacle shining low in the east. Dawn finds them very high in the south, now as little as 1° apart depending on where you are.

This Week’s Planet Roundup

Mercury glimmers low in the east-northeast during dawn. It’s bright, growing from magnitude –0.9 to –1.3 this week, but it gets a little lower each morning. Look for it about 30° lower left of brilliant Venus.

Venus (magnitude –4.5, moving from eastern Taurus into the top of Orion’s Club) rises in deep darkness more than 1½ hours before the very beginning of dawn. As dawn gets under way, Venus blazes brightly in the east. To its right or lower right is Orion. The brightest star high upper left of Venus is Capella.

In a telescope Venus is a very thick crescent, shrinking this week from 28 to 25 arcseconds tall while waxing from 42% to 47% sunlit — just short of dichotomy.

Mars rises in the east around 11 p.m. daylight saving time, bright (magnitude –1.2) yellow-orange in Pisces like a far-off bonfire. Where will it come up? Watch the horizon below the Great Square of Pegasus. By dawn Mars shines grandly high and bright in the south, a high-blown firespark.

In a telescope this week Mars grows from 14½ to 15½ arcseconds in apparent diameter, as big as it appears at some oppositions! But we’re still speeding toward it along Earth’s faster orbit around the Sun, and we have a long way to go. Around this year’s opposition in early October, Mars will be 22.6 arcseconds wide.

Mars is still very gibbous, 87% sunlit. Look for its white South Polar cap and for subtler dark surface markings. To get a map of the side of Mars facing you at the date and time you’ll observe, use our Mars Profiler. The map there is rectangular; remember to mentally wrap it onto the side of a globe. (Features near the map’s edges become very foreshortened.)

An extraordinarily high-res image of Mars, taken July 17th by Enrico Enzmann and Damian Peach when Mars was 12.9 arcseconds in diameter. South is up. Peach says, “Mars this morning is looking very hazy with airborne dust.” They used a 76cm (30-inch) Ritchey-Chrétien telescope with a Canon ME20 camera to acquire video frames for stacking.

The darkest horizontal streak near center is Sinus Sabaeus, ending on its right (“following”) end with two-pronged Sinus Meridiani. Upper right of that is Margaritifer Sinus.

Jupiter and Saturn (magnitudes –2.7 and +0.2, respectively) are a little past opposition. They loom in the southeast in twilight and pass highest in the south around midnight daylight-saving time. Jupiter is brightest; Saturn is 8° to its lower left or left.

Farther to Jupiter’s right is the Sagittarius Teapot. High to the planets’ upper left, the brightest star is Altair.

Keep up with the telescopic interplay of Jupiter with its moons and their shadows, and find all the transit times of Jupiter’s Great Red Spot, in the Celestial Calendar section of the current month’s Sky & Telescope.

Jupiter on July 26, 2020 (non-Great-Red-Spot side)Jupiter’s non-Red-Spot side, imaged on July 26th by Christopher Go in the Philippines. South is up. Note the chaos in and around the dark North Equatorial Belt, especially on the left (celestial west; preceding) side. The smoother South Equatorial Belt is divided by a bright cloud line all along its middle. The central-meridian longitude (System II) at the time of the picture was was 221°. Writes Go, “I was fortunate to get enough data for Juno’s target area for PJ28,” the next close swingby (perijove) of NASA’s Jupiter orbiter Juno.Saturn on July 9th as imaged by Christopher Go. South is up. The ring tilt has lessened enough that a bit of the globe peeks up from behind the rings’ south edge. On the globe, a small white storm at latitude 63° north, and a slight dent in the edge of the North Equatorial Belt almost dead center, have been brought out by boosting the image’s brightness contrast and color contrast, and by edge enhancement. This processing has added artifacts around sharp edges in the rings.

Uranus (magnitude 5.8, in Aries) is high in the east by 2 a.m. daylight-saving time, some 20° to the celestial east of Mars.

Neptune (magnitude 7.8, in Aquarius) is high in the south-southeast at that time, about 30° west of Mars. Finder charts for Uranus and Neptune.

All descriptions that relate to your horizon — including the words up, down, right, and left — are written for the world’s mid-northern latitudes. Descriptions that also depend on longitude (mainly Moon positions) are for North America.

Eastern Daylight Time, EDT, is Universal Time (also known as UT, UTC, GMT, or Z time) minus 4 hours.

Want to become a better astronomer? Learn your way around the constellations. They’re the key to locating everything fainter and deeper to hunt with binoculars or a telescope.

This is an outdoor nature hobby. For an easy-to-use constellation guide covering the whole evening sky, use the big monthly map in the center of each issue of Sky & Telescope, the essential magazine of astronomy.

Jumbo Pocket Sky Atlas coverThe Pocket Sky Atlas plots 30,796 stars to magnitude 7.6, and hundreds of telescopic galaxies, star clusters, and nebulae among them. Shown here is the Jumbo Edition for easier reading outdoors at night. Sample chart.

Once you get a telescope, to put it to good use you’ll need a detailed, large-scale sky atlas (set of charts). The basic standard is the Pocket Sky Atlas (in either the original or Jumbo Edition), which shows stars to magnitude 7.6.

Next up is the larger and deeper Sky Atlas 2000.0, plotting stars to magnitude 8.5; nearly three times as many. The next up, once you know your way around, are the even larger Interstellarum atlas (stars to magnitude 9.5) or Uranometria 2000.0 (stars to magnitude 9.75). And read how to use sky charts with a telescope.

You’ll also want a good deep-sky guidebook, such as Sky Atlas 2000.0 Companion by Strong and Sinnott, or the bigger (and illustrated) Night Sky Observer’s Guide by Kepple and Sanner.

Can a computerized telescope replace charts? Not for beginners, I don’t think, and not on mounts and tripods that are less than top-quality mechanically, meaning heavy and expensive. And as Terence Dickinson and Alan Dyer say in their Backyard Astronomer’s Guide, “A full appreciation of the universe cannot come without developing the skills to find things in the sky and understanding how the sky works. This knowledge comes only by spending time under the stars with star maps in hand.”

Audio sky tour. Out under the evening sky with your
earbuds in place, listen to Kelly Beatty’s monthly
podcast tour of the heavens above. It’s free.

“The dangers of not thinking clearly are much greater now than ever before. It’s not that there’s something new in our way of thinking, it’s that credulous and confused thinking can be much more lethal in ways it was never before.”
            — Carl Sagan, 1996

“Facts are stubborn things.”
            — John Adams, 1770

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