The Night Sky This Month: August 2017

Rupes Recta (the Straight Wall)
Image Credit: the Straight Wall by JAXA/SELENE

The Summer Triangle, whose brightest stars mark out the constellations of Cygnus, Aquila, and Lyra, continues to dominate the night sky in August, and in the northern hemisphere appears virtually overhead near midnight, with an upside-down version low in the sky also visible from the southern hemisphere. Near the southern horizon this month can be seen the beautiful constellation of Scorpius and its neighboring companion Sagittarius to its east, with the bright planet Saturn found between these two zodiac constellations, and seen up until the late evening.

In the meantime, to the north-east of the star Vega in Lyra can be found the night sky’s 5th largest constellation of Hercules, while in the northern sky are the five circumpolar constellations that are ever-present in the northern hemisphere throughout the year, namely Ursa Major, Ursa Minor, Cassiopeia, Cepheus, and Draco.

Finally, US residents along a 70 mile wide band running from Oregon to North and South Carolina can expect to see a total solar eclipse on August 21st, with those people located elsewhere able to see a partial eclipse as the celestial phenomenon moves from west to east.

The Moon

August offers lunar observers two opportunities to observe Rupes Recta (the Straight Wall), a major lunar feature that Sir Patrick Moore once described as “Neither is it a wall nor is it straight!” Look for this gentle scarp that poses as a wall 1 or 2 days after the first quarter moon, or one or two days before the third quarter moon. The best views can be had on the evening of August 30th, or on the evening of August 14th.

Full Moon: 7th
Last Quarter: 15th
New Moon: 21st
First Quarter: 29th

The Planets

– Mercury starts the month low above the western horizon, shining at magnitude +0.4, and given clear conditions, the planet will show an angular diameter of about 8 second of arc just after sunset. Binoculars might be needed to see Mercury, but be sure to wait for well after sunset before trying to spot the planet. On August 26th, Mercury will pass between the Sun and Earth and will appear as a tiny black dot as it completes its leisurely transit across the face of the Sun .

– Venus remains visible as a predawn object in the east during August, rising as it does about 3 hours before dawn. While the planet will dim slightly from magnitude -4 to -3.9 due to its angular diameter shrinking from 14.5 to 12.5 seconds of arc, its illumination will increase from 74% to 83%, which explains the planets’ fairly constant brightness through the month. Look for Venus shortly before sunrise on August 2nd, when it reaches its highest elevation close to the open star cluster M35 in the constellation Gemini.

– Mars is still close to the Sun, and will therefore remain invisible for all of August.

– Jupiter can still be seen low above the south-western horizon in the constellation Virgo, four months after opposition. As August starts, the King of the planets will set at about 11:30 BST and its angular diameter will reduce from 34 to 32 seconds of arc, while its brightness will dim from magnitude -1.9 to -1.7. Jupiter also starts the month lying about eight degrees to the westward of the star Spica, which reduces to about 4 degrees as the month wears on, to pass Spica on September 11th as it dips southward toward the ecliptic. During the next three years, Jupiter will remain close to the southern horizon, reaching an elevation of only 25 degrees in 2018, and only about 18 degrees or so during 2019 and 2020 before resuming its journey northwards. Nonetheless, given good seeing conditions and clear skies, even small telescopes will easily reveal the planets’ equatorial bands during the next three years.

– Saturn reached a point of opposition on July 11th, which means that it still culminates near its highest possible elevation due south soon after sunset. Starting the month at magnitude +0.3, it will dim slightly to magnitude 0.4, while its angular diameter remains fairly constant at about 17 seconds of arc. The planet also ceases its retrograde motion towards Scorpio, and resumes its eastward motion towards Sagittarius on August 25th. Although the ring system is tilted towards our line of sight by more than 26 degrees, and will reach maximum tilt in October this year, Saturn will be only about 17 degrees or so above the horizon when it is due south. At this low elevation it may become difficult to obtain clear views of the ring system, so using an atmospheric dispersion corrector is recommended.

Meteor Showers

August sees one meteor shower, the Perseids, which peaks on the mornings of August 12th and 13th. Given clear skies, northern observers can expect to see about 50 or so meteors per hour originating from a point between the constellations Perseus and Cassiopeia in the hours before dawn on August 12th. This year however, a waning gibbous moon will rise well before midnight, meaning that the best views of the shower can be had before moon rise, although many meteors may be washed out by the moon’s glare.

Deep Sky Objects

NGC869 and NGC884The constellations Cygnus, Ursa Major, Hercules and Virgo are still prominent in the northern hemisphere, and all of these constellations are home to many easy targets for both binoculars and small telescopes, which can be found by following their links. In addition, the constellation Perseus contains several open star clusters that might be worth observing at this time of year, including NGC 869/NGC 884.

NGC 869 is the western-most of the two clusters shown in this image, in which north is up. Collectively, the two clusters are also known as the Double Cluster; with the other cluster designated NGC 884. NGC 869 is thought to be about 13 billion years old, while NGC 884 is a little younger, with an estimated age of about 12.5 billion years. The two clusters are both members of the Perseus OB1 association and physically very close, being separated by only a few hundred light years. Look for this spectacular sight just before dawn at low power, when the pair of clusters is at their highest elevation above the north-eastern horizon. For observers in latitudes that take in southern England, the two clusters become visible at about 28 degrees above the north-eastern horizon at about 22:45 BST.

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