The Night Sky This Month: August 2018

Summer Triangle
Image Credit: derekshirlaw.co.uk

The three bright stars of the Summer Triangle (Deneb, Altair and Vega) continue to dominate the night sky in August, and appears in the eastern sky at nightfall, high overhead around midnight, before moving ever westward throughout the night. It also provides a useful springboard for leaping across the celestial heavens and exploring other constellations, which are explored further in the link provided.

A line from Vega to Deneb, for instance, points towards the constellation of Andromeda where the great galaxy M31 can be found, while directly north of Vega are located the two Bear constellations, Ursa Major and Ursa Minor, which are circumpolar and so circle the celestial north pole throughout the whole year. As the poet-scientist Erasmus Darwin (1731-1802), who was also the grandfather of Charles Darwin, wrote:

“Onward the kindred Bears, with footsteps rude, Dance round the pole, pursuing and pursued.”

Moon

Favorable illumination on the night of August 20th offers lunar observers an excellent opportunity to view the craters Tycho and Copernicus, two of the biggest and most prominent crates on the near side of the Moon. At only about 108 million years old, the crater Tycho is thought by most planetary scientists to have been formed when a remnant of the impactor that created the asteroid Baptistina slammed into the Moon.

Similarly, the 800-milion year old crater Copernicus was likely to have been formed during the same break-up event, which also may have caused the Chicxulub impact that wiped out the dinosaurs about 65 million years ago.

Phases

Last Quarter: 4th
New Moon: 11th
First Quarter: 18th
Full Moon: 26th

Planets

Mercury becomes visible after the 20th, having passed between the Sun and Earth, and reaches its greatest eastern elongation from the Sun on the 26th of the month. Look for the little planet about 18 degrees eastward of the Sun after this date, when it will rise at about 5 AM, and shine at about magnitude zero.

Venus is still visible low above the western horizon soon after sunset, but it will sink progressively lower as the month wears on. Even though its illumination will also reduce from about 57% to less than 29%, its angular diameter will increase from 20 to 29 seconds of arc, which means that Venus will attain a magnitude of -4.6. Look for the planet as it approaches the bright star Spica in Virgo; by month’s end, it will be less than one degree below Spica, but note that both will be less than about 10 degrees above western horizon during the last days of August.

Mars continues its retrograde motion through the constellation Capricornus, and it will reach a point of closest approach to Earth since 2003 on the night of July 30th/31st, when its magnitude will have reduced to -2.2. While Mars’ angular diameter reaches 24 seconds of arc during the first week of August (but reduces to 21 seconds of arc by month’s end), the planet is now enshrouded in an almost planet-wide dust storm, which might make it difficult to discern any surface detail. Moreover, as seen from the UK, Mars will reach an altitude of only about 14 degrees when it transits the meridian, further hindering observations.

Jupiter may be shining relatively brightly at magnitude -2.1 soon after sunset, but it is sinking progressively faster toward the southern part of the ecliptic as it moves westward through the constellation Libra. As seen from the UK, Jupiter will remain at or below about 15 degrees above the horizon, meaning that obtaining clear views of the King of the planets may be difficult during much of the month.

Saturn was in opposition on the 27th of July, and it will therefore be visible in the south for most of the month, albeit an elevation of only about 15 or so degrees. Its angular diameter will also reduce slightly from 18 seconds of arc to 17 seconds of arc, which will cause it to dim slightly from magnitude +0.2 to magnitude +0.4. Look for Saturn close to the topmost star in the Teapot asterism as it moves westwards to within a few degrees of both the Lagoon Nebula (M8), and the Trifid Nebula (M20).

Neptune reaches its point of closest approach to Earth during the first week of September, and will therefore be visible for all of August and September. Although its angular diameter will only be about 3.7 seconds of arc, it will culminate at an elevation of about 27 degrees, which should make it an easy binocular target in the constellation Aquarius. Look for Neptune to the left of the star Lambda Aquarii, but note that with an 8-inch telescope, it may just be possible to spot Neptune’s moon Triton, given dark skies and good seeing conditions.

Meteor Showers

The Perseid Meteor Shower is expected to peak on the 12th/13th, although the best views are expected during the pre-dawn hours of the 12th. Note that while the radiant lies between the constellations Perseus and Cassiopeia, most meteors in this shower appear to originate at a point about 50 or so degrees away from the actual radiant. Fortunately, the peak occurs only a few days after New Moon, and hence, there will be no bright moonlight to hinder views of the shower.

Deep-Sky Objects

Some prominent constellations at this time of year include Ursa Major, Lyra, and Cygnus, all of which are home to a great many spectacular objects that are easy to spot with binoculars and small telescopes, a few of which are listed below.

M81/M82, located in Ursa Major, are a pair of interacting galaxies that are easy to find with a small telescope. The close interaction between the two galaxies has set off a spurt of star formation in M82, hence its designation as a “starburst” galaxy. At low magnifications, both galaxies can be seen in the same field of view.

M56, situated about 33,000 light years away in the constellation Lyra, is an 8th magnitude globular star cluster with a diameter of about 60 light years. Although M56 is neither the biggest, nor the brightest globular cluster visible from the Northern hemisphere, it is nevertheless an easy target for small to medium telescopes; look for it at about the midpoint between the stars Albireo and Gamma Lyrae.

Albireo (Beta Cygni) is actually a pair of stars that marks out the head of the Swan, and while there are many double stars that show a marked color contrast, this pair of stars is without any doubt among the most beautiful double stars in the entire sky. Note that while some large binoculars can resolve the pair, the best views are had with telescopes, and the bigger the aperture, the more striking the color contrast becomes.

Related Articles