Moon and Phases
August offers two opportunities, on the 11th, and again on the 25th, to view the Straight Wall (Rupes Recta), which was discovered by Christiaan Huygens in the 1670’s. While this 100 km long fracture in the Moon’s crust might appear to be a dramatic cliff, it is in fact just a gentle 15-degree slope that is no higher than about 300 meters. Fractures such as the Great Wall, which Sir Patrick Moore once described as “Not a Wall, and not quite straight!” occur when part of a basin floor, the Mare Nubium floor in this case, subsides when the Mare is flooded with lava. Look for the Wall on the edge of the Mare Nubium, just to the westward of the heavily cratered southern highlands.
New Moon: 2nd
First Quarter: 10th
Full Moon: 18th
Last Quarter: 24th
Continuing to dominate the night sky in August is the famous Summer Triangle asterism, whose three stars are among the first to reveal themselves in the twilight sky before a myriad of other summer stars later join them. Brightest amongst this asterism is Vega (the plunging vulture) in the constellation Lyra, which is a bluish-white star of magnitude 0.03 and a mere 25 light years distant from Earth. Deneb (the tail of the swan) in Cygnus, on the other hand, is the group’s faintest member, with the blue supergiant located 1,500 light years away and with a magnitude of 1.25, while Altair (the flying eagle) in Aquila, a 0.77 magnitude white dwarf found 17 light years, completes this beautiful and unmistakable trio of stars.
The Summer Triangle also provides a useful means for locating and identifying many of the other nearby stars and constellations in the summer sky. This includes Delphinus (Dolphin) and Sagitta (Arrow), both of which somewhat resemble the creature/objects they depict. An imaginary line drawn from Deneb and Altair also leads to the constellation Sagittarius (Archer), which contains another distict asterism called ‘the Teapot’, with the The Milky Way representing the “steam” emenating from its spout. Due of its proximity in the direction of The Milky way, Sagittarius contains a multitude of notable deep sky objects, including the Omega Nebula, the Lagoon Nebula, and the Trifid Nebula, as well as seven globular clusters , and four open star clusters.
The Perseid Meteor Shower
Although the smallest planet reaches its furthest point from the Sun on the 16th of the month, it will spend the month low on the western horizon, making it difficult to spot. It will also lose some of its shine during the month, dimming from magnitude -0.2 to +1.1, even though its angular diameter will grow from 6 to 9.5 seconds of arc.
The start of August will see Venus very low on the western horizon about 40 minutes or so after sunset, but despite its magnitude of -3.8, it will be too close to Sun to be spectacular. One way to find Venus during the first days of the month is to look for it close to the star Regulus in Leo. On the 27th, Venus and Jupiter will be separated by less than 0.5 degree, but both planets will be very low on the horizon, and being close to the Sun, the pairing might be difficult to observe.
Mars will start August shining at magnitude -0.8 low on the south-western horizon as it moves from Libra into Scorpius, and then onto Ophiuchus towards the end of the month. Mars will also dim considerably during the month, with its magnitude dropping from -0.8 to 0.3 as its angular diameter shrinks from 13 to about 10.5 seconds of arc. However, despite its low elevation and loss of brightness, it might still be possible to see the Red planet with the aid of a small telescope, provided of course that the seeing is good.
Jupiter becomes visible low on the western horizon as the month starts, but gradually sinks into the Sun’s glare as the month wears on. During the month however, Jupiter will stay at magnitude -1.7, and form some interesting pairings and groupings with Mercury, the crescent Moon, and Venus. On the 1st of August, Jupiter will form a line with Mercury, Regulus (in Leo) and Venus low on the western horizon, but note that this happens within an hour of sunset, so do not use optical aid until the Sun has fully set.
Saturn is low on the south-western horizon during August, and although its ring system is still tilted towards us, its elevation that never rises above about 20 degrees might make it difficult to spot in the heat haze just after sunset. The planet’s magnitude will decrease from +0.3 to +0.5, while its angular diameter will shrink slightly from 17.5, to 16.7 seconds of arc. On the 23rd of the month, Saturn will be in a close grouping with Mars and the star Antares low on the south-southwestern horizon in Scorpius. Mars will be at magnitude -0.3, while Saturn will shine at magnitude +0.4, and provided seeing conditions are good, it might just be possible to spot Saturn’s largest moon, Titan, with a small telescope.