Skylights: August 2021

August 2021  :  Jim Hendrickson

Welcome to August. Even though the nights have been getting longer since the solstice in June, this month the shifting onset of night really becomes apparent, with the last 8pm sunset occurring on August 4, and nights will be 74 minutes longer at the end of the month than at the beginning. The last sunset in the 8pm hour occurs on August 4, and we won’t return to sunsets this late again until May 17, 2022. With darkness coming earlier, and nights getting a bit cooler,and hopefully a bit less humid, our skywatching conditions should be improving.

August is perhaps best known for the annual Perseid meteor shower, which peaks on the night of the 11th-12th. With the radiant being circumpolar, and minimal interference from the light of a crescent Moon, conditions are optimal for an all-night meteor display. The Perseids are also known to have a wide stream, so even a week or two before and after peak, you’re likely to see a few Perseids.

Our tour around the solar system this month starts with Mars. It has put on a good show for us over the past year and a half, and August will be the last month to view the red planet in the evening sky as it’s on its way to conjunction with the Sun on October 8. At a distance of 2.6 AU, Mars’s tiny 3.6 arcsecond disk is no larger than Uranus, and doesn’t offer much in the way of surface detail. While Mars lacks the brilliance and splendor that it did a year ago, it is worth watching its motion as it joins with two other objects in our solar system.

First, watch for the 2.3% illuminated waxing crescent Moon near Mars on August 9. The Moon will be just 3.3° north of Mars, appearing at approximately the 2 o’clock position.

Then, on the 18th, Mercury is just 10 arcminutes (1/6°) to the west of Mars, appearing below and to the right. Mercury is nearly four times brighter than Mars, and both planets show only tiny gibbous phases, 5.2 and 3.6 arcseconds respectively.

Although Mercury attains a significant eastern elongation from the Sun (23° by month’s end), this apparition is not very favorable, owing to the shallow angle of the ecliptic in the western sky after sunset during this time of year. On the 31st, it sets just 50 minutes after the Sun. This presents us with some challenging, yet rewarding, opportunities to spot it near some other celestial objects this month. The very young (0.4d, 0.3% illuminated) waxing crescent Moon is 3.3° to its right on the 8th. On the 11th, it is just over 1° to the 1 o’clock position from Regulus, and, as mentioned above, is its meeting with Mars on the 18th.

While Mercury and Mars leave the sky before twilight ends, Venus shines a bit longer, though its visibility is also limited by the shallow angle of the ecliptic. 

On the 10th, the 2.4 day waxing crescent Moon pairs with Venus in what should be one of the month’s more memorable sights. After this, Venus continues to progress eastward through Virgo. Its motion becomes apparent during the second half of August as it approaches the first magnitude star Spica, which it passes early next month. 

Through a telescope, Venus grows slowly from 12.7 arcseconds to 15.1 arcseconds, and its gibbous phase narrows slightly from 82% to 73%, as its distance closes from 1.31 AU to 1.11 AU during the month of August.

The best planets for viewing this month are Saturn and Jupiter, which both reach opposition in August, Saturn on the 2nd, and Jupiter on the 19th. This means that you won’t need to wait long after sunset to observe them, and they will be visible all night, as they are opposite the Sun in the sky. 

Although opposition is typically the best time to observe the outer planets, because it is at this point that they are at their closest - and therefore their largest and brightest - Saturn generally appears best before or after opposition due to its rings and the way the shadows are offset. At opposition, the shadow of the rings on the planet and the planet on the rings are in close alignment, minimizing their visibility. However, when looking at Saturn about a month or so before or after opposition, the shadow angles are offset, and give Saturn a more dramatic, three-dimensional appearance. This does not mean, however, that you should wait to observe Saturn, as it is always a joy to observe whenever it is visible.

Back to the Moon: the 6 day waxing crescent passes near the wide double star Zubenelgenubi (Alpha Librae) on the 14th. The best view will be just before Moonset, when they will be separated by less than 1/2°.

The Full Sturgeon Moon occurs on the 21st, with moonrise at 19:41. On this night, the Moon will be just 4.5° directly below Jupiter, and both objects will transit at about the same time, 40 minutes past midnight.

On the 18th-19th, watch the waxing gibbous Moon graze Nunki (Omicron Sagittarii). For some observers just south of us, the Moon will occult the star, but for us it will be just 1.5 arcminutes from the northern limb of the Moon at 18 minutes past midnight.

Uranus lies within the constellation of Aries, and rises around 11:30pm at the beginning of the month, and just before 10pm by the end of the month. It is located within a triangle of 5th and 6th magnitude stars Pi, Omicron, and Sigma Arietis. These stars, similar in brightness to Uranus, should aid in locating it in binoculars.

If you have never seen Uranus, or think that it is difficult to find, there is an opportunity on the 27th-28th when the waning gibbous Moon passes as close as 1.6 degrees to its south. Wait until after midnight, and aim your binoculars at the Moon. Place the Moon towards the right of the binocular field of view and you will find two 6th magnitude stars about three Moon diameters apart from each other, and oriented in a roughly 2 and 8 o’clock angle. Bisect this line, and move northward about 1/2° (one Moon diameter) and you should find a similarly bright object. 

Our planetary tour now brings us to the Circlet asterism in Pisces. It is here that we’re at the hopping-off point to find two solar system objects. The first object we find is the third largest object in the asteroid belt, 2 Pallas. Pallas is within its retrograde loop, and due to its highly inclined orbit, it is moving on a distinctly southwesterly trajectory. During August, it is tracking along a line that is closely parallel to the line connecting Theta and Gamma Piscium. Shining at 9th magnitude, you’ll need binoculars or a small telescope to locate it, at 2.45 to 2.18 AU from Earth.

Finally, we journey to our solar system’s outermost planet, Neptune. Just a bit brighter than Pallas, Neptune’s motion amongst the background stars is much slower, given its 29.06 AU distance. For the entire month, it can be found within a quadrilateral of 6th and 7th magnitude stars that is located 7° south of the Circlet. Start at 5th magnitude Kappa Piscium, a nice wide double, and move south-southeast just over 2° to find an asterism of 4 6th and 7th magnitude stars. Two of the stars at the western side of this asterism point due south. Follow this line 2.7° until you encounter another star of similar brightness that lies almost exactly in the line. Here is the westernmost point of the 1.5° long diamond-shaped asterism we’ll use to find Neptune. The line bisecting the long axis of the diamond is roughly along the path Neptune moves along. Note that the eastern two stars are somewhat dimmer than the western ones, and Neptune will be slightly dimmer still. As the month progresses, Neptune moves from near the easternmost star, to just past the line connecting the short axis of the diamond asterism. After observing Neptune two or three times in August, you’ll become familiar with this star pattern, from which Neptune won’t venture too far from for the remainder of the year.