Sky Notes for August 2015 Geoff Mitchell
As we head into August the evening twilight noticeably fades earlier throughout the month. The August night sky shows the summer constellations prominently in the south east from early evening. The bright planet Venus is into our daytime skies and reaches inferior conjunction on August 15th. Similarly Jupiter also slips into daylight skies during the month reaching conjunction on August 26th.
Look to the NW from half an hour to two hours after sunset or similarly to the NE hours before sunrise i.e. when the Sun just below the horizon during the summer months when extremely high clouds at 80 km altitude known as Noctilucent Cloud (NLC) may be seen. NLC’s show a bluish colour and also show filamentary structure. August marks the end of the season for seeing NLC`s .
Our own Milky Way galaxy stretches from the constellation of Auriga [The Charioteer], marked by the bright star Capella and through the constellations of Perseus and Cassiopeia, Cygnus, Sagitta, Aquila, Scutum and down into Sagittarius. Perseus is now positioned low in the north east and from July 23rd to August 20th observers may also catch the Perseid meteor shower, with maxima occurring in the early hours of August 13th. This year is very favourable, with moonless conditions all night. If you have clear conditions on the evening of the 12th and a dark site, keep a look out from late evening for these bright meteors with persistent trails . Meteors originate from the dust debris trail of comets and these sand grain size meteoroids burn up in Earths upper atmosphere causing the streak of light we see as a meteor. In the case of the Perseid shower, these are seen to peak around August 12th each year as Earth passes through the meteoroid stream from Comet Swift Tuttle (1862 III). Each time the comet orbits the Sun, another stream is laid down, resulting in a complex set of streams, rather like the stands of a rope – Meteor activity can therefore vary from year to year. Due to perspective the meteors appear to radiate from a single point in the sky, the Radiant, in the case of the Perseids this is in the constellation Perseus hence the name – Perseid. If you plan to watch the Perseids then warm clothing is recommended and a sun lounger is also useful. Look at an elevation of 60° / overhead and look at 45° from the radiant (for example due South, as Perseus is rising in the NE early evening). Avoid looking at the radiant , as meteors can appear some distance from the radiant and so can be be missed. Observing with a group of people looking in different directions will increase the chances of someone seeing a meteor at any one time. Visually around 30 to 50 meteors / hour at maxima in the early hours of August 13th `may` be recorded , although the frequency of the meteors is not predictable , with longer periods of seeing nothing followed by a burst of a few being quite normal. Very bright meteors of magnitude -5m are possible and are called `Fireball’s or Bolide’s, and these can be bright enough to light up the ground / cast shadows.
In the south the summer constellation of Cygnus [The Swan] now seen as evening twilight fades. The Milky Way can be seen as a faint band of stars stretching low in the east down through constellations of Aquila [The Eagle], Scutum [The Shield] and towards Sagittarius [The Archer] and Scorpius [The Scorpion] in late evening skies, use binoculars to see the myriad of stars in these rich star clouds, best seen on clear dark moonless evenings from darker locations outside the town. Sagittarius is best seen in June / July, the constellation is known for the `Teapot` asterism of stars has rich star fields and some fine star clusters located above the spout of the Teapot asterism; however you do need a good southern horizon and finder chart to spot some of these. Pluto is located just above `the handle of the teapot asterism` and just think of NASA’s New Horizon space probe successful flyby Pluto in mid July , 4.5 light hours away !
Hercules is noted for the Globular cluster M13 containing some 750,000 stars, a nice view in a small telescope The bright star Vega in the constellation of Lyra [The Lyre] is seen low above the north east horizon and Altair in the constellation of Aquila low in the east. Vega, Altair and Deneb, in the constellation of Cygnus form the `Summer Triangle` asterism, a useful sign post for the summer skies.
Look low in the south early evening to find a yellowish star and the distinctive `T` shaped asterism of stars of the `head` of Scorpius , the yellow star is in fact the ringed planet Saturn which reached opposition on May 23rd. Although rather low as seen from the UK, its ring system is now wide open, a classic view of this gem of the solar system. Small telescopes will show the rings and the brightest moon Titan. Larger telescopes show up to six or so fainter moons and any white oval features on the planets disk. Look below the `T` head of Scorpius to see the `blood red` coloured star Antares. Antares name means `The rival of Mars`, a red super giant star, with a mass of some 20 solar masses. It has a diameter that, on the scale of our solar system, would be greater than the orbit of Mars. High in the south the bright orange star Arcturus in the constellation of Bootes [The Herdsman] is the brightest star in the northern hemisphere of the sky.Planets in August
Mercury returns low in the evening twilight from mid month but is poorly placed
Venus is lost to the day time skies, with inferior conjunction on August 15th before re appearing before dawn.
Mars is now positioned in the dawn skies but presents a small image
Jupiter heads towards the daytime skies by late August, with conjunction on August 26th.
Saturn now rises by early evening low in the south in Libra / Scorpius – a good time to see this ringed `gem`.
Uranus rises by late evening – best views in autumn skies.
Neptune now rises early evening – best views in autumn skies.
Moons phases in August
New Moon August 14th Moonless, best time for deep sky observing.
First Quarter August 22nd Best days to see shadow details in lunar craters (early evening)
Full Moon August 29th Best days to see bright ray craters like Copernicus / Tycho.
Last Quarter August 7th Moon visible in daytime skies. Do not look directly at the Sun.
Perseid range July 23rd to August 20th, Maxima August 13th (pre dawn) ZHR 80+ / hour, visually 30 to 50 per hour – Fast meteors with bright persistenttrails (see notes / www.lutonastrolink.org.uk for details
Delta Aquarids, range July 15th to August 20th, maxima August 6th, ZHR 10/ hr
The Highlights of the Month
Perseid Meteor Shower, very favourable in moonless skies maxima August 12th/ 13th (see notes)
Saturn very low in south , rings open at 24° DE [tilt] showing Saturn’s North Pole.
Crescent moon and Spica visible early evening August 19th low in the western twilight.
Noctilucent cloud – watch the NW skies from 30 minutes to 2 hours after sunset (see text)
Milky Way star fields, visible from dark sites on moonless evenings – look during a Perseid Meteor Watch Remember the binoculars if your on holiday in a dark site and trace out the star fields and dark rifts through Cygnus , Aquila ,Scutum and down into Sagittarius .
DSLR astrophotography – Sky photography on moonless evening – 28mm /50mm lens – manual focus to infinity. Set ISO / ASA 1600, exposure 30s, remote cable release , tripod mounted – Have a go at capturing some of the star fields through Cygnus around Deneb ( NGC 7000 N.American Nebula) / Scutum – you may also capture a Perseid meteor too !
More detailed sky notes and LAS Newsletters, Finder charts are available to LAS members via the Members` page on the LAS Website www.lutonastrolink.org.uk Artificial satellite and International Space Station visible passes and bright Iridium flares – check the home page for posting s of details of favourable observing times.