Download The Night Sky This Month - Usk Astronomical Society

The Night Sky
(September 2016)
BST (Universal Time plus one hour) is used this month.
©
23:00 hours BST early in the month
22:00 hours BST in the middle of the month
21:00 hours BST at the end of the month
September
The General Weather Pattern
September may very well be a drier month than the previous two and it can be a quiet, calm month. However at
night it can be quite cool and humid. Any winds carried from the west by depressions usually bring to Wales a
mixture of wet and windy weather with cool temperatures. During clear, still nights mist and dew usually present
a problem.
From Earth
Throughout this month, the ecliptic is at a very shallow angle of about 15º at sunset. The minimum angle will be
around the Autumn Equinox which is on the 22nd this year. In this configuration, the angle of separation of a
planet from the Sun translates into lower altitude at sunset, and planets in close line-of-sight and to the east of the
Sun will be harder to see. The opposite is true of the dawn, for instance at the end of the month, Mercury rises at
an angle of approximately 45° about 100 minutes before the Sun and reaches about 12° above the horizon before it
fades into the twilight.
The nights are getting longer, and thank goodness, the short, hot, bright nights of summer are nearly over! This is
a comfortable time of year to observe in the evening, but unfortunately the skies in autumn are not the most
exciting for beginners. This might be a nice time to take a look at the Andromeda Galaxy, and Cygnus is overhead
with the black hole, Cygnus X1, with which to impress your friends. There are signs of the glories to come late
evening and early morning as the winter sky looms over the eastern horizon.
The Milky Way
Recent research suggests that 60% of Europeans and 33% of mankind no longer see the Milky Way due to
worsening light pollution. Throughout September, in the hours of darkness, the Milky Way lies within 20° of the
zenith. On two occasions each night, around 23:00 and 05:00, the plane of the Milky Way can be found at the
zenith. Consequently, this is a wonderful time of the year to observe our Galaxy. For instance, dark regions of
Conditions apply as to the use of this matter. D J Thomas
dust and gas which obscures light from more distant stars can be seen along the galactic plane from dark-sky
locations.
There are four types of nebulae recognised today and found within our Galaxy, consequently most can be observed
along the band of the Milky Way. For the same reason, star clouds such as open clusters are found in this region.
Sun
Mid-month on the 16th the Sun moves from Leo into Virgo. If you have any news of sunspot activity, other
members would be interested, so let us know. Don’t forget to ask experienced members for help if you want to
observe the Sun. On the 22nd the Sun passes into the southern celestial hemisphere; the centre of the solar disc
crosses the celestial equator at the equinox (14:21UT). This position is known as the first point of Libra.
The annular solar eclipse of September 1st is not visible from the UK.
Moon
The New Moon is on 1st at about 09:00 in the constellation of Leo.
The First Quarter is on 9th at about 11:50 in the constellation of Sagittarius.
The Full Moon on 16th at about 19:05 in the constellation of Pisces is commonly called the Harvest Moon*.
The Last Quarter is on 23rd at about 09:55 in the constellation of Taurus.
The Moon is at perigee (nearest Earth) on the 22nd and at apogee (most distant from Earth) on the 10th.
* The names of moons were, at one time, associated with the names of the lunisolar months. The Romans, with
the Julian calendar, helped to decouple that tradition. It is only in recent times that naming full moons has taken
hold in urban myth. The Harvest Moon is supposed to be the full moon nearest the autumnal equinox, with the
fallacious myth that it is used to illuminate the night when the harvest is gathered in the dark.
A penumbral lunar eclipse will be taking place when the Moon rises on the evening of the 16 th. This type of
eclipse occurs when the Moon enters the penumbra of the Earth’s shadow, but not the umbra. A photo-opportunity
presents itself to astrophotographers, for the shadow at this time will be difficult to observe with the naked eye.
The Planets
Mercury is at inferior conjunction on the 12th and sets with the Sun early in the month. By the end it will be
rising about 1hr 40mins before the Sun; due east, and be at greatest western elongation on the 28 th. Between the
middle of September and the middle of October is a good time, this year, for observing Mercury.
Venus is in the west in the evening twilight throughout the month and is unfavourable.
Mars is just west of south at dusk throughout this month and is poorly placed for observation. In the first days
of the month, however it makes a near-equilateral triangle with Saturn and Antares, but low down.
Jupiter sets in the evening twilight at the start of the month and is lost to us as it moves to conjunction on the
26th.
Saturn is in Ophiuchus, low in the west as the Sun sets and as it moves towards conjunction at the end of
November; it is way past its best. In the beginning of September Saturn, Mars and Antares form a triangle low in
the south-south-west. Observe Saturn as soon as you can this month.
Uranus rises around 19:20 at the end of the month and is becoming better placed to observe as it moves
towards opposition in October. At a magnitude of 5.70 it may well be seen with binoculars. A small telescope
might show a blue hue, but since Uranus usually has few features, little else may be seen with a larger amateur
instrument. Late in the evenings in the last few days of September, it can be found in the constellation of Pisces at
RA 1h 26m 34s, Declination 8º 23' 47"; in the south-east.
Neptune rises an hour or so before Uranus and is slightly better placed for observers. The near full Moon
occults Neptune on the evening of the 15th, revealing the planet again at 20:55 in the south-eastern twilight. On the
2nd Neptune is at opposition, so at the beginning of the month at around 01:00, it can be found culminating in the
constellation of Aquarius at RA 22h 49m 53s, Declination -8º 23' 15". It has a magnitude of 7.82.
Meteors
September is a poor month for meteor showers, but sporadic meteors are more evident at this time of year.
Sporadics are loners, not associated with any one part of the sky, entering our atmosphere from all directions and
therefore they require much more patience to observe than do showers which emanate from one part of the sky.
Piscid meteor showers have weak meteors with multiple-radiants which are at their maximum on the 8th and 20th
Sept and 13th October this year.
Constellation Culminations from Usk
A celestial body or region of the sky is said to culminate when it crosses an observer’s meridian (an imaginary line
drawn overhead and through both poles). All other things being equal objects are usually best observed in this
position as the light from them travels through the least amount of atmosphere.
Constellation
Convenient Culminations Midnight Culminations
Observability
Aquila
21:00 Late September
Early August
Whole constellation
Conditions apply as to the use of this matter. D J Thomas
Sagitta
Cygnus
Delphinus
Vulpecula
Equuleus
Capricornus
Microscopium
Aquarius
Cepheus
Lacerta
Piscis Austrinus
Pegasus
21:00 Late September
20:00 Mid-October
20:00 Mid-October
20:00 Mid-October
20:00 Late October
20:00 Late October
20:00 Late October
19:00 Mid-November
19:00 Mid-November
19:00 Mid-November
19:00 Mid-November
19:00 Late November
Early August
Mid-August
Mid-August
Mid-August
Late August
Late August
Late August
Mid-September
Mid-September
Mid-September
Mid-September
Late September
Whole constellation
Whole constellation at zenith
Whole constellation
Whole constellation
Whole constellation
Whole but poor; low in the murk
Unfavourable and partially hidden
Whole constellation
Whole - upper culmination - north
Whole - upper culmination - zenith
Whole - v poor; v low in the murk
Whole constellation - high
Cygnus (Pronounced sig’ nus)
In Welsh
Yr Alarch nm. literally the Swan.
From the Latin for ‘the Swan’, the constellation Cygnus lies
immediately above our heads in mid-September, at about 22:00
BST. Locating it is easy at this time, the asterism known as the
Northern Cross lies at its core.
The cross stretches from
Deneb, the tail star, through the body and neck to Albireo at the
head. The arms of the cross attach to Sadr, the body of the
Deneb
swan.
Deneb, the dimmest of the three stars that comprise the 'summer
triangle', is in fact one of the most luminous of visible stars.
With an absolute magnitude of -8.38 it is approximately
200,000 times brighter than the Sun. But, at an estimated
distance of 2,600 light-years, it has a visible magnitude of 1.25.
Cygnus X-1
Next to Eta Cygni, in the neck, lies a blue supergiant, HDE
226868, at a distance of about 6,100 light years. It was
discovered in 1964 that this region is also a source of powerful
X-rays. Not only that, but HDE 226868 also wobbles under the
influence of a companion with a mass some 9 to 10 times that of
the Sun. There is considerable evidence that Cygnus X-1, as the
Albireo
hidden, compact companion is designated, is a black hole.
Consequently, you may find its position, but will not be able to
observe it!
Albireo is the primary star of a magnificent double with a separation of 34 arcseconds from its companion star β2
Cygni. Even a small telescope can separate them so that you can distinguish their individual gold and blue colours.
The Summer Triangle
Lines drawn between the bright stars Deneb, Vega and
Altair form the Summer Triangle, a prominent formation
of stars, which is known as an asterism not a
constellation.
Deneb, in the tail of Cygnus, is a very luminous bluewhite supergiant and, although there is some uncertainty,
is believed to lie at a distance of about 2,600 light years.
Vega, in the constellation of Lyra, is 25 light years away.
It was the pole star around 14000 years ago; due to the
precession of the Earth.
Altair in Aquila is about 17 light years away.
Conditions apply as to the use of this matter. D J Thomas