Observing

  • 01-09-2015

    Stargazing in September 2015


    The September Night Sky

    The General Weather patterns

    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. 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 23rd 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. 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/h2>

    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 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.

    The 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.

    The 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 16th. 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 28th. 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

    Monthly Constellations

    Constellation Convienient Culmination Midnight Culmination Observability
    Aquila 21:00 Late September Early August Whole constellation
    Sagitta 21:00 Late September Early August Whole constellation
    Cygnus 20:00 Mid-October Mid-August Whole at zenith
    Delphinus 20:00 Mid-October Mid-August Whole constellation
    Vulpecula 20:00 Mid-October Mid-August Whole constellation
    Equuleus 20:00 Late October Late August Whole constellation

    Cygnus (pronounced sig’ nus)

    From the Latin for ‘the Swan’, the constellation Cygnus lies immediately above our heads in mid –September, at about 22:00. Locating it is easy at this time, the asterism known as the Northern Cross lies at its heart. 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 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 -8.73 it is approximately 60,000 times brighter than the Sun. But, at an estimated distance of 1,800 light years, it has a visible magnitude of 1.25

    Next to Eta Cygni, the star 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 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 hidden, compact companion was 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 Cyg. Even a small, decent telescope can separate them so that you can distinguish their individual gold and blue colours.


    David J Thomas

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