The night sky in August 2018

posted 29 Jul 2018, 14:31 by Pete Collins
by Anne Holt

Sunrise        1st:   05.24         31st:   06.16
Sunset         1st:   21.05         31st:   20.01

Astronomical darkness
1st:  00.38  to  02.02         31st:  22.14 to 04.04

New Moon:   11th  at 10.57
Full Moon:    26th  at 15.26

Lunar perigee:  10th at 19.06
Lunar apogee:   23rd at 12.25

August's full Moon is known as the Sturgeon Moon because these fish in the American Great Lakes are said to be easiest to catch at this time. Other names are the Green Corn Moon and the Grain Moon.  Some North Americans call it the Red Moon as it sometimes appears this colour when it rises through the summer haze.

Highlights

Nights are getting longer by almost 4 minutes each day and we have increasing amounts of astronomical darkness during August - 2 and three quarter hours on 1st and nearly 6 hours on the night of 31st August/1st Sept. 
The new Moon on 11th is a Supermoon, occurring close to Lunar perigee.  Of course a new Moon can't be seen but the thin waning crescents, rising at 02.55 on the morning of 9th and at 04.05 on the 10th,  should appear larger than usual.
There is a small partial eclipse on the morning of 11th but it is only visible from the far north of Scotland,  Orkney, Fair Isle and Shetland.  From Thurso on the coast of northern Scotland, the edge of the Moon will cross the face of the Sun between 09.30 and 10.00.  From Lerwick, in Shetland, the eclipse will be slightly longer - 09.30 to 10.15.  However, it will be a very small 'bite' out of the Sun  the maximimum is only 2.5% when seen from the most northerly part of the Shetland Isles.
And, of course, we have the year's best meteor shower, the Perseids, free this year from Moon interference.

Constellations

When it finally gets dark enough, the Milky Way is now at its best.  From a dark sky site it can be seen stretching right across the sky and down to the southern horizon, passing almost overhead around midnight.

The Milky Way passes through the Summer Triangle, which is now high in the sky, with Deneb and Vega particularly prominent.  Alberio, a beautiful yellow and blue double star at the head of Cygnus the swan, is very well placed for telescopic observation.

The Plough and its host constellation Ursa Major are now very low in the Northern sky which means that the W asterism of Cassiopeia is riding high in the south east and very easy to spot.

Pegasus and Andromeda are now well above the horizon for most of the night and Perseus, followed by Auriga, are rising soon after midnight.

Planets

Mercury:  in Libra, mag 2.9. Not visible for most of August.  On 1st it sets a few minutes before the Sun and is separated from it by only 14 degrees.  The angular distance decreases as it approaches inferior conjunction on 9th.  It then becomes a morning object, in Cancer, but is still too low to be seen until the last few days of the month. On 20th it will be at mag 1.4 and be a few degrees above the horizon at 05.15.  On 26th it reaches greatest western elongation, 18.3 degrees from the Sun but still only 8 degrees above the horizon at dawn - though brighter at mag - 0.1.  On 30th it moves into Leo and on 31st it rises at 04.25,  reaching 8 degrees above the eastern horizon at 05.46, half an hour before sunrise, and brighter at mag -0.7.

Venus:  in Leo, mag -4.2. Could be seen in early August, on 1st it sets at 22.22, about 90 minutes after sunset.  It is only 7 degrees above the horizon at dusk but should be visible, given an unobstructed view,  as it is so bright.  It moves into Virgo on 2nd.  The angle of the ecliptic is decreasing so the planet is getting lower in the sky, even though it reaches greatest eastern elongation on 17th.  On this day it is 49.5 degrees from the Sun but only 4 degrees above the western horizon at dusk, setting about an hour after sunset.   On 31st it will have brightened to mag -4.4 but will be only 3 degrees above the horizon as the sky begins to darken, setting about 45 minutes after the Sun.

Mars:  in Capricorn, mag -2.8. Still very bright, and large when seen through a scope, but rather low in the sky.  On 1st it culminates at 00.57, only 10 degrees above the southern horizon.  In the first few days of August it's disc is 24 arcseconds in diameter, however it will be difficult to see, or image, much detail through a scope - not only is it low but there are currently huge dust storms blowing across the surface.  It fades slightly during the month, on 23rd, when the 82% lit Moon passes 6 degrees to the north, Mars will be at mag -2.3.  Its apparent motion is retrograde for most of the month but after 28th it resumes prograde, or direct, motion appearing to move eastwards against the background stars. On 31st it culminates at 22.36, 10 degrees above the southern horizon  It sets at  01.59 and will have faded to mag -2.1.

Jupiter:  in Libra,  mag -2.1. Quite low in the west as the sky darkens now. On 1st it is only 15 degrees above the horizon at 21.30, setting at 23.35. On 17th the 45% lit Moon passes about 5 degrees north of the planet. By month end it will have faded slightly to mag -2.0, be only 11 degrees in the SW a few minutes before 20.30 as the sky darkens, and set at 22.00.

Saturn:  in Sagittarius, mag 0.2. An early evening object, not easily seen as it is currently sitting north of the teapot asterism, where the Milky Way is rather bright.  It is also quite low - on 1st it is only 12 degrees above the southern horizon as the sky darkens, just before 22.00. It culminates, a degree higher, at 22.41 and sets at 02.33.  On 31st it culminates only half an hour after sunset and may be seen, 13 degrees above the southern horizon, just before 21.00.  It sets at 00.30 and will have faded to mag 0.4. 

Uranus:  in Aries, mag 5.8. On 1st it rises at 23.17 and reaches 33 degrees in the SE before the sky begins to brighten.  It is currently the only planet reaching a good altitude.  On 4th it is about 4 degrees north of the third quarter Moon.  It gets even better as the month progresses and on 31st it culminates at 04.32, 48 degrees above the southern horizon, it will also be marginally brighter at mag 5.7.  Because it is so well positioned, high in the sky at the moment, it could possibly be seen with the naked eye from a dark sky site - with the usual proviso that they eye in question must have 20/20 vision and know exactly where to look.  Though I've yet to meet anyone who has seen it without any optical aid.  It should be an easy binocular target, and a decent amateur scope will show a small blue/green disc. A much larger scope will be needed to show any markings.

Neptune:  in Aquarius, mag 7.8. Reasonably high in the sky, though lower than Uranus.  On 1st it rises at 22.07 and reaches 29 degrees in the SE before the sky begins to brighten.  On 27th the just past full Moon passes about 6 degrees to the SE just before midnight.  By month end the planet culminates at 01.38, 29 degrees above the southern horizon.  It could possibly be seen, through decent binoculars, in the late evening in the second part of the month - though probably not from Manchester.  A good amateur scope should show its bright blue disc.

Minor planets

Ceres and Pluto are too low to be easily spotted this month.

Eris, Haumea and Makemake are possible targets for keen amateur photographers with good equipment.

Eris:  in Cetus, mag 18.8. On 1st it rises at 02.22 and is 23 degrees above the horizon at dawn.  Its position improves during the month, on 31st it rises at 22.21 and culminates at 04.17,  34 degrees above the southern horizon.

Haumea:  in Bootes, mag 17.4. Starts the month 27 degrees above the western horizon as the sky darkens, setting at 02.21.  By 31st it is 23 degrees above the horizon at dusk, setting not long after midnight.

Makemake:  in Coma Berenices,  mag 17.2. Setting about 15 minutes before Haumea throughout August, but lower in the sky - 22 degrees just before dawn on 1st, 18 degrees on 31st.

For more information, including exact positions on any day see:
https://in-the-sky.org/ephemeris.php      (for minor planets, look under asteroids)

Meteor Showers

One (very) major shower, this month.

The Perseids, active July 13th to August 26th.  Peak on the night of 12th/13th but good rates can usually be seen a day or so either side of this.  ZHR at peak is given as 100 but this is under ideal conditions - from a dark sky site with the radiant directly overhead.  Usual rates under dark skies more likely to be in the region of 50 to 75, and as low as 10 from a light polluted urban area.  Best seen after midnight (01.00 BST) when the radiant is high in the sky,  but it is worth looking earlier as the shower sometimes includes Earth grazers - rare, slow moving meteors streaking across the horizon leaving bright persistent trails, which are occasionally seen when the radiant is very low in the sky.  The Perseids happen when Earth passes through debris left by comet 109P/Swift Tuttle.  This year the Moon is out of the way, it sets at 21.32 on 12th. All we need is for clouds to stay out of the way, as well.

Minor Showers

Kappa Cygnids.  active 3rd to 25th, peak 18th.  However, this shower may have altered and be active between 6th and 18th with a peak on 14th.  ZHR 5, but sometimes many more.  According to the IMO this shower does not always behave as it is supposed to.  They are slow moving, often very bright, meteors, parent comet unknown.  The Moon sets before midnight on both the possible peak dates.

Aurigids,  active August 28th to Sept 5th, peak 31st/Sept 1st.  ZHR 6, again sometimes more, but not predicted for this year.  These are bright, slow moving meteors.  80% Moon rises at 22.13, so will interfere.

There are a couple of showers whose peak is actually at the end of July, but which usually have reasonable rates for a couple of days after, so might be worth looking on the first couple of days of August.

Alpha Capricornids:  active to August 15th, ZHR 5.  Slow moving, yellow tinged meteors often leaving trails.  Moon rises soon after 23.00 on 1st, but the shower often includes fireballs which may be visible despite this.

Southern Delta Aquariids,  active to August 15th.  medium paced, faint meteors. so more likely to be washed out by moonlight.

Comets

In August we have a couple of comets which may be visible in binoculars.

21P/Giacobini-Zinner.
Currently circumpolar.  Starts the month at about mag 9 (sources vary - there's a surprise) moving from Cassiopeia into Draco.  Back into Cassiopeia mid month before moving through Camelopardalis and ending the month in Auriga, heading towards Capella, when it is predicted to reach mag 7.2.  Or maybe even brighter.

PanSTARRS (2017S3)
Visible in the morning sky in the first few days of August. Starts the month in Auriga, then quickly moves into Gemini and getting too low to be seen before dawn. The magnitude increased rapidly during July, from 12 to 9.  Starts August at mag 8 and could brighten to 7.

comet positions also given in in-the-sky.org


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