A monthly look at astronomical events in the sky and on Earth
August the 8th is the first night since early May that the sky goes completely dark – for a short time centred on 1 a.m. This increase in the length of the night is noticeable in the late evenings in August and heralds the “shoulder season” for astronomical observations. By the 15th, astronomical twilight ends at 2316 BST, and by the end of the month it’s at 2214. Just taking the dog for its last walk or strolling home from the pub, August is the month when one begins to notice the stars again. And with it being predominantly warm, August is probably the month with the best combination of evening comfort and darkness. So break out those 7 x 50s, dust off that ‘scope and get out there!
Best seen as a thin waxing crescent low in the west after sunset on Mon 5th to late gibbous low in the south on Mon 12th. It doesn’t climb much above 13° altitude throughout this period due to the angle of the ecliptic at this time of year. Jupiter will be seen about 1.5° below it on the 9th.
Mercury may be spotted low (6 – 10°) in the ENE very early in the morning at around 0440 to 0500 between the 8th and 15th of the month.
Venus and Mars
Are both on the far side of the sun and completely unobservable this month.
Jupiter and Saturn
Both can be seen low in the south by about 2100 BST. Jupiter is in Scorpio and fairly near the star Antares; Saturn is 30° to its left in Sagittarius. The low altitude of these planets is not conducive to seeing them at their best. However, the YAS has organised a special observing event for Saturday the 31st August where we will be bringing an armoury of high power telescopes to bear on Saturn. This is probably your best chance this year to see this spectacular planet. See the Events page for more details and to book a place.
Uranus and Neptune
Visible only after midnight. Wait until later in the year for these two.
Perseid meteor shower 11th – 13th
Normally the northern hemisphere’s favourite meteor show, the Perseids this year will be a bit of a challenge. The problem is the moon. As the shower reaches its peak, not only date-wise but hour-wise, the waxing gibbous moon will reduce the number of meteors seen. Despite this, if you are prepared to stay up very late or even go to bed then get up very early, the Perseids are definitely worth observing.
The graph below shows the daily Perseid numbers observed in 2017, and this year’s peak will be on the same day – the 13th. The numbers for 2017 were perfectly normal and typical of this shower.
Data from Hong Kong Space Museum
The zenithal hourly rate (ZHR) is what a single observer would expect to see each hour if the sky was perfectly clear and completely dark and the radiant was overhead. You’ll see the ZHR rises, especially from the 9th, reaches a peak on the 13th then quickly reduces.
From the UK, the radiant – at the northern end of the Perseus constellation – rises in the NE while it’s still light, but by 3 a.m. has risen to a very respectable 60° altitude. After that, the sky begins to brighten as dawn approaches. The number of meteors to be seen is roughly equal to the sine of the radiant altitude.
- The ZHR column shows how the ZHR numbers change day by day.
- The AHR (actual hourly rate) column shows the number of meteors actually expected to be seen by an observer in York.
- The Moon Alt column gives the altitude of the Moon. Negative numbers mean it’s below the horizon.
- The Sun Alt column shows how far below the horizon the sun is. Less than 18 means some sky glow.
So there are five one-hour slots centred on the times given when it’s worth giving it a try.
On the 11th, expect to see a meteor every 2½ minutes or so; on the 12th, one every 2 minutes; and on the 13th, if you wait until between 0230 and 0330, one every minute.
You don’t need to identify the constellation of Perseus to see these meteors. The best way is to set your chair or lounger to face north east (the opposite area of sky to where the moon is) and just relax. Good luck!
Non-Solar System Objects
July was the month to focus on the many nebulae and clusters in Scorpio and Sagittarius. In August we should turn our attention to the summer triangle and surrounding area. In case you don’t know the summer triangle, it is formed by the stars Altair in Aquila, Vega in Lyra, and Deneb in Cygnus. During the first week of August – when the moon is not around – but particularly from the 22nd onwards, this area of sky through which the Milky Way passes on its route northwards is riding high in the south and is peppered with deep sky splendours. The Astronomy Now chart below shows the best of these. Look high in the south at 2300BST. Each object is described in the table below.
The Summer Triangle area of sky (Chart courtesy of Astronomy Now)
|Albireo||3||β Cygni. A beautiful and bright 3rd magnitude double star with amber and sapphire components 35 arc seconds apart. Use a magnification of x30 to see them at their best.|
|ε Lyrae||4.7||The famous "double double" comprising two pairs of stars. The pairs are separated by about 3 arc minutes but the components of each pair are only about 2.4 arc seconds apart. This object is a good test for a 4-inch telescope; you'll need a magnification ox x200 or more.|
|M57||9||The Ring Nebula. Another famous object. It's a planetary nebula that looks like a smoke ring. Although small and faint, it's easy to find, half way between the two stars shown on the chart and will stand a fair degree of magnification. In fact, it is about 4 arc minutes in diameter, nearly six times the apparent diameter of Jupiter.|
|M27||7.5||The Dumbbell Nebula, another famous planetary nebula. This showpiece of the northern hemisphere never fails to impress, but do try to view it only when the sky is very dark. At 8 arc minutes in diameter, it's easily visible even in binoculars. Not to be missed,|
Just lying on a lounger and with binoculars, sweeping the star fields in this area of the Milky Way, stretching right across the sky from SW to NE will reveal many beautiful star clusters. I find doing just that is amazingly relaxing. Do try it; it’s a tonic!
But if you would like the opportunity to see these objects with the array of society telescopes, we’ll be observing them at the event (previously mentioned) on Saturday 31st August. Go to the Event Page on our web site for more info.
Passes of the International Space Station (ISS)
The ISS is visible on only four nights in August: on the 1st and 3rd at around 10 p.m. and then on the 29th and 31st at around 5 a.m. See the Heavens-Above web site for more details.
For up-to-date news on space missions, rocket launches etc. scroll https://www.space.com/32286-space-calendar.html
Clear skies and good viewing.
John Rowland 25 July 2019