A monthly look at astronomical events in the sky and on Earth
Compiled by John Rowland
For most people, the realisation that the days are getting shorter comes as July rolls into August. That last walk with the dog, when suddenly, some stars appear that you didn’t notice the night before, or you realise you’re turning the lights on the inside before your favourite TV program ends instead of after it. All signs that “the season” is just around the corner. And what’s nice about August is that it’s often reasonably warm when it’s dark. What bliss.
Regarding sunset times (for York), here are some facts and figures about the darkening skies. Please note that all 4-figure times given in this article are in BST.
|Date||Sunset||Civil Twilight Ends||Nautical Twilight Ends||Astronomical Twilight Ends|
- Civil twilight ends when Sun drops to 6° below the horizon
- Nautical twilight ends when Sun drops to 12° below the horizon
- Astronomical twilight ends when Sun drops to 18° below the horizon
Things do change fast in August, and by month’s end, it gets completely (astronomically) dark well before bedtime. Great!
So, what has August 2021 got to offer us? Very briefly, the oppositions of Jupiter and Saturn, a very favourable Perseid meteor shower, and the chance to see some deep sky objects. There is also evidence of solar activity.
The Sun has at last shown signs of life! A few sunspots have been recorded in July, which have prompted some to declare that Solar Cycle #25 really has started. Whilst we’re not expecting anything dramatic any time soon, it’s worth checking spaceweatherlive.com to see whether anything’s going on.
August’s new moon is at 1450 on the 8th. The best evenings to observe the Moon are from Tuesday 10th (slender crescent) through to Thursday 19th (late gibbous). However, for this entire period, the Moon doesn’t get much higher than 20° above the horizon due to the angle of the ecliptic. The Moon and Venus are near each other in the sky at sunset (2043) on the 11th but very low down. For those wishing to spot the earliest crescent moon, the best (but challenging) day will be Monday 9th after the Sun sets at 2048 (See the Crescent Moon Watch page for more details.)
Also please register on this link if you would like to join other members at Roulston Scar (Sutton Bank, Thirsk) on this date (9/8/2021) to try to spot the earliest crescent moon.
Mercury is too low at sunset to be observable this month.
Venus is gradually catching up with us following its journey from behind the Sun but due to the low angle of the ecliptic at this time of year, Venus is only about 10° above the western horizon at sunset. This situation will continue right through until the end of the year.
Mars is on the far side of the Sun and completely unobservable this month. We’re going to have to wait until its next opposition in December 2022 before it presents itself once more.
Jupiter and Saturn
Both these gas giants reach opposition this month (Saturn on the 2nd and Jupiter on the 20th). That means that they are due south and highest in the sky at 0100 hrs on their respective opposition days. When considering observing these planets, factors to consider are:
1. Observing date and distance to the planet
Obviously, the closer to the opposition you observe, the nearer (and of course larger) the planet is in your telescope eyepiece. However, leaving it to a month after opposition will only reduce Jupiter’s diameter by 3.3%, and Saturn’s diameter by 1.5%. Leaving it two months reduces Jupiter by 11% and Saturn by 5%. So don’t think you have to rush. One month after opposition, the planets are due south and reach their highest point in the sky (culminate) two hours earlier; two months – four hours earlier. How much more convenient it is, observing at 9-10 pm than at one in the morning!
2. Observing time and altitude*
*by “altitude” I mean angular distance above the horizon.
If you observe any object before or after culmination, its altitude will of course be lower. For these two planets, as their altitude only reaches 23.2° (Jupiter) and 17.5° (Saturn), it is important to observe them within one hour on either side of culmination. Outside those limits, not only is their brightness reduced by 50% or more, but atmospheric turbulence increases significantly, resulting in very poor ‘seeing’. This deterioration of brightness and clarity is far more significant than the effects of increased distance to the planets.
So the moral of the story is to observe Jupiter and Saturn somewhere within the two hour period centred on their culmination. And remember, the time of culmination gets earlier by about two hours per month. (Good S&T article on atmospheric extinction here.)
Uranus and Neptune
Both these planets are morning objects. Best wait until September or October.
The Perseid Meteor Shower
If you’re into killing wild birds with guns, then you’ll be looking forward to the glorious twelfth, the start of the red grouse shooting season. If however, you’re into less lethal pursuits, you’ll be pleased to know that the Perseid meteor shower – the second most prolific shower of the year – peaks in the late evening of that day, Thursday 12th of August and on into the small hours of Friday 13th. This year promises to be a good show as the Moon is well out of the way.
So, how many meteors can we expect to see? Well, it depends on two factors. Firstly, the night you observe; on either side of the peak night, meteor numbers drop quite quickly. Secondly, the time you observe; on any given night, the time is a critical factor because it affects the altitude of the radiant, and this in turn significantly affects the number of meteors seen.
I have been concerned to give you the most reliable estimate of meteor numbers, and to this end have done considerable research. Apparently, peak Perseid numbers have been struggling to reach 80 (per hour) over the last few years apart from the once in twelve years highs caused by the influence of Jupiter. The last “Jupiter” high was in 2016. In fact, over the last four years, the peak numbers averaged 74 per hour. Please, take no notice of exaggerated ZHR predictions of 100/hr or more that you may see in the popular press. Even Wikipedia is quoting Patrick’s out of date figure of 120 from a book of his from 2011. It is also very rare for meteor prediction sites (including Wikipedia) to indicate rates on the nights leading up to and following the peak night.
The table below has been compiled using data from the RAS, the International Meteor Organisation’s most helpful secretary Tracie Louise Heywood, and the authoritative Öpik/Kresák/Richardson table that links relative meteor numbers with radiant altitude. The numbers given are statistically the most likely numbers but should not be taken as a firm prediction. Numbers can be greater or lesser overall and greater or lesser on any given night.
Most Probable Actual Meteor Numbers Expected
By Date and Time
The radiant will be in the NE in the late evenings and rise slowly, ending up high in the east by 0400.
0400 is the start of nautical twilight, so the sky will start to brighten from then on. However, many of the Perseids are very bright, and even after that you should be guaranteed a good show
The best night for people who work Mondays to Fridays would be the night of Fri/Sat 13/14th when the numbers will be maybe 2/3rds of the night before. But if you are able to, my recommendation is that on the Thursday 12th evening, you prepare a lounger, facing east, cover it with a waterproof then go to bed and set your alarm for 0315 or 0330, get up, take your quilt to the lounger and just lie there looking up. Relax (but try not to fall asleep) and enjoy!
Do please use the “Leave a Reply” facility below to let us know how you got on.
Deep Sky Objects
Meteors or no meteors, if you do go the whole hogg and venture out onto your lounger in the small hours of Friday 13th, barring clouds, you should be rewarded with an impressive sky. The summer triangle of Altair, Vega and Deneb will be in the west, the milky way including the superb Cygnus star clouds will stretch from there all the way across the sky above you through Cassiopeia. Perseus and on past the Pleiades and down towards Orion rising in the east. And to top it off, the Perseus double cluster and M31, the Andromeda galaxy will be right there, straight above you. This is the sort of occasion that if you are not in awe of the wonders of the heavens, you never will be.
As far as recommendations for specific DSO for telescopic viewing during August, try these, with a little help from Astronomy Now.
The Summer Triangle area of sky (Chart courtesy of Astronomy Now)
|Albireo||3.0||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||9.0||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,|
And last but by no means least, if you haven’t seen it before (or even if you have) the incredible Great Globular Cluster in Hercules, M13, is still favourably placed high in the SW to W and is definitely worth you giving it some attention.
Man’s Space Activities
As the Telegraph recently put it with typical droll, “You wait ages for one, then two billionaires go into space at once”! And so they did. The Branson Bezos space race is well under way with both men having realised their first-step-dreams. But of course, Elon Musk has outdone both Branson and Bezos by achieving real “delivery” to the ISS. We will be hearing much more from the three rival members of this triumvirate in the years to come.
Spotting the International Space Station
The ISS is not visible during August until the last three days, and even then, it’s very low in the sky. Wait until September, when there’ll be many opportunities to spot it. The Heavens Above website gives all the details. The link is preset to return the data for sightings from York. Use the left and right chevron buttons to change the date range. All times are BST.
General Space News
There’s always something of interest going on in space. See the space.com website for the latest news. And for a detailed timetable of projected launches and other space-related activities, the Wikipedia website is an excellent place to start as it has links to each project/mission mentioned.
Last Month’s Challenge
The challenge was to identify the “Cat in the Sky” (below).
Well done, Freda Rockliffe, who was the first to correctly identify it as being part of the Pillars of Creation structure in M16, the Eagle Nebula. See below.
This Month’s Challenge
Identify this crater. Some body (one of four) obviously made an impression here.
If you can identify it, let us know by using the Leave a Reply facility as your messenger.
To enable greater participation, replies to the challenge may not be displayed until August 15th, when the winner (if any) will be announced.
I wish you clear skies and good viewing. Stay safe.