Dunsink Lane, Dublin 15, D15 XR2R

August 2022 Sky Notes

Many thanks to our colleague John Flannery for providing the below information on what to watch out for in the skies this month! John is a long-time amateur astronomer with an interest in the history and lore of the sky. He is a member of the Irish Astronomical Society.

Celestial fireworks dash across the sky this month when one of the best annual meteor showers, the Perseids, peak on the night of August 12/13th.  Unfortunately, the display must contend with a full moon on the same date this year. 

The moon’s light overwhelms any faint meteors but do not let that put you off, as chances are you will see some brighter members even during a short watch of thirty minutes or so. 

Take advantage too of the dark-of-the-moon periods later in the night during the first week of August to catch any early arrivals as the shower is active for a couple of weeks. The rates will be lower though than the night of Perseid maximum.

What is a meteor shower?

As a comet nears the Sun its surface ices begin to sublimate, and a tail composed of gas and dust forms. Over time, the dust – composed mostly of sand grain sized particles – gradually spreads along the comet’s orbit. On occasion, we ford those streams and get a meteor shower as Earth sweeps up more material than usual.

The grit plunges into our atmosphere at high speed and burns up due to friction at a height of about eighty kilometres. An analogy is to rub your hands together rapidly and notice how your palms warm up. The same thing happens to a meteor except the mote is rubbing against air molecules. We then see a flash of light as the dust particle is vaporised. 

The Perseids are associated with comet Swift-Tuttle, which has a 130-year long orbit and last returned to the inner solar system in 1992. I remember that well as the following year there was an exceptional return of the Perseids predicted. The night of maximum was overcast from Ireland, but we were still amazed by the flashes of bright fireballs through the clouds from our observing location in Glendalough.

In 1866, the Italian astronomer Giovanni Schiaparelli showed that the Perseids and the comet – which had been discovered in 1862 – followed the same orbit. It was amongst the first proofs of the cometary origin of meteors.

Where and when to look

Showers are named for the area of sky or constellation the meteors appear to originate. The meteors travel in parallel but appear to diverge from a point called the radiant. The effect is due to perspective, like railway lines receding in the distance.

Perseus can be found above the northeast skyline after dark under the ragged “W” of Cassiopeia, and it rises higher in the sky as the hours pass. This also means your chances of seeing a meteor increases too: most only become visible a short distance away from the actual radiant and your local horizon will block a certain number early on.

Meteors that do not appear to be Perseids are called sporadics, though they could also belong to one of the less well-known streams also active around this time. Sporadics can appear any night you look up during the year and are often just random crumbs swept up. 

Be wary of any rates of a hundred or more meteors mentioned in some sources though as often that will be a theoretical value known as the Zenithal Hourly Rate. The ZHR is based on perfectly dark and clear skies, no meteors missed, and the radiant source overhead. The true number of meteors seen is usually much lower.

Meteor observing at Dunsink

Dunsink Observatory is part of NEMETODE , a network of cameras across Ireland and the UK set up by professional and amateur astronomers.

The goal is to catch a brilliant meteor that potentially might drop some meteorites. If a fireball is recorded by two or more sites, then astronomers can triangulate its track and predict if a meteorite fall is likely, and the general area of where to look. 

Only eight meteorite falls have been recovered in Ireland to date and the last was in August 1999 when about 450g of material was collected around Leighlinbridge in Carlow.

Who knows when the next spectacular fireball might be seen, but every clear night, there is always a chance of seeing a beautiful bright meteor and making a wish. 

The Moon this month

First quarter moon falls on August 5th, full moon on the 12th, last quarter on the 19th and new moon on 26th. This month’s full moon is the last of the so-called super moons for 2023. 

The planets this month

Mercury is too deep in the solar glare to be seen this month, but Venus rises about an hour before the Sun during August and is a brilliant morning “star”.

Mars rises before midnight mid-month and moves from Aries into Taurus during the second week of August. It lies in the same low-power binocular field of view as the planet Uranus during the first week of the month.

Jupiter and Saturn are both visible in the evening sky, with the latter reaching opposition on August 14th in Capricornus. Opposition is when a planet is opposite the Sun in the sky, and it rises at sunset (like the full moon). Saturn is visible throughout the hours of darkness currently and is at its brightest for the year.

Dates for the diary

August 6th – The bright star Dschubba (labelled with the Greek letter delta on star atlases) in Scorpius is occulted, or hidden, by the Moon tonight at 22:35pm (Summer Time). 

The moon’s bright limb covers the star first so that will be a bit harder to see in binoculars. The star’s reappearance from behind the moon’s unlit edge (right-hand side) at 11:22pm should be more obvious. 

Historically, occultations helped refine the moon’s orbit about the Earth and were an early means of computing longitude. Because there is no lunar atmosphere, a star suddenly winks out when it is occulted, rather than gradually fading from view. 

August 7th – All this week has been an opportunity to see the planets Mars and Uranus in the same binocular field of view in Aries. The gap will have widened too far by the seventh though, but any planetarium app will guide you to distant ice giant’s location.

August 12th – Tonight is full moon and it also the last of this year’s four super moons. August’s full moon in the Celtic year was known as the Dispute Moon when disagreements were resolved.

August 12th – The peak of the Perseid meteor shower is tonight.

August 14th – Saturn is at its brightest during opposition tonight in Capricornus.

August 15th – Jupiter lies a little to the upper left of the waning moon in the late evening sky.

August 16th – Mars passes closes to the Pleiades, or Seven Sisters, star cluster in Taurus over the next few days.

August 18th – Mars lies a little to the lower left of the moon as they rise just before midnight. The gap between the two narrows during the early hours of the 19th. 

August 19th – The first quarter moon lies between the Pleiades and the Hyades, a V-shaped group marking the head of Taurus the Bull.

August 24th – The phenomenon of the “Rolling Sun” of Croagh Patrick may be seen around this date.

August 26th – Venus lies to the right of a thin crescent moon in the morning sky.

August 28th – Look for a young moon low in the western sky after sunset.

August 29th – The launch window opens today for NASA’s Artemis-1 mission that will slingshot an uncrewed Orion capsule around the moon before it returns to Earth and a splashdown in the Pacific Ocean. 

The giant Space Launch System rocket that will ferry astronauts to the Moon has been a long time under development and suffered numerous delays, but NASA are confident a successful test will quickly lead to humans returning to the Moon by 2024 after a more than fifty-year hiatus. The first crewed mission will be a lunar flyby, followed by a landing at the Moon’s South Pole in 2025.

August 30th – Mars, now bright, lies midway between the Pleiades and Aldebaran, the orange giant marking the fiery eye of Taurus the Bull. The gathering does not troop over the eastern horizon until around midnight though.