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Scotland’s Sky in August, 2018

Perseid meteor shower peaks in planet-rich sky

The maps show the sky at midnight BST on the 1st, 23:00 on the 16th and 22:00 on the 31st. An arrow depicts the motion of Mars. (Click on map to enlarge)

The maps show the sky at midnight BST on the 1st, 23:00 on the 16th and 22:00 on the 31st. An arrow depicts the motion of Mars. (Click on map to enlarge)

The persistent twilight that has swamped Scotland’s night sky since May is subsiding in time for us to appreciate four bright evening planets and arguably the best meteor shower of the year.

The Perseid shower returns every year between 23 July and 20 August as the Earth cuts through the stream of meteoroids that orbit the Sun along the path of Comet Swift-Tuttle. As they rush into the Earth’s atmosphere at 59 km per second, they disintegrate in a swift streak of light with the brighter ones often laying down a glowing train that may take a couple of seconds or more to dissipate.

The shower is due to peak in the early hours of the 13th at around 02:00 BST with rates in excess of 80 meteors per hour for an observer under ideal conditions – under a moonless dark sky with the shower’s radiant point, the place from which the meteors appear to diverge, directly overhead. We should lower our expectations, however, for although moonlight is not a problem this year, most of us contend with light pollution and the radiant does not stand overhead.

Even so, observable rates of 20-40 per hour make for an impressive display and, unlike for the rival Geminid shower in December, we don’t have to freeze for the privilege. Indeed, some people enjoy group meteor parties, with would-be observers reclining to observe different parts of the sky and calling out “meteor!” each time they spot one. Target the night of 12th-13th for any party, though rates may still be respectable between the 9th and 15th.

The shower takes its name from the fact that its radiant point lies in the northern part of the constellation Perseus, see the north map, and climbs from about 30° high in the north-north-east as darkness falls to very high in the east before dawn. Note that Perseids fly in all parts of the sky – it is just their paths that point back to the radiant.

Records of the shower date back to China in AD 36 and it is sometimes called the Tears of St Lawrence after the saint who was martyred on 10 August AD 258, though it seems this title only dates from the nineteenth century.

Sunrise/sunset times for Edinburgh change this month from 05:17/21:20 BST on the 1st to 06:15/20:10 on the 31st. The Moon is at last quarter on the 4th, new on the 11th, at first quarter on the 18th and full on the 26th.

A partial solar eclipse on the 11th is visible from the Arctic, Greenland, Scandinavia and north-eastern Asia. Observers in Scotland north of a line from North Uist to the Cromarty Firth see a thin sliver of the Sun hidden for just a few minutes at about 09:45 BST. Our best place to be is Shetland but even in Lerwick the eclipse lasts for only 43 minutes with less than 2% of the Sun’s disk hidden at 09:50. To prevent serious eye damage, never look directly at the Sun.

Vega in Lyra is the brightest star overhead at nightfall and marks the upper right corner of the Summer Triangle it forms with Deneb in Cygnus and Altair in Aquila. Now that the worst of the summer twilight is behind us, we have a chance to glimpse the Milky Way as it flows through the Triangle on its way from Sagittarius in the south to Auriga and the star Capella low in the north. Other stars of note include Arcturus in Bootes, the brightest star in our summer sky, which is sinking in the west at the map times as the Square of Pegasus climbs in the east.

Of the quartet of planets in our evening sky, two have already set by our map times. The first and brightest of these is Venus which stands only 9° high in the west at Edinburgh’s sunset on the 1st and sets itself 68 minutes later. By the 31st, these numbers change to 4° and 35 minutes, so despite its brilliance at magnitude -4.2 to -4.4, it is becoming increasingly difficult to spot as an evening star. It is furthest east of the Sun (46°) on the 17th.

Jupiter remains conspicuous about 10° high in the south-west as darkness falls and sets in the west-south-west just before the map times. Edging eastwards in Libra, it dims slightly from magnitude -2.1 to -1.9 and slips 0.6° north of the double star Zubenelgenubi on the 15th. A telescope shows it to be 36 arcseconds wide when it lies below-right of the Moon on the 17th.

The two planets low in the south at our map times are Mars, hanging like a prominent orange beacon only some 7° high in south-western Capricornus, and Saturn which is a shade higher above the Teapot of Sagittarius almost 30° to Mars’ right. Mars stood at opposition on 27 July and is at its closest to the Earth (57.6 million km) four days later. A planet-wide dust storm has hidden much of the surface detail on its small disk which shrinks during August from 24 to 21 arcseconds as its distance increases to 67 million km. Although Mars dims from magnitude -2.8 to -2.1, so it remains second only to Venus in brilliance. Catch the Moon near Saturn on the 20th and 21st and above Mars on the 24th.

Finally, we cannot overlook Mercury which is a morning star later in the period. Between the 22nd and 31st, it brightens from magnitude 0.8 to -0.7, rises more than 90 minutes before the Sun and stands around 7° high in the east-north-east forty minutes before sunrise. It is furthest west of the Sun (18°) on the 26th.

Diary for 2018 August

Times are BST

4th            19h Last quarter

9th            01h Mercury in inferior conjunction on Sun’s near side

11th          11h New moon and partial solar eclipse

13th         02h Peak of Perseids meteor shower

14th         15h Moon 6° N of Venus

17th         12h Moon 5° N of Jupiter

17th         19h Venus furthest E of Sun (46°)

18th         09h First quarter

21st         11h Moon 2.1° N of Saturn

23rd         18h Moon 7° N of Mars

26th         13h Full moon

26th         22h Mercury furthest W of Sun (18°)

28th         11h Mars stationary (motion reverses from W to E)

Alan Pickup

This is a slightly revised version, with added diary, of Alan’s article published in The Scotsman on July 31st 2018, with thanks to the newspaper for permission to republish here.
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