The Night Sky This Month: October 2018

Comet 21P/Giacobini-Zinner
Image Credit: Greg Ruppel/ESA

October offers early risers in the northern hemisphere the chance to spot the Comet 21P/Giacobini-Zinner, a periodic comet which returns every 6.6 years, and this time around will pass within a 36.3 million mile (58.5m kms) distance of the Earth. It can be seen during the wee hours for much of the month, moving along a vertical track between the star Procyon in Canis Minor and Sirius in Canis Major, and extending south of the latter on around the 16th. Under dark, clear skies, it might be possible to view the comet without optical aid, but the use of binoculars is recommended for the best views.

Moon Phases

Last Quarter: 2nd
New Moon: 9th
First Quarter: 16th
Full Moon: 24th

Planets

The coming month is generally not a good time to view the planets, all being low on the horizon or not visible at all. Nonetheless, some details of what can be expected from the planets is provided below:

Mercury might just be visible very low on the western horizon at a point just to the left of of where the Sun had set during the last few days of last month. Note that since the little planet will have an angular diameter of only about 6 seconds of arc, the use of binoculars is recommended for the best views, its magnitude of -0.2 notwithstanding. Mercury will be close to the Sun at the end of October, though, so do not use binoculars to spot the planet until the Sun has set completely.

Venus will not be visible to observers in the UK during October. It will however become visible low on the eastern horizon at about mid-November.

Mars is now rapidly moving eastwards through the constellation Capricornus, shining at magnitude -1.3 at the start of the month, but during the last days of October reducing to about magnitude -0.6 when it transits the meridian. Note that while its angular diameter is 16 seconds of arc as the month starts, this reduces to about 12 seconds of arc at the month’s end when the Red planet will reach an elevation of only about 14 degrees above the horizon. However, the dust storm that had engulfed the planet has now largely cleared up, meaning that major surface features may be visible with small telescopes that are fitted with atmospheric dispersion correctors.

Jupiter is still visible low above the western horizon as the month starts, shining at magnitude -1.8. However, its angular diameter reduces from 32.6 seconds of arc to 31.4 seconds of arc, which might make it even more difficult to get clear views of the King of the planets through the murk of the atmosphere.

Saturn too, will only reach an elevation of about 14 degrees above the south-western horizon soon after sunset at the start of Ovtober, but its angular diameter will reduce from 16.5 seconds of arc to about 15.7 seconds of arc, which will reduce its brightness from magnitude +0.5 to magnitude +0.6 as the month wears on. Although the ring system is still tilted towards us by about 25 degrees, the planets’ low elevation will make it difficult to get clear views of the rings.

Neptune was in opposition during the first week of September, so it is still well placed for observation with binoculars and small telescopes. Look for the planet to the upper left of the bright star Lambda Aquarii in the constellation Aquarius, where it will be shining at magnitude +7.9. Note though that Neptune will have an elevation of about 27 degrees when it transits the meridian.

Meteor Showers

October sees two meteor showers, some details of which are given below:

-The Draconids meteor shower results from debris of the comet 21P/Giacobini-Zinner, and is expected to peak during the early evening of the 7th or 8th of October. Although the expected maximum hourly rate is still being debated, there will be no moonlight on the expected peak nights, so whatever number of meteors do arrive, most should be clearly visible to observers who have the benefit of dark, clear skies. For observers in most of the USA, the shower’s peak will occur during daylight hours.

– The Orionids meteor shower is expected to peak on the night of October 21st, but since the radiant (in Orion) will only be about 15 degrees above the eastern horizon for observers in the vicinity of London, observers north of the city should not expect to see more than about six or so meteors per hour during the peak. Moreover, the waxing gibbous Moon will be about 85% illuminated during the peak, which will further impede views of the shower.

Deep-Sky Objects

The constellations Lyra and Cygnus are still prominent at this time of year, and both contain many spectacular targets for binoculars and small telescopes, including the three examples below:

– Epsilon Lyrae (The double-double star), located just to the upper leftward of Vega, is the brightest star in the constellation Lyra and the fifth brightest star in the entire night sky. While this pretty two pairs of binary stars can be seen without optical aid under dark skies, the best views are had with binoculars or small telescopes.

– M56, found about 33,000 light years distant midway between the stars Albireo and Gamma Lyrae, is a pretty magnitude 8 globular cluster stretching over about 60 light years.

– Brocchi’s Cluster is situated in the constellation Cygnus, just to the lower left of the star Albireo. This asterism is also known as the “Coathanger”, and it is an easy target for binoculars and small telescopes. Part of the asterisms’ attraction is its relative isolation from other bright objects that are largely obscured by a dark dust lane between itself and objects behind it.

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