The three top sights to observe or image this month
DON’T MISS: Jupiter moon events
BEST TIME TO SEE: Multiple events, dates and times as specified.
RECOMMENDED EQUIPMENT: Small/medium telescope
On Monday 4 October, there’s a rare opportunity to see the shadows of Ganymede and Callisto in transit simultaneously. The event begins in daylight at 18:00 BST (17:00 UT) as Callisto’s shadow starts its passage. Ganymede itself appears on disc at this time, but exits from view at 19:19 BST (18:19 UT) under darkening skies. Ganymede’s shadow begins its transit at 19:50 BST (18:50 UT) under dark-sky conditions. Callisto’s shadow is conveniently positioned on Jupiter’s central meridian at this time.
Both shadows pose for an image either side of the central meridian at 20:53 BST (19:53 UT). By the time Callisto’s shadow exits the disc at 22:25 BST (21:25 UT), Ganymede’s has largely caught up and will leave transit an hour later at 23:25 BST (22:25 UT). The dark nature of both shadows makes this an ideal event to observe through smaller instruments.
Just after midnight BST on 20 October, it’s possible to see Io and Callisto in transit at the same time. This event starts at 00:30 BST on 21 October (23:30 UT on 20 October) with both moons lined up as if about to start a race. If it were a race, the clear winner would be Io. Being closer to Jupiter, Io orbits at a much faster pace and, given a flat southwest horizon, it should be possible to see the inner moon striving ahead of the outer moon Callisto. This event occurs near to Jupiter setting and, sadly, concludes with the planet beneath the UK’s horizon.
You need a telescope to observe these moon events, but if the sky is clear on the evening of 15 October, there’s a meeting between our 77%-lit waxing gibbous Moon and mag. –2.5 Jupiter that should be striking to the naked eye. Both objects will appear at their closest in the early evening as the sky is darkening.
On a plus point, Jupiter is now increasing in altitude as seen from the UK. This year it’s in Capricornus and able to reach a height approaching 22° as seen from the centre of the UK. In 2022, that figure increases to around 36°.
October meteors
BEST TIME TO SEE: From 20:30 BST (19:30 UT) on 8 October
RECOMMENDED EQUIPMENT: Naked eye
There’s good news and bad news for meteor watching in October. The bad news concerns the annual Orionid shower, which reaches its peak on 21 October. With a peak ZHR of 20 meteors per hour, the Orionids is a popular event to observe. During 2021 however, the peak occurs with a just-past-full Moon in the sky and many of the shower’s trails will be lost to its glare.
The good news concerns a lower-rate shower named the Draconids (also known unofficially as the Giacobinids, in reference to the parent comet 21P/ Giacobini-Zinner). This shower has a very low ZHR peak value of five meteors per hour, but has shown increased activity over the past few years. The shower put on spectacular displays in 1933 and 1946, with ZHR rates measured at thousands of meteors per hour. Enhanced rates were also seen in 1998, 2005, 2011 and 2012. The 2012 event consisted mostly of very faint trails, which were detected by the Canadian Meteor Orbit Radar facility.
More recently, in 2018 the shower put on an impressive display equating to a ZHR of 150 meteors per hour over a four-hour period. Although there are no predictions for enhanced activity in 2021, the whole point of meteor observing is to record what actually happens regardless of predictions. It’s only with recorded data that future predictions can be refined.
Why is this good news? Well, that pesky Moon, which will wipe out a lot of the Orionid shower, is new on Wednesday 6 October and will not interfere with the 2021 Draconids. Draconid meteors are best seen in the evening on 8 October. The shower trails are especially slow, the meteoroids entering Earth’s atmosphere at 21km/s – less than one-third the speed of November’s Leonid meteorids.
Occultation of Eta Leonis
BEST TIME TO SEE: 3 October from 04:40 BST (03:40 UT) to 05:20 BST (04:20 UT)
RECOMMENDED EQUIPMENT: Small/medium telescope
Mag. +3.5 Eta (η) Leonis is occulted by a 13%-lit waning crescent Moon on the morning of 3 October, in an event suitable for binoculars or a small telescope. It should also be possible to see Eta Leonis disappear and reappear from behind the Moon’s disc with the naked eye.
Eta Leonis is part of the Sickle asterism, which forms the head and foreleg of Leo, the Lion. Regulus (Alpha (a) Leonis) sits at the foot of the Sickle’s backwards question mark shape; Eta Leonis immediately north of Regulus.
Eta Leonis disappears behind the Moon’s illuminated edge at 04:49 BST (03:49 UT), reappearing from behind the dark limb at 05:12 BST (04:12 UT). The star’s line of passage behind the Moon’s disc is close to the Moon’s northern edge, so there will be a considerable variation in terms of what you will see, depending on your location. North of a line running from Sunderland through St Bees in Cumbria, Ramsey on the Isle of Man, Dundalk in the Republic of Ireland and Lettermore in County Galway, you will see a near miss; south and you’ll see it. Along the line described, there’s an opportunity to see the star clip the Moon’s edge, which is known as a ‘grazing lunar occultation’.