Mercury joins Venus early- evening

May

Follow the fascinating observing opportunities the Moon offers, writes Tony Cook.

Take the chance to catch Mercury joining Venus in the early-evening sky, while Mars is a real challenge to spot pre-dawn. We feature a pair of galaxy icons and take a deep-sky tour of late-spring’s diverse targets.

When: soon after sunset

How to observe

: Venus is a magnificent sight soon after sunset in the west- northwestern sky throughout May. It holds station relative to the twilight, located around 15 degrees high some 40 minutes after sunset (a clenched fist held at arm’s length spans about 10 degrees across the knuckles). Venus brightens only marginally during May, shining just shy of magnitude –4 by the end of the month. Venus lies in Taurus at the start of May, close to the Hyades and Pleiades clusters. It moves steadily eastwards to enter Gemini by the early-evening of 19 May, when it lies 1.5 degrees west of Messier 35, Gemini’s superb open cluster, and a slim crescent Moon lies under five degrees west. By the next evening Venus moves to within a degree of M35 and on 23 May sports a northerly declination of 25.1 degrees, farthest north for this apparition, and sets 10 minutes before midnight BST. By now Mercury is above the north-western horizon from UK shores, having sped east of the Sun following superior conjunction on the far side of the Sun on 14 May. It reaches greatest eastern elongation on 15 June.

From London on 23 May at 21:40 BST, about 40 minutes after sunset, magnitude –1.3 Mercury lies just three degrees high (actually, Mercury is reduced in brightness to around +0.9 by significant extinction). Across an uninterrupted north-western horizon free of hazy conditions a pair of binoculars may spot it. During the last week of May, Mercury climbs slowly, by the end of the month reaching an altitude of around seven degrees for the same degree of twilight, though it fades to magnitude –0.6 (+0.5 with extinction).

Spot Eta Aquarids before dawn

When: predicted peak on 6/7 May

How to observe: A moon-free Eta Aquarid meteor shower is a fine sight when viewed from more southerly climes, but its shooting stars, a product of debris from Halley’s Comet (1P/Halley), the most famous naked-eye periodic comet, are not easy to see from the UK owing to a low radiant in the pre-dawn east-southeastern sky. South of the celestial equator a yield of 20 meteors an hour, on average, is common, though the radiant’s unfavourable position at mid-northern latitudes gives a short observing window, with observed rates less abundant. The Moon rises at 01:43 in Sagittarius, with glare from its 75 per cent waning gibbous phase becoming an increasing distraction towards dawn.

The Eta Aquarid radiant lies in north-western Aquarius, close to the Y-shaped ‘Water Jar’ asterism, formed from the stars eta (η), zeta (ζ), gamma (γ) and pi (π) Aquarii. It climbs to an altitude of about 15 degrees from the south of England by the time the pre-dawn twilight starts to lighten the sky at around 04:00.

Catch Jupiter early in May

When: early-evening sky

How to observe: Jupiter remains visible in the western sky after sunset throughout May. At the start of the month the dominant gas giant planet appears in the ebbing twilight from around 21:00 BST in the south of England (21:40 in Scotland). Jupiter is located around 40 degrees above the western horizon across the UK and can be observed through a telescope or imaged for about an hour or so. It sets at about 02:00 (02:35 in Scotland).

On 1 May at 21:30 Ganymede’s dark shadow is on Jupiter’s cloud tops, with Europa itself having just started a transit. By the end of the month Jupiter lies under 20 degrees high at 22:00 from the south of England, as reasonable darkness begins to descend at the start of nautical twilight when the Sun lies between 6 and 12 degrees below the horizon. Jupiter sets at 00:15. In Scotland, Jupiter is just about a write-off, deeply embedded in strong twilight lying around 15 degrees high at the end of civil twilight (when the Sun is six degrees below the horizon).

Pre-dawn Mars is a real challenge

When : late month’s pre-dawn sky

How to observe: Mars can be an infuriating planet to observe and image, coming to precious oppositions just every 26 months (average synodic period of 779.94 days) and often that opposition is unfavourable in terms of the red planet’s apparent diameter or position in the sky. Even at a favourable, perihelic, opposition Mars is at its best for a frustratingly small period of time before a painfully slow fade back to relative obscurity.

Mars was last at opposition on 16 January 2025 and the next does not take place until 19 February 2027. Mars passed through conjunction with the Sun on 9 January this year and so far has not emerged into the UK’s pre-dawn sky (more southern climes saw Mars reappear late March into April). Now it is back for the long haul to opposition. Mars is a very challenging target in the pre-dawn sky for those intrepid observers looking to spot it as early as possible in this apparition. On 31 May from London, Mars rises at 03:40 and lies 4 degrees high above the east-northeastern horizon (azimuth 72 degrees) shortly after 04:00 BST, the beginning of civil twilight. Mars’ magnitude of +1.3 is knocked back to +3, its brightness severely curtailed by the overwhelming air masses we have to look through this close to the horizon. Try sweeping with binoculars across a flat horizon if local conditions are favourable, but remember to make sure the Sun remains below your horizon (sunrise is at 04:50).

Saturn also emerges into the pre-dawn sky ahead of Mars this month, the ringed wonder lying 27 degrees farther west along the ecliptic. Saturn rises at about 03:00 from London and sits ten degrees up (azimuth 98 degrees) shortly after 04:00, its brightness of magnitude +0.9 reduced by atmospheric extinction to magnitude +1.6.

The BAA's MOONWATCH

Follow the fascinating observing opportunities the Moon offers, writes Tony Cook.