What to see in the sky in April: Venus and Mercury to meet in a conjunction

April marks 60 years since the first manned flight in history

On April 12th, 1961 – sixty years ago this month – we fulfilled a dream we had for millennia as Soviet cosmonaut Yuri Gagarin became the first human in space.

Less than 600 people have made the trip into orbit since that time, yet spaceflight nowadays does not garner headlines like the early missions. Trips are expected to become even more routine in the next decade as commercial ventures like Blue Origins and Virgin Galactic offer sub-orbital flights to the public.

For those of us earthbound though, we can journey far beyond our local confines each clear night just by looking up.

Following Orion as he slips towards the southwestern skyline these evenings are his faithful canine companions. The constellation of the lesser dog (Canis Minor) has only the prominent star Procyon but the greater dog (Canis Major) hots Sirius, the brightest diamond in the sky.


The name Sirius comes from the Greek “sparkling” or “scorching” and its brilliance does catch the eye, while another old designation translates as “of a thousand colours”.

That is a very apt description as Sirius never gets very high from Ireland’s latitude and shows a range of hues. The kaleidoscope effect is due to different wavelengths of the star’s light being smeared out as it passes through a denser mass of atmosphere when we see it low down.

All stars twinkle – or scintillate – to some extent, but the effect is dramatic with Sirius. The upshot is that starlight is scattered and recombined in a haphazard way while passing through our turbulent atmosphere before reaching telescopes and detectors. Astronomers living at the bottom of that ocean of air therefore need to somehow sharpen the view.

Professional observatories do have a trick up their sleeve and that is by using adaptive optics. The technique employs actuators mounted behind the mirrors of giant telescopes to rapidly flex their surface and compensate for atmospheric blurring. It allows us get images from the ground almost as good as those returned by observatories in orbit.

The diamond-like sparkling of Sirius is worth admiring though. Its rainbow colours are beautifully enhanced when you defocus your binoculars slightly while viewing the star and especially so when Sirius is just after rising or nears setting.

Sirius is also bright because it is nearby. The star lies 8.6 light years away and is about twice the mass of our sun and a little more than three times its diameter.

Other stars in Canis Major would blaze brighter than Sirius if they were as close but the constellation’s lucida has no compare at present in our sky for apparent brightness. You need to go back about 90,000 years to find a star that outshone Sirius, and fast-forward more than 200,000 years for when it will be demoted as top dog.

Huddled in the glare of Sirius is a companion aptly nicknamed the Pup. The star is a white dwarf only as big as the Earth, yet a teaspoonful of its material weighs as much as a family car. White dwarves are the corpses of stars that have evolved past old age and puffed off most of their material to expose the underlying core.

Sirius was important to some early civilisations as its first appearance in the morning sky before sunrise - what we call its helical rising - coincided with the annual flooding of the Nile river in ancient Egypt. This was a crucial time as when the flood waters receded, they left behind fertile soil.

The stifling heat around the beginning of August in ancient Rome led to that time of year becoming known as the dies caniculares (days of the dog). Sirius was then above the horizon during daylight hours, so it was believed the star’s heat was added to that of the Sun to give a run of scorching temperatures. The interval from early July to mid-August of Dante’s “great scourge of canicular days” is still referred to as the dog days of summer in modern times.

The Moon this month

Easter Sunday this year is on April 4th. That moveable feast is generally the first Sunday following the first full moon after the spring equinox. The actual calculation of Easter is slightly more intricate, but the aide-mémoire is effective.

The last quarter moon on April 4th coincides with Easter Sunday, while the new moon is on April 12th, and first quarter on the 20th. The full moon on the 27th is classed as a supermoon but next month’s full moon is the closest of the year.

The planets this month

Mercury swiftly emerges from the solar glare towards the end of April to become visible in our evening sky. The planet does not set until more than an hour after the Sun at the end of the month.

The innermost planet can be found a little below the Pleiades star cluster on the evening of April 30th. Scan the west-northwest skyline with binoculars after 9.30pm and you should catch Mercury’s star-like point which can appear slightly pinkish or pale whiskey in colour in the twilight.

Venus makes a welcome return this month and gleams low in the west-northwest after sunset during the second half of April. Venus and Mercury are less than two moon-widths apart on the evening of the 25th but sink fast in the twilight so find a clear horizon to spot them. Once you find Venus, use binoculars to glimpse nearby fainter Mercury.

Mars hangs around in our evening sky until the early hours - especially with summertime now in effect. It has now faded a little but is still a reasonably bright object as it charges through Taurus, before crossing into Gemini on the 25th. The Moon passes by Mars on the evening of the 17th.

Jupiter rises just over an hour before the Sun at the beginning of April but appears nearly two hours beforehand by the end of the month. It can be found in the east-southeast at this time amongst the dim stars of Capricornus.

The waning Moon is close to Jupiter on the morning of April 7th and later in the month, on the 26th, the planet crosses into Aquarius.

Saturn can be found in Capricornus and rises 90 minutes before the Sun on the 1st with that interval almost doubling by the end of April. The moon is a couple of days after last quarter when it is near Saturn on the 6th.

The Lyrid meteor shower reaches maximum on April 22nd, but rates are generally low. There have been surprise outbursts however, with the last occasion being in 1982.

Spaceflight this month

Two missions reach the International Space Station during April, with the first scheduled to lift off from Baikonur on the 9th and ferry three members of the latest Expedition crew to their home for the next six months.

The second operational Crew Dragon mission will be launched by SpaceX on April 22nd from the Kennedy Space Centre. Four astronauts from three different space agencies will replace the current Crew-1 participants who return to Earth in early May.

The ISS itself is only visible in the evening sky from Ireland during the first couple of days of April and then starts a series of morning passes from the end of the month.

The core module of China’s next generation space station Tianhe (“Harmony of the heavens”) is provisionally scheduled for launch at the end of April. The next few years will see it accommodate various crews and a gradual expansion of the facility as additional sections are bolted on.

John Flannery is a long-time amateur astronomer with an interest in the history and lore of the sky along with astronomical phenomena observable with the unaided eye. He is a member of the Irish Astronomical Society.