The Sky Above You – April 2024
By Duncan Lunan
The Moon will be New on April 8th, and Full on April 23rd. The Moon passes Jupiter on the early evening of the 10th, and no other visible planets. Next month, unusually, there will be no planets visible in the morning and evening sky. The April edition of Astronomy Now urges us to take the opportunity to spot the crescent Moon as soon as possible after the 8th, because soon after the spring equinox the Moon’s orbit makes a large angle with the horizon, and the Moon will be much lower in the sky around the Autumn equinox in September.
On April 8th there will be a total eclipse of the Sun on a track crossing North America, and if they don’t check for updates, some people who expect to see it may miss it. According to an article in EarthSky on April 2nd (Dave Adalian, ‘April 8th Eclipse Maps Are Wrong Along the Edges’, quoting one by James Carter on Forbes.com (‘Why Your Total Eclipse Map Is Now Wrong (and Where to Find the New One)’, March 30th), the apparent diameter of the Sun has been remeasured and found to be greater than previously measured, by 0.02 arc seconds. It narrows the eclipse track by approximately 2000 feet, very little when the track is 115 miles wide, but enough to make a big difference in the Cité Jardin of Montreal, for example, where the park—where many people may go to watch from the Olympic Village and the Olympic Stadium – will now lie outside the track. I’m giving the full references for this story because the revised map was issued on April 1st, and if the articles it accompanies hadn’t been published on March 30th and April 2nd, one might have one’s doubts.
There’s growing excitement about whether Venus, Jupiter and Comet Pons-Brooks will be visible during it, all of them near the Sun. Images of the comet near the M31 galaxy in Andromeda, which is visible to the naked eye in good conditions, suggest that it might be. The comet has had outbursts, on July 20th, October 5th, November 1st and 14th, December 14th and January 18th, bringing it to near naked-eye brightness. It may be visible from here at predicted maximum brightness, in Taurus, low down in the west on April 21st, after passing below Jupiter on April 12th, and I’ve heard from a friend in Australia who has high hopes, literally. Closest approach to Earth will be on June 2nd, when it will no longer be visible from here.
After the Nova-C ‘Odyssey’ lander fell silent on February 29th, the causes of the crash near the lunar south pole became more clear. The much-vaunted fix of replacing the main laser altimeter with an experimental NASA one had almost worked, but the spacecraft computer could not accept the data due to a missing phrase in the software patch. Without it, using only optical data, Nova-C nearly achieved a soft landing, but it touched down at six metres per second vertically, two metres horizontally, broke a landing leg and fell over. The last transmitted image showed the crescent Earth to the left of the Sun. There were hopes that it might survive the lunar night and resume signals 14 Earth days later, and these were boosted when the Japanese SLIM lander did survive the night, sending more pictures of its landing site, though it wasn’t designed to. Still more remarkably, in late March SLIM reawakened after a second lunar night, and continues to add to its coverage of the landing site, though it remains stuck in its nose-down position.
The SOHO spacecraft, which has been orbiting at the Sun-Earth Lagrange 1 point since 1995, has discovered its 5000th Sun-grazing comet – a remarkable achievement, because it wasn’t intended to do that, and only a few comets passing very close to the Sun had been observed previously. They’re thought to be fragments of much larger comets which broke up in close passes, and many of them don’t survive doing it again. The Parker probe, much closer to the Sun and getting closer on each pass, has encountered multiple Coronal Mass Emissions, high-energy clouds of particles emitted from the Sun during eruptions and causing auroral displays when they enter the atmosphere after overloading the magnetic field.
The planet Mercury is invisible this month, as are Venus and Mars, still out of sight beyond the Sun, and as all the planets will be by the end of next month. Mercury reaches inferior conjunction on this side of the Sun on April 11th, and although the April edition of Astronomy Now suggests that it might be visible at the beginning of the month, as usual serious caution is needed if searching for it so near sunset. NASA’s Marshall Space Flight Center has proposed a new mission, Mercury-Scout, which would achieve a close orbit around Mercury and photograph features of interest at 1-metre resolution or better. Mercury-Scout would be propelled by solar sail, reacting to light pressure. Although the principle was formulated more than a century ago, and demonstrated with the Mariner probes and Echo satellites of the 1960s, the only interplanetary sail to date has been Japan’s Ikaros, which entered high orbit around Venus in December 2015. Venus will be occulted by the Moon in daylight on April 7th, seen from Central America, Mexico and northern and eastern USA. There’s good news meanwhile about NASA’s VERITAS probe to Venus, which is scheduled for 2031 launch after a pause in funding last year.
In the continuing study of water below the surface of Mars, ESA’s Mars Express continues to play a major role. Its MARSIS penetrating radar booms weren’t deployed until well after its arrival in December 2023, and there was concern at the time because one of them wasn’t fully locked. But in operation it’s been a success, and has now discovered a body of ice below the surface which is large enough to be called an ocean.
When Mariner 9 reached orbit around Mars in 1971, one of the first features to emerge from the planet-wide dust storm was an elaborate structure of linked chasms named Noctis Labyrinthis, the labyrinth of night. It lies east of the Tharsis Ridge which bears three great volcanoes, dwarfed only the still larger Olympus Mons further west. To appreciate the huge size of these features it helps to visualise them superimposed on the continental USA. Its origin has always been obscure, but had to be connected with the downward flow of terrain from the Tharsis Ridge and Valles Marineris, the huge rift valley to the east, whose floor has been scoured by enormous floods in the remote past. Now it turns out, from a study comparing images of the region from Mariner 9 to the present, that between Noctis Labyrinthis and the head of Valles Marineris the chaotic landforms are the remains of another shield volcano, 200 miles across at the base, which has been erased by a network of outflow channels linking the Ridge to the Valley.
The intriguing question is, where did the water come from? We now know that huge amounts of it were deposited below the crust of Mars in its early history – see above. There’s a long-standing idea that the Tharsis Ridge might have been uplifted by shockwaves from the huge impact which formed the Hellas Basin, on the far side of the planet, possibly changing the tilt of its axis. Could that also have released enough water to break up the Noctis volcano? That would have been one hell of a spectacle. It’s well known that Ailsa Craig in the Firth of Clyde is the granite plug of an ancient volcano, part of the chain that runs across central Scotland through Dumbarton Rock, Stirling, Arthur’s Seat and the Bass Rock in the Firth of Forth. The conical mountain which surrounded the Craig has long since been washed away by the sea (though as my sister remarked, they must have used a very strong detergent). But its destruction is nothing to what happened on Mars.
Jupiter, in Taurus, is moving towards the Hyades cluster, setting at 10.00 p.m., and the Moon passes Jupiter on March 10th, when Uranus will be 2 degrees above the Moon to the left. On March 14, Jupiter and Uranus were at heliocentric conjunction - in the same direction as seen from the sun - for the first time since April 21, 1941, just under 83 years ago. On April 20th they reach conjunction as seen from here, separated by the diameter of the Full Moon.
There’s an argument going on about the composition of the seafloor on the ‘water moon’ Europa. The James Webb Space Telescope has confirmed the existence of oxygen, emanating from water deposits on the surface, but there’s not as much of it as expected. It suggests that the ocean floor, up to 200 miles below Europa’s ice crust, may be a solid plate without the volcanic vents which might support the evolution of life, and there may be fewer outbreaks to the surface than anticipated, with less possible evidence of life for NASA’s Europa Clipper probe and ESA’s Jupiter Ice Moons Explorer (JUICE, now en route) to find when they get there. But outbreaks do occur: they’ve been detected from here, and the Galileo spacecraft flew through one of them in 2014. But for them, Europa would be covered in sulphur from the volcanoes on Io, the next large moon inward. Sulphur is found along the cracks in Europa’s ice, and it’s not clear whether those a recent deposits or whether it migrates towards the cracks and into the interior, again possibly forming an energy source for life. The argument continues.
Saturn, in Aquarius, is already no longer visible. It’s increasingly clear from the data compiled by the Cassini Saturn orbiter that there’s sub-surface water below the south pole of the icy moon Enceladus, erupting in geysers due to tidal forces and resurfacing the hemisphere of Enceladus nearest them. The difference between the two hemispheres was striking even in the images obtained by the Voyager spacecraft in 1980-1981, even though the closest images were lost due to a temporary jamming of Voyager 2’s camera platform. Cassini’s flybys through the plumes identified salt and organic compounds, though it wasn’t possible to identify them, but it could be done with the right instrumentation and calls for a follow-on mission are growing.
Uranus, on the left side of Aries, sets at 10 p.m.. Uranus appears near the Moon on the 8th.
Neptune too has disappeared from the night sky for now.
The Lyrid meteors from Comet Thatcher (no relation) peak on the night of 22nd/23rd April, but will be spoiled by the Full Moon, (‘ruined’ according to Nigel Henbest’s Stargazing 2024).
ESA’s Euclid space telescope, launched in July 2023 to search for evidence of dark matter and dark energy, has been experiencing a build-up of ice on its supercold mirror as individual molecules of water escape from its structure. The problem was anticipated and engineers have successfully heated the mirror and cooled it down, driving off the ice, and expect to do so again during the spacecraft’s life.
The Voyager 1 spacecraft, in interstellar space 45 years after launch, has been sending back garbled data due to a memory fault which has developed in its Flight Data System. Engineers have now managed to command a full download from the FDS, and its hoped that the fault can be found within that and remedied, though both Voyagers are expected finally to run out of power around 2030.
(Below) The night sky about 9 p.m. (GMT/UT). mid-April 2024
From a drawing by Jim Barker
Uranus, Jupiter and Comet Pons-Brooks are all just off the map, in that order, to the right of the Hyades in Taurus (Aldebaran is the brightest star in the cluster).
The Sky Above You
By Duncan Lunan
About this Column
I began writing this column in early 1983 at the suggestion of the late Chris Boyce. At that time the Post Office would allow 1000 free mailings to start a new business, just under the number of small press newspapers in the UK at the time. I printed a flyer with the help of John Braithwaite (of Braithwaite Telescopes) offering a three-part column for £5, with the sky this month, a series of articles for beginners, and a monthly news feature. The column ran from May 1983 to May 1993 in various newspapers and magazines, but never in more than five outlets at a time, although every one of those 1000-plus papers would have included an astrology column. Since then it’s appeared sporadically in a range of publications including The Southsider in Glasgow and the Dalyan Courier in Turkey, but most often, normally three times per year, in Jeff Hawke’s Cosmos from the first issue in March 2003 until the last in January 2018. It continues to appear monthly in Troon's Going Out and Orkney News. Enquries from other outlets welcomed!
The monthly maps for the column were drawn for me by Jim Barker, based on similar, uncredited ones in Dr. Leon Hausman’s “Astronomy Handbook” (Fawcett Publications, 1956). Jim had to redraw or elongate several of them because they were drawn for mid-US latitudes, about 40 degrees North, making them usable over most of the northern hemisphere. The biggest change needed was in November when only Dubhe, Merak and Megrez of the Big Dipper, as the US version called it, were visible at that latitude. In the UK, all the stars of the Plough are circumpolar, always above the horizon. We decided to keep an insert in the January map showing the position of M42, the Great Nebula in the Sword of Orion, and for that reason, to stick with the set time of 9 p.m., (10 p.m. BST in summer), although in Scotland the sky isn’t dark then during June and July.
To use the maps in theory you should hold them overhead, aligning the North edge to true north, marked by Polaris and indicated by Dubhe and Merak, the Pointers. It’s more practical to hold the map in front of you when looking south and then rotate it as you face east, south and west. Some readers are confused because east is on the left, opposite to terrestrial maps, but that’s because they’re the other way up. When you’re facing south and looking at the sky, east is on your left.
The star patterns are the same for each month of each year, and only the positions of the planets change. (“Astronomy Handbook” accidentally shows Saturn in Virgo during May, showing that the maps weren’t originally drawn for the Hausman book.) Consequently regular readers for a year will by then have built up a complete set of twelve.
©DuncanLunan2013