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The night sky for April 2016

Northern Hemisphere

Ian Morison tells us what we can see in the northern hemisphere night sky during April 2016.

Highlights of the month

April - still a great month to view Jupiter.
This is still a great month to observe Jupiter. It lies in the southern part of Leo, but still reaches an elevations of ~48 degrees when crossing the meridian during the evening. An interesting observation is that the Great Red Spot appears to be diminishing in size. At the beginning of the last century it spanned 40,000 km across but now appears to be only ~16,500 km across - less than half the size. It used to be said that 3 Earths could fit within it, but now it is only one. The shrinking rate appears to be accelerating and observations indicate that it is now reducing in size by ~580 miles per year. Will it eventually disappear?

The features seen in the Jovian atmosphere have been changing quite significantly over the last few years - for a while the South Equatorial Belt vanished completely but has now returned to its normal wide state.

April: Look for the Great Red Spot on Jupiter
The list below gives some of the best evening times during April to observe the Great Red Spot which should then lie on the central meridian of the planet.

April 3rd - 22:00 BST: Ganymede emerges from Jupiter's shadow
During the early evening, Jupiter will apear to have just 3 Gallilean satellites: Io and Callisto to its right and Europa to its left. Ganymede is hiding in Jupiter's shadow but will emerge just after 22:00 BST later in the evening.

April 6th: just before dawn - the Moon occults Venus
On the 6th of the month, the Moon and Venus will lie close together low in the eastern sky before dawn. At 08:28 BST, as observed from the centre of the UK, Venus will disappear behind the disk of the very thin crescent Moon whose phase will be just 2%. This will be quite an observing challenge and will need binoculars or a small telesocpe to observe along with a good low eastern horizon. BUT BEWARE NOT TO OBSERVE CLOSE TO THE SUN! If possible stand in the shadow of a wall to the left of your position. Ideally, using an equatorial mount, locate the Moon when it rises at 06:20 BST and continue tracking as it approaches and then occults Venus. As seen from the centre of the UK, it will emerge around 20 minutes later as it briefly passes behind the Moon's northern dark limb. The occultation will not be visible from Scotland and, in the northern part of the UK, Venus will be seen to graze along the Moon's rough northern edge. Venus will take ~60 seconds to disappear and ~70 seconds to emerge. NOTE: to show the occultation graphically, I have had to remove the Sun's glare - this will be a very difficult observation.

April 8th: 45 minutes after sunset - Mercury and a thin crescent Moon
Looking west after sunset and as darkness falls, Mercury will be seen just 6 degrees to the right and slightly up from the a very thin waxing crescent Moon.

April 16th - mid evening: A waxing Moon nears Jupiter
During the evening the Moon will be seen gradually nearing Jupiter, closing in to a separation of just over 4 degrees at 22:00 UT.

April 21st all night: The Moon at apogee
On the 21st the Moon, one day from full, reaches apogee, that is at its furthest distance from the Earth. So, on the following day, it will not appear as big - or as bright - as when the full Moon is at perigee, its closest approach to the Earth. Perhaps surprisingly, its angular diameter at apogee is 12% smaller that at perigee and, should a solar eclipse occur near apogee, the Moon's full shadow may not reach the Earth giving rise to what is called an annular eclipse.

April 16th and 29th: Two Great Lunar Craters
These are two good nights to observe two of the greatest craters on the Moon, Tycho and Copernicus, as the terminator is nearby. Tycho is towards the bottom of Moon in a densely cratered area called the Southern Lunar Highlands. It is a relatively young crater which is about 108 million years old. It is interesting in that it is thought to have been formed by the impact of one of the remnents of an asteroid that gave rise to the asteroid Baptistina. Another asteroid originating from the same breakup may well have caused the Chicxulub crater 65 million years ago. It has a diameter of 85 km and is nearly 5 km deep. At full Moon - seen in the image below - the rays of material that were ejected when it was formed can be see arcing across the surface. Copernicus is about 800 million years old and lies in the eastern Oceanus Procellarum beyond the end of the Apennine Mountains. It is 93 km wide and nearly 4 km deep and is a clasic "terraced" crater. Both can be seen with binoculars.

Observe the International Space Station
Use the link below to find when the space station will be visible in the next few days. In general, the space station can be seen either in the hour or so before dawn or the hour or so after sunset - this is because it is dark and yet the Sun is not too far below the horizon so that it can light up the space station. As the orbit only just gets up the the latitude of the UK it will usually be seen to the south, and is only visible for a minute or so at each sighting. Note that as it is in low-earth orbit the sighting details vary quite considerably across the UK. The NASA website linked to below gives details for several cities in the UK. (Across the world too for foreign visitors to this web page.)

Note: I observed the ISS three times recently and was amazed as to how bright it has become.

Find details of sighting possibilities from your location from: Location Index

See where the space station is now: Current Position

The Planets

Southern Hemisphere

Haritina Mogosanu from the Carter Observatory in New Zealand speaks about the southern hemisphere night sky during April 2016.

This campfire story is dedicated to Stuart @astronomyblog

Welcome to the month of April. My name is Haritina Mogosanu and tonight I'm your starryteller from Space Place at Carter Observatory in Aotearoa New Zealand.

I love the Milky Way. The Milky Way is the most spectacular feature of the Southern Hemisphere but to say that is such an understatement. The Milky Way is so striking here and I believe that in the absence of a polar star (which I found hard to find in the Northern Hemisphere anyway), people could even orient themselves by the Milky Way. And why not? We can easily see the Milky Way from Wellington, which according to Lonely Planet is the the coolest little capital in the world. But is still a city, which means that it does come with light pollution and from most of the cities of the world we are lucky to see just the brightest stars. Yet I have noticed when walking home at night from the Observatory, from my street I can still see the Galaxy. I call it My City of Stars. There are times when I look up and gaze straight at the center of it. This time of the year just after sunset I can see from the centre to the edge from Scorpius to Taurus, in one glorious panorama.

So in April, my beautiful City of Stars is stretching through the night sky from northwest to southeast. Allow your gaze to wander along this celestial tapestry and you will see the brightest stars. Let's start from West. Lining up onto the celestial river are:

Nearby comes Canopus -0.72, the second brightest star in the sky. Canopus is not in the white band of the Milky Way. Standing tall, Canopus is high in the sky. Canopus is a circumpolar star from Wellington, which means that it goes around in circles in 23 hours and 56 minutes, riding the celestial Ferris wheel of the Southern Skies, a giant wheel that never stops, day after day, in a sidereal time cycle, as long as the Earth is turning.

Besides Canopus, there are other stars lighting the gondolas of the big wheel but not each and every gondola has a bright star inside. If Canopus is on the top of the big wheel then just imagine that the diameter of the wheel is from Canopus to the horizon. Looking clockwise from Canopus in the 4 o'clock position on the wheel is the Lone Star, Achernar. Achernar marks the end of the grand river Eridanus, the river-asterism that flows all the way from Orion to the southern world. At 0.4 magnitude it shines bright in a region that seems devoid of other stars. Lower down, a peacock (Pavo) takes a ride on the wheel. It's main star, which carries the mundane name of Alpha Pavonis (which literally means the brightest star in Pavo), is in the 7 o'clock position on the giant turning wheel, almost as if is just hanging on the side.

Following the imaginary curve of the wheel, two very bright stars show up closer to the 10 o'clock position. Firstly, the third brightest star in the sky and our closest neighbour, Alpha Centauri, and then Beta Centauri. They point up at the Southern Cross which is even higher than them in the sky at this time of the year. And one of my favourites, the hypergiant Eta Carinae is somewhere in between Canopus and the Southern Cross. All these stars make the imaginary big wheel.

The sky looks almost devoid of stars anywhere inside my celestial Ferris wheel, with two exceptions. Let's split it in two with a diametral line that links the Alpha and Gamma Crucis, stars of the Southern Cross to lonely Achernar. On the same side as the pointers of the Southern Cross, you will find the Small Magellanic Cloud, a beautiful bright galaxy, that looks to the untrained eye (like mine) like a cirrus cloud hanging in space, 200,000 light years away. On the other side of the semicircle, another galaxy, the Large Magellanic Cloud compensates its loneliness by its size, from 150,000 light years away. These so called clouds that neighbour our galactic presence are visually two thirds away from the Southern cross and one third from Achernar. There is nothing else too bright within the big wheel, maybe because the wheel is inhabited by this giant spider, the Tarantula Nebula that has its nest inside the Large Magellanic Cloud. You can see its beautiful wisps through a telescope, although it is very faint. The tarantula nebula is a star-forming region, also known as 30 Doradus, and according to NASA is one of the largest star forming regions, located close to the Milky Way. About 2,400 massive stars in the center of 30 Doradus produce intense radiation and powerful winds as they blow off material into space.

While the Large Magellanic Cloud is enormous on a human scale, it is in fact less than one tenth the mass of our home galaxy. It spans just 14,000 light-years compared to about 100,000 light-years for the Milky Way and it is classified as an irregular dwarf galaxy. The ESO astronomers believe that its irregularity, combined with its prominent central bar of stars suggests to astronomers that tidal interactions with the Milky Way and fellow Local Group galaxy, the Small Magellanic Cloud, could have distorted its shape from a classic barred spiral into its modern, more chaotic form.

Crux, the Southern Cross, is no stranger to the northern hemisphere and it was entirely visible as far north as Britain in the fourth millennium BC. The Greeks could see it too but since then, the precession of the equinoxes, the wobble of Earth, its gyroscopic dance on the orbit has changed the skies a lot so that now Crux is only visible in the Northern Hemisphere from as far south as 25 degrees latitude north. Florida Keys, Puerto Rico, the islands of the Caribbean, as well as Hawaii are its northern limit of visibility. Near the Southern Cross, there is a dark patch of dust that masks the light that comes from the stars behind it and that is known as the coalsack. Inside the coalsack, the Jewel Box is one of my favourite sights that I visit over and over with the telescope.

Lower down on the path of the Milky Way the two pointers look now as if they are hanging from the Southern Cross. First comes Beta Centauri then the famous Alpha Centauri. For Maori they are also known in a different time of the year as the rope of an anchor. Here in Aotearoa, the Maori have three names for the same asterisms (groupings of stars) at different times of the year. What we know as Scorpius is now called Manaia Ki Te Rangi, the guardian of the skies. The messenger between the earthly world of mortals and the domain of the spirits, Mania also resembles to a seahorse and its symbol is used as a guardian against evil. Often you will see Maori people wearing a greenstone in Maori named pounamu Manaia as a taonga, a necklace.

Lower on the Horizon, at a magnitude of +0.95, red giant Antares shines as the brightest star in Scorpius. Right next to it, its rival, Ares by its Greek name, or Mars as we all know it better, is challenging the giant's red hue with its own red glimmer. This is how Antares got its name, as being the rival of Ares, Ant-Ares, the rival of Mars.

As the Milky Way splits the sky into two sectors, through the northeastern horizon runs the ecliptic, a lower arch, the plane of our solar system bearing the zodiacal constellations. They intersect the Milky Way right on the horizon. First to set on the western horizon, is Taurus and of it, just Aldebaran is left gleaming faintly as it passes beyond the edge of the world. The arch of the ecliptic climbs through Gemini, holder of the two bright stars Castor and Pollux, then higher up, Cancer is almost invisible to the untrained eye, a good peripheral vision training object. Leo, with the Royal Star Regulus is now host to the bright planet Jupiter, then comes Virgo with its bright star Spica, then Libra with Zubenelgenubi and Zubeneschamali the severed claws of Scorpius repurposed into a balance for Justice by the Roman Emperor Julius Caesar. Finally the arch curves down onto the western horizon where Scorpius with red Antares is carrying Red Mars. They appear around 10 PM followed by Saturn about forty minutes later. Mars will brighten steadily through the month as we catch up on it. Its distance shrinks from 118 million km away at the beginning of April to 88 million km away at the end of the month. It remains a small object in a telescope. According to our very own Alan Gilmore who received a lot fan mail about the subject, as probably did all of us, in the mid-month a telescope needs to magnify 130 times to make Mars look as big as the Moon does to the naked eye.

Saturn rises after 10:20 pm NZDT at the beginning of April; around 7:20 NZST by month's end. This also means that daylight saving starts soon and with it we will get an extra hour of sleep. Saturn is straight below Antares. If you have never seen Saturn through a telescope, the hunting season is about to open. A small telescope shows Saturn as an oval, the rings and planet blended. Larger telescopes separate the planet and rings and may show Saturn's moons looking like faint stars close to the planet. The best comment that I hear over and over from people looking through the telescope at Saturn for the first time after the ubiquitous wow is how much Saturn looks like... Saturn. Titan, one of the biggest moons in the solar system, orbits about four ring diameters from the planet. Saturn is 1400 million km away mid-month. Mercury might be seen setting in the bright twilight mid-month. It looks like a lone bright star on the northwest skyline.

This almost concludes our Night Sky South report for April 2016 but before I leave you with the peace of the night sky, I just want to quickly show you only two deep sky objects visually close to Jupiter, currently the luminary of the night sky. Jupiter is in Leo. Neighbouring Leo are Sextans and Hydra. Sextans is a "minor" equatorial constellation, a designation that made me smile. This constellation was actually invented by the famous stellar cartographer Johannes Hevelius to celebrate his sextant, a beloved instrument he used to map the sky. A copy of his famous maps adorns the ceiling of our beautiful library inside Space Place at Carter Observatory. Unknown to Hevelius, inside the celestial Sextant there is a bright galaxy NGC 3115, also known as the Spindle Galaxy. According to NASA, this field lenticular galaxy, several times bigger than the Milky Way, holds the nearest billion-solar-mass black hole to Earth whereas our supermassive black hole at the center of the Milky Way galaxy, called Sagittarius A, has a mass only equal to about 4 million suns.

The other object that I want to show you is inside the largest of the 88 constellations in the sky, Hydra, and close to the current position of Jupiter. The remains of a dying star form a planetary nebula called NGC 3242 and nicknamed "The Ghost Of Jupiter". A planetary nebula is a slowly dying star, a star that is not too big not too small, anything say in the range of 0.8 - 8 solar masses. Planetary nebulae are beautifully coloured and it is believed that they may play a crucial role in the chemical evolution of the Milky Way, blowing out their chemical elements to the interstellar medium. Now these are the same chemical elements that make our bones, construct our skin, and basically are both the building bricks of who we are and what keeps us alive. And all these chemical elements we have on Earth have all been through the hearts of stars. I get many comments a lot of times from people telling me how small and daunted, dwarfed and insignificant they feel when they look at the stars. And that they deliberately avoid looking up. It took me many years to get my head around this but when I look up to the sky, I know for sure that I am made of stardust, and that makes me glow every day.

From Space Place at Carter Observatory here in the southern hemisphere I wish a you clear and dark skies so that we can always see the stars and remember that we are made of the same stars dust as they are.

Special Thanks go to the amazing Rhian Sheehan, Peter Detterline, Chief Astronomer of the Mars Society, Alan Gilmore from University of Canterbury and to Toa Nutone Wii Te Arei Waaka from the Society for Maori Astronomy and Traditions.

Previous episodes:

The Night Sky This Month is one part of the Jodcast. The full show contains the latest news, interviews with astronomers, answers to listener questions and more.

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