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THE NIGHT SKY FOR APRIL 2017

Northern Hemisphere

Ian Morison tells us what we can see in the northern hemisphere night sky during April 2017.

The Stars and Galaxies

Constellations and Stars

Orion setting in the west, with upper part of Taurus and the Pleiades still visible. Also above Orion Gemini is visible with the stars Caster above and Pollux below. Also below Gemini is Procyon the only bright star in Canis Minor. Leo is visible in the south with a wonderful region called the Realm of the Galaxies just behind the tail. Between Leo's tail and the bright star Arcturus (in Bootes) is Virgo and Coma Berenices with many Messier objects in that region. High overhead is Ursa Major with the Plough. The central star of the handle, Mizar, is a double star (with Alcor) and observable in a telescope is faint red star up to the right of the pair.

The Planets

  • Jupiter
    comes into opposition on April 7th, lying in Virgo initially some 6 degrees above its brightest star, Spica. Visible all night, It will be due south at an elevation of 34 degrees at around midnight UT. The size of Jupiter's disk decreases slightly from 44.2 to 43.6 arc seconds as February progresses with its magnitude reducing very slightly from -2.5 to -2.4. With a small telescope one should be easily able to see the equatorial bands in the atmosphere, sometimes the Great Red Spot and up to four of the Galilean moons as they weave their way around it.
  • Saturn
    rises around midnight (UT) and will be highest in the pre-dawn sky. Lying in the western part of Sagittarius, its diameter increases from 17 to 18 arc seconds during the month as it brightness increases slightly from magnitude +0.4 to +0.3. It will be high enough in the south-east in the hours before dawn to make out the beautiful ring system which, at over 26 degrees to the line of sight, are nearly as open as they ever become. If only it were higher in the ecliptic; its elevation this year never gets above ~18 degrees and so the atmosphere will hinder our view of this most beautiful planet. [Note: I have just acquired a ZWO Atmospheric Dispersion Corrector which uses two contra-rotating prisms to combat the dispersion of the atmosphere at low elevations.]
  • Mercury
    passed through superior conjunction on March 7th and, on April 1st, will lie ~14 degrees above the western horizon at nightfall when it is at its greatest elongation, some 19 degrees, from the Sun. Then at magnitude -0.2, it brightness drops to magnitude +3 by the 18th of the month as it falls back towards the Sun. Mercury passes through inferior conjunction on the 20th and will reappear in the predawn sky by the end of the month. With an angular size of to 7.5 arc seconds on the 1st, increasing to 11 arc seconds on the 18th, no details would be expected to be seen on its disk.
  • Mars
    As April begins, Mars lies in Aries but moves into Taurus on the 12th of the month. In early April, Mars has an elevation of ~20 degrees above the western horizon at sunset, but this reduces to ~11 degrees by month's end. On the 16th, it lies 4 degrees below the Pleaides cluster and then passes between the Pleiades and Hyades clusters on the 25th when it lies some 9 degrees to the right of Aldebaran. Its brightness falls slightly during the month from magnitude +1.5 to +1.6 whilst its angular diameter falls from 4.2 to 3.9 arc seconds. No details would be expected to be seen on its salmon-pink surface.
  • Venus
    rises in the east about an hour before sunrise on the first of the month and then climbs a little higher each morning as April progresses. On April 1st, the disk, forming a slender crescent nearly one arc minute tall, is just 2 percent lit shining with a magnitude of -4.2. By the end of the month, Venus has its maximum brightness of magnitude -4.7 with its angular size reduced to 39 arc seconds and its illuminated fraction increased to 26 percent. It will then have an elevation of ~13 degrees at sunrise. In daytime when still high in the sky it can be imaged in the infrared as the blue light from the sky is filtered out.

Highlights

April - a great month to view Jupiter.This is a great month to observe Jupiter which comes into opposition on April 7th. It is moving down the ecliptic and, at the start of April, lies in Virgo some 6 degrees above Spica (Alpha Virginis). It now reaches an elevations of ~36 degrees when crossing the meridian. 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 JupiterThe night sky page gives a list of 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. The times are in UT.

1st to 7th April - early evening after dusk: Mercury at its highest in the skyIf clear on the evenings of the first week of April, Mercury will be seen above the western horizon after sunset. Then it will have an elevation of some 18 degrees - so an excellent week to observe a somewhat elusive planet.

April 7/8th - all night: The waxing Moon closes on Regulus in LeoDuring the darkness hours of the night of the 7/8th of April, a waxing Moon closes on Regulus and is within 2 degrees as dawn approaches on the 8th.

10th April - all night: The Moon, one day before full, passes Jupiter in Virgo. If clear on the evening of the 10th and looking first to the south-east, one will see the Moon, one day before full, passing just 2 degrees above Jupiter in Virgo.

22nd April - after midnight: The peak of the Lyrid Meteors. Without any moonlight to hinder our view and from a dark rural location one, if clear, would have a chance of observing the peak of the Lyrid meteor shower with up to 10 meteors visible each hour. As one might expect, the shower's radiant is close to Vega in Lyra.

25th April - 1 hour after sunset: Mars passes between the Hyades and Pleiades Clusters. Looking low in the west after sunset on the evening of the 25th, Mars will be seen to lie directly between the Hyades (to its left) and Pleiades clusters - if clear, a nice photographic opportunity.

April 28th - a daylight occultation of AldebaranBefore dusk on the evening of the 28th, Aldebaran is occulted by a very thin crescent Moon. It will disappear behind the Moon's dark limb at 19:11 BST as seen from London and 19:07 BST as seen from Edinburgh and reappear at the bright limb at 20:07 BST as seen from London and 19:57 BST from Edinburgh. As darkness falls, Aldebaran will be seen to lie just below the Moon.

March 5th and 18th: An interesting valley on the Moon: The Alpine ValleyThese are two good nights to observe an interesting feature on the Moon if you have a small telescope. Close to the limb is the Appenine mountain chain that marks the edge of Mare Imbrium. Towards the upper end you should see the cleft across them called the Alpine valley. It is about 7 miles wide and 79 miles long. As shown in the image is a thin rill runs along its length which is quite a challenge to observe. The dark crater Plato will also be visible nearby. You may also see the shadow cast by the mountain Mons Piton lying not far away in Mare Imbrium. This is a very interesting region of the Moon!

Southern Hemisphere

Claire Bretherton from the Carter Observatory in New Zealand speaks about the southern hemisphere night sky during April 2017.

Kia ora and welcome to the April Jodcast from Space Place at Carter Observatory in Wellington, New Zealand.

New Zealand daylight saving ends the first weekend of the month, bringing our southern hemisphere summer abruptly to an end. On the bright side, the lighter mornings and darker nights will make it much easier to get out and do some observing.

One of the first objects you'll see in the evening twilight is bright golden Jupiter, rising in the east soon after the sun sets at the beginning of the month. It slowly crosses the sky through the course of the night, before disappearing in the west at sunrise. By the end of the month it will be rising well before dusk. Jupiter reaches opposition on April the 8th, meaning it will lie directly opposite the Sun in the sky and be overhead at midnight. At this time Jupiter is also at its closest to Earth, but the difference in angular size and brightness is not really noticeable to the naked eye.

Jupiter is always worth a look through a small telescope or good binoculars, revealing up to four of its large Galilean moons. The full Moon passes close to Jupiter on the 10th/11th of the month.

Jupiter sits in the constellation of Virgo, just to the left of the brightest star Spica. Last month I mentioned that Virgo is home to the Virgo Cluster of Galaxies, containing up to 2000 members. This month we'll take a closer look at some of those galaxies in more detail.

The cluster's centre lies around 54 Million light years away and it extends nearly 8 degrees across the sky. Many of the brighter galaxies are included in Messier's catalogue of non-cometary fuzzy objects and are easily visible with a small telescope. Perhaps the most famous member is the giant elliptical galaxy Messier 87, located close to the cluster centre. The second brightest galaxy in the northern part of the cluster, with an apparent magnitude of 9.59, M87 is easily observed with a modest 60mm telescope, and is in reach of a good pair of binoculars under excellent conditions, visible as a faint, hazy patch of light. Small telescopes may reveal the galaxy's elliptical shape, brightening towards the centre.

M87 is one of the most massive and luminous galaxies in the local Universe, estimated to contain the mass of around 2.7 trillion Suns, some 200 times that of the Milky Way, but only around one sixth of this mass is in the form of an estimated 1 trillion stars. It is also distinctive for its large number of globular clusters, with over 6 times as many as our own Galaxy. Close to its core is a 3.5 billion solar mass black hole, one of the most massive known, orbited by a fast moving disk of ionized gas, which is a strong source of radiation, particularly at radio wavelengths. In fact M87 is one of the brightest radio sources in the sky.

In 1918 American astronomer Heber Curtis detected a "curious straight ray" extending from the galaxy's centre, which we now know is a jet of energetic plasma blasted out from the nucleus of M87 at relativistic speeds. The central black hole of M87 is actually offset from the core by around 25 parsecs in the direction opposite to the jet, suggesting that the jet may be responsible for accelerating the black hole away from the galaxy's centre. An alternative theory suggesst that the displacement my have been caused by a merger with another galaxy.

To find M87, draw a line from Denebola at the tail of Leo to Vindemiatrix (or Epsilon Virginis), and the galaxy can be found just over half way along, close to Virgo's border with the Coma Berenices constellation.

Around 1.5 degrees back towards Leo is the M84/M86 galaxy pair, located just 17 arcminutes apart. Whilst M86 can be seen in 10x50 binoculars under good conditions, larger binoculars or a small telescope will be needed to pick up its companion easily. The two are visible in the same field of view, and in a 20cm or larger telescope a number of fainter galaxies can be seen nearby, including NGC 4435, NGC 4388, NGC 4402 and NGC 4438.

M86 and M87 are thought to be moving towards each other for their first galactic encounter.

The brightest galaxy in the cluster is the 9.4 magnitude Messier 49, located a little above M87 in our evening skies. M49 was the first member of the Virgo cluster to be found, and only the second galaxy to be discovered outside our local group. Messier 49 is interacting with the nearby dwarf irregular galaxy UGC 7636, which has a trail of debris covering 1 x 5 arcminutes of the sky.

M49 can be seen in large binoculars and small telescopes, with slightly larger telescopes picking up a bright core and large halo, but an otherwise featureless view.

Further around to the north in our evening sky, to the other side of Denebola is the main body of Leo, with the constellations brightest star Regulus or Alpha Leonis marking the lion's heart. Regulus is at the top of an upside down question mark as we see it here in New Zealand, which marks the head and mane of the Lion. With an apparent magnitude of 1.35, the star is the 21st brightest in the night sky, but it is in fact a system of four individual stars arranged in two pairs.

Regulus A is a spectroscopic binary comprising of a hot, young, blue-white main sequence star with a tiny companion of less than 0.3 solar masses, which is probably a white dwarf. Regulus B and C make a second pairing located 177 arc-seconds away from Regulus A. Resolving the BC pair from Regulus A is a good challenge for binocular observers, and certainly achievable with a small telescope.

A little below Regulus is another well known double star called Algieba or the Mane. First discovered by William Herschel in 1782, Algieba comprises a yellow-orange giant primary and a yellow-white giant secondary at magnitudes 2.3 and 3.5 respectively. The pair have an angular separation of around 4 arcseconds, so you won't be able to resolve them in binoculars, but with a telescope of aperture around 8cm or greater you should be able to split them.

Like its neighbouring Virgo, Leo is also home to a number of bright galaxies including the Leo Triplet, a small group of interacting galaxies consisting of spiral galaxies M65, M66 and NGC 3628. Often known as the M66 group, the Leo Triplet is located around 35 million light years away, and provides a fantastic opportunity to study galaxy interaction in our local Universe. Each of the three main members show signs of tidal disturbance with NGC 3628 exhibiting an impressive tidal tail extending for over 300,000 light years.

The triplet is located fairly close to Denebola, or Beta Leonis and around half way between Chertan, or Theta Leonis and Iota Leonis. Most small telescopes should be able to pick up the group, but M66, the brightest of the three, and M65, the second brightest, should also be visible in large binoculars.

This month the new moon falls on the 27th, so around this time will be the best opportunity for galaxy spotting.

Moving around towards the southeast, our winter constellation of Scorpius is rising in our evening skies at the beginning of the month and by around 11pm is joined by cream-coloured Saturn, sat a little below. By the end of the month Saturn will be rising shortly after 8pm. The Moon will be just to the left of Saturn on the night of the 16th.

Venus is moving quickly into our morning skies, rising a little over an hour before the Sun at the start of the month and 3 hours before at the end, when it will be joined in the dawn skies by faint Mercury sat lower and to the right of bright Venus.

Wishing you clear skies, and happy galaxy hunting, from the team here at Space Place at Carter Observatory.

Compiled by Ian Morison