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« March 2021


The Night Sky April 2021


Compiled by Ian Morison




This page, updated monthly, will let you know some of the things that you can look out for in the night sky.  It lists the phases of the Moon, where you will see the naked-eye planets and describes some of the prominent constellations in the night sky during the month.


New(ish)

The author's: Astronomy Digest

which, over time, will provide useful and, I hope, interesting articles for all amateur astronomers.   A further aim is to update and add new material to link with the books recently published by Cambridge University Press and which are described on the home page of the digest.   It now includes over 100 illustrated articles.



Image of the Month

M16-Infrared

The Eagle Nebula in Infrared
Image: NASA, ESA, HSTI processed by Luis Romero

Perhaps the most famous (and beautiful) image captured by the Hubble Space Telescope is of the Eagle Nebula associated with the open star cluster M16 and termed 'The Pillars of Creation' seen lower right. It was taken in the light of the Sodium II line - translated to red, the H-alpha line translated to green and the O III line translated to blue.   So this is a 'false colour' image whose true colour should really be predominatly red.   The image above has been captured by the HST in the near infrared whose light can penetrate the thick dust clouds to bring out the internal structure of the region that lies at a distance of ~6,500 light years.

Pillars of Creation

Image: NASA, EASA HSTI

Highlights of the Month


April 6th - before dawn: Jupiter, Saturn and the crescent Moon

Saturn
Saturn, Jupiter and a crescent Moon.
Image: Stellarium/IM

Given clear skies and a low horizon towards the southeast, one should see Saturn (above) and Jupiter (over to the left) of a 24 day old waning crescent Moon.

April 15th - evening: Mars and the Moon in Taurus.

Mars
Mars and the Moon
Image: Stellarium/IM

If clear in the evening of the 15th and looking towards the west, Mars will lie above a very thin waxing crescent Moon, just 3.3 days after new.

April 25th - after sunset: Mercury and Venus

Mercury
Mercury and Venus.
Image: Stellarium/IM

If clear after sunset on the 25th, and given a very low horizon towards the west-northwest, it should be possible to spot both Mercury and Venus at the start of their apparitions.   Binoculars may well be needed to help remove the Sun's glare but please do not use them until after the Sun has set.

April 29th - before dawn : the Moon close to Antares.

Moon
The Moon and Antares
Image: Stellarium/IM

If clear before dawn on the 29th of the month, looking towards the south southest, one should see the 17 day old Moon lying just 3 and a half degrees up to the right of Antares in Scorpio.   At magnitude 1.1, Antares is the third brightest star that lies along the ecliptic - slightly fainter than Aldebaran and Spica.

April 30th - after sunset : Mercury and Venus.

Mercury
Mercury and Venus
Image: Stellarium/IM

Just after sunset, if clear, Mercury will be seen having climbed higher above Venus since the 25th when they were at their closest.   A very low horizon will be needed and binoculars may well be needed to pick them out of the Sun's glare - but please do not use them until the Sun has set.

Learn the Mare on the Moon.

Mare on the Moon
Mare on the Moon
Image:Ian Morison

Why not use the annotated image of the full Moon to learn the locations of the Moon's Mare.  You can see some of them with your unaided eye and binoculars will enable you to spot them all.


NGC 891 imaged with the Faulkes Telescope

NGC 891
Edge-on galaxy NGC 891
Image: Danial Duggan
Faulkes Telescope North.

Galaxy NGC 891, imaged by Daniel Duggan.
This image was taken using the Faulkes Telescope North by Daniel Duggan - for some time a member of the Faulkes telescope team.   NGC 891 is an edge-on spiral lying in the constellation Andromeda at a distance of 27 million light years.   We think that this is very much as our own galaxy might look when seen edge-on.

Learn more about the Faulkes Telescopes and how schools can use them: Faulkes Telescope"

















Observe the International Space Station

The International Space Station
The International Space Station and Jules Verne passing behind the Lovell Telescope on April 1st 2008.
Image by Andrew Greenwood

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 Moon

3rd Quarter Moon
The Moon at 3rd Quarter. Image, by Ian Morison, taken with a 150mm Maksutov-Newtonian and Canon G7.
Just below the crator Plato seen near the top of the image is the mountain "Mons Piton".   It casts a long shadow across the maria from which one can calculate its height - about 6800ft or 2250m.
`
new moon first quarter full moon third quarter
April 12th April 20th April 27th April 4th

Some Lunar Images by Ian Morison, Jodrell Bank Observatory: Lunar Images

A World Record Lunar Image

World record Lunar Image
The 9 day old Moon.

To mark International Year of Astronomy, a team of British astronomers have made the largest lunar image in history and gained a place in the Guinness Book of Records!   The whole image comprises 87.4 megapixels with a Moon diameter of 9,550 pixels.  The resolution of ~0.4 arc seconds allows details as small as 1km across to be discerned!   The superb quality of the image is shown by the detail below of Plato and the Alpine Valley.  Craterlets are seen on the floor of Plato and the rille along the centre of the Alpine valley is clearly visible.  The image quality is staggering! The team of Damian Peach, Pete lawrence, Dave Tyler, Bruce Kingsley, Nick Smith, Nick Howes, Trevor Little, David Mason, Mark and Lee Irvine with technical support from Ninian Boyle captured the video sequences from which 288 individual mozaic panes were produced.   These were then stitched together to form the lunar image.

Plato and the Alpine valley
Plato and the Alpine Valley.

Please follow the link to the Lunar World Record website and it would be really great if you could donate to Sir Patrick Moore's chosen charity to either download a full resolution image or purchase a print.









The 8 day old Moon

Lunar Image
The 8 day old Moon imaged by Ian Morison.

This image was taken by the author on a night in March 2018 when the Moon was at an elevation of ~52 degrees and the seeing was excellent.   This enabled the resolution of the image to be largely determined by the resolution of the 200 mm aperture telescope and the 3.75 micron pixel size of the Point Grey Chameleon 1.3 megapixel video camera.   The use of a near infrared filter allowed imaging to take place before it was dark and also reduced the effects of atmospheric turbulence.   The 'Drizzle' technique developed by the Hubble Space Telescope Institute (HSTI) was used to reduce the effective size of the camera's pixels to allow the image to be well sampled.   Around 100 gigabytes of data, acquired over a 2 hour period, was processed to produce images of 54 overlapping areas of the Moon which were then combined to give the full lunar disk in the free 'stitching' program Microsoft ICE.   A further HSTI development called 'deconvolution sharpening' was then applied to the image.   The Moon's disk is ~6,900 pixels in height and has a resolution of 0.6 to 0.7 arc seconds.   Interestingly, as seen in the inset image, the rille lying along the centre of the Alpine Valley is just discernable and this is only ~0.5 km wide!   [Due to size limitations the large image is 2/3 full size.]









The Planets

 A montage of the Solar System
A montage of the Solar System. JPL / Nasa

Jupiter

Jupiter
A Cassini image of Jupiter . Nasa

Jupiter. As April begins and given a low horizon towards the southeast Jupiter, rising at 05:17 BST, may be glimpsed just before dawn shining at magnitude -2 and having an angular size of 34.7 arc seconds.   By month's end, it rises at 03:29 BST, about two hours before the Sun, when its magnitude will have increased slightly to -2.2 and its angular size to 37.4 arc seconds.   Sadly, its low elevation of ~10 degrees as dawn approaches will greatly hinder our view of the solar system's giant planet.



Saturn

Saturn
The planet Saturn. Cassini - Nasa

Saturn precedes Jupiter into the sky, rising at ~05:00 BST at the beginning of the month.   Again, a low horizon towards the south-east will be needed to see them both.   It is then shining with a magnitude of +0.75 and its disk is 15.9 arc seconds across with the rings spanning some 37 arc seconds.   By month's end, it rises at 03:00 BST with a slightly increased brightness of magnitude +0.71 and a 16.7 arc second disk.   We will have to wait a while to see this most beautiful planet at its best.




Mercury

Mercury.
Messenger image of Mercury Nasa

Mercury passes behind the Sun on the 18th of April and reappears, just up to the right of Venus, in the west northwest after sunset on the 25th of the month shining at magnitude -1.6 and with an angular size of 5.3 arc seconds.   It rapidly climbes up into the sky and by the 30th of the month will lie 4 degrees above Venus.   Mercury will reach greatest elongation east from the Sun on May 17th.   As the ecliptic is at a steep angle to the horizon at this time of the year, it will then have an elevation of 16 degrees as the Sun sets - so this will be one of Mercury's best apparitions this year.






Mars

Mars showing Syrtis major.
A Hubble Space Telescope image of Mars.
Jim Bell et al. AURA / STScI / Nasa

Mars, having passed between the Hyades and Pleoades clusters last month is now high in the constellation of Taurus.   Shining at magnitude +1.3 at the beginning of April, it has a angular size of 5.5 arc seconds so no details will be visible on its salmon pink surface.   Mars passes into Gemini on the 23rd of the month as its magnitude falls to +1.5   Mars will be best seen high in the west soon after nightfall and will still be visible in the evening sky until August before it passes behind the Sun in October.




Venus

Venus
Venus showing some cloud structure

Venus. Around the 18th of the month Venus, having passsed behind the Sun, will appear again just north of west after sunset.   Shining at magnitude -3.9, binoculars may well be needed to spot it - but please do not use them until after the Sun has set.   By month's end it will still only have an elevation of ~5 degrees at sunset.   Venus will grace the evening sky for the rest of this year and reaches its greatest elongation east from the Sun on October the 29th but will be highest in the evening sky at the beginning of December.





The Stars

The mid evening April Sky

FebruarySky
The April Sky in the south - mid evening.

This map shows the constellations seen in the south in mid-evening.

The constellation Gemini is now setting towards the south-west and Leo holds pride (sic) of place in the south with its bright star Regulus.  Between Gemini and Leo lies Cancer.   It is well worth observing with binoculars to see the Beehive Cluster at its heart. Below Gemini is the tiny constellation of Canis Minor whose only bright star is Procyon.  Rising in the south-east is the constellation Virgo whose brightest star is Spica.   Though Virgo has few bright stars it is in the direction of of a great cluster of galaxies - the Virgo Cluster - which lies at the centre of the supercluster of which our local group of galaxies is an outlying member.   The constellation Ursa Major is high in the northern sky during the evening this month and contains many interesting objects.

The constellation Gemini

Gemini
Gemini - click on image to enlarge

Gemini - The Twins - lies up and to the left of Orion and is in the south-west during early evenings this month. It contains two bright stars Castor and Pollux of 1.9 and 1.1 magnitudes respectivly. Castor is a close double having a separation of ~ 3.6 arc seconds making it a fine test of the quality of a small telescope - providing the atmospheric seeing is good! In fact the Castor system has 6 stars - each of the two seen in the telescope is a double star, and there is a third, 9th magnitude, companion star 73 arcseconds away which is alos a double star! Pollux is a red giant star of spectral class K0. The planet Pluto was discovered close to delta Geminorum by Clyde Tombaugh in 1930. The variable star shown to the lower right of delta Geminorum is a Cepheid variable, changing its brightness from 3.6 to 4.2 magnitudes with a period of 10.15 days

Gemini
M35 and NGC 2158
This wonderful image was taken by Fritz Benedict and David Chappell using a 30" telescope at McDonal Observatory. Randy Whited combined the three colour CCD images to make the picture

M35 is an open star cluster comprising several hundred stars around a hundred of which are brighter than magnitude 13 and so will be seen under dark skies with a relativly small telescope. It is easily spotted with binoculars close to the "foot" of the upper right twin. A small telescope at low power using a wide field eyepiece will show it at its best. Those using larger telescopes - say 8 to 10 inches - will spot a smaller compact cluster NGC 2158 close by. NGC 2158 is four times more distant that M35 and ten times older, so the hotter blue stars will have reached the end of their lives leaving only the longer-lived yellow stars like our Sun to dominate its light.

Gemini
The Eskimo Nebula, NGC2392, Hubble Space Telescope

To the lower right of the constellation lies the Planetary Nebula NGC2392. As the Hubble Space Telescope image shows, it resembles a head surrounded by the fur collar of a parka hood - hence its other name The Eskimo Nebula. The white dwarf remnant is seen at the centre of the "head". The Nebula was discovered by William Herschel in 1787. It lies about 5000 light years away from us.

The constellation Leo

Leo
Leo - click on image to enlarge

The constellation Leo is now in the south-eastern sky in the evening. One of the few constellations that genuinely resembles its name, it looks likes one of the Lions in Trafalger Square, with its main and head forming an arc (called the Sickle) to the upper right, with Regulus in the position of its right knee. Regulus is a blue-white star, five times bigger than the sun at a distance of 90 light years. It shines at magnitude 1.4. Algieba, which forms the base of the neck, is the second brightest star in Leo at magnitude 1.9. With a telescope it resolves into one of the most magnificent double stars in the sky - a pair of golden yellow stars! They orbit their common centre of gravity every 600 years. This lovely pair of orange giants are 170 light years away.

Leo also hosts two pairs of Messier galaxies which lie beneath its belly. The first pair lie about 9 degrees to the west of Regulus and comprise M95 (to the east) and M96. They are almost exactly at the same declination as Regulus so, using an equatorial mount, centre on Regulus, lock the declination axis and sweep towards the west 9 degrees. They are both close to 9th magnitude and may bee seen together with a telescope at low power or individually at higher powers. M65 is a type Sa spiral lying at a distance of 35 millin klight years and M66, considerably bigger than M65, is of type Sb. Type Sa spirals have large nuclei and very tightly wound spiral arms whilst as one moves through type Sb to Sc, the nucleus becomes smaller and the arms more open.

M65 and M66
The galaxies M65 and M66
M65 M66
M65 - Type Sa spiral, 9.3 magnitude M66 - Type Sb spiral, 8.9 magnitude

The second pair of galaxies, M95 and M96, lie a further 7 degrees to the west between the stars Upsilon and Iota Leonis. M95 is a barred spiral of type SBb. It lies at a distance of 38 million light years and is magnitude 9.7. M96, a type Sa galaxy, is slightly further away at 41 million light years, but a little brighter with a magnitude of 9.2. Both are members of the Leo I group of galaxies and are visible together with a telescope at low power.

M95 and M96
The galaxies M95 and M96
M95 M96
M95 - Type SBb spiral, 9.7 magnitude M96 - Type Sa spiral, 9.2 magnitude

There is a further ~9th magnitude galaxy in Leo which, surprisingly, is in neither the Messier or Caldwell catalogues. It lies a little below lambda Leonis and was discovered by William Herschel. No 2903 in the New General Catalogue, it is a beautiful type Sb galaxy which is seen at somewhat of an oblique angle. It lies at a distance of 20.5 million light years.

NGC2903
The 8.9th magnitude, type Sb, Galaxy NGC2903

The constellation Virgo

Virgo
Virgo

Virgo, rising in the east in late evening this month, is not one of the most prominent constellations, containing only one bright star, Spica, but is one of the largest and is very rewarding for those with "rich field" telescopes capable of seeing the many galaxies that lie within its boundaries. Spica is, in fact, an exceedingly close double star with the two B type stars orbiting each other every 4 days. Their total luminosity is 2000 times that of our Sun. In the upper right hand quadrant of Virgo lies the centre of the Virgo Cluster of galaxies. There are 13 galaxies in the Messier catalogue in this region, all of which can be seen with a small telescope. The brightest is the giant elliptical galaxy, M87, with a jet extending from its centre where there is almost certainly a massive black hole into which dust and gas are falling. This releases great amounts of energy which powers particles to reach speeds close to the speed of light forming the jet we see. M87 is also called VIRGO A as it is a very strong radio source.

M87 MERLIN images
The Giant Elliptical Galaxy M87 HST image showing the jet

Below Porrima and to the right of Spica lies M104, an 8th magnitude spiral galaxy about 30 million light years away from us. Its spiral arms are edge on to us so in a small telescope it appears as an elliptical galaxy. It is also known as the Sombrero Galaxy as it looks like a wide brimmed hat in long exposure photographs.

The Sombrero Galaxy
M104 - The Sombrero Galaxy

The constellation Ursa Major

Ursa Major
Ursa Major

The stars of the Plough, shown linked by the thicker lines in the chart above, form one of the most recognised star patterns in the sky. Also called the Big Dipper, after the soup ladles used by farmer's wives in America to serve soup to the farm workers at lunchtime, it forms part of the Great Bear constellation - not quite so easy to make out! The stars Merak and Dubhe form the pointers which will lead you to the Pole Star, and hence find North. The stars Alcor and Mizar form a naked eye double which repays observation in a small telescope as Mizar is then shown to be an easily resolved double star. A fainter reddish star forms a triangle with Alcor and Mizar.

Ursa Major contains many interesting "deep sky" objects. The brightest, listed in Messier's Catalogue, are shown on the chart, but there are many fainter galaxies in the region too. In the upper right of the constellation are a pair of interacting galaxies M81 and M82 shown in the image below. M82 is undergoing a major burst of star formation and hence called a "starburst galaxy". They can be seen together using a low power eyepiece on a small telescope.

M81 and M82
M81 and M82

Another, and very beautiful, galaxy is M101 which looks rather like a pinwheel firework, hence its other name the Pinwheel Galaxy. It was discovered in1781 and was a late entry to Messier's calalogue of nebulous objects. It is a type Sc spiral galaxy seen face on which is at a distance of about 24 million light years. Type Sc galaxies have a relativly small nucleus and open spiral arms. With an overall diameter of 170,000 light it is one of the largest spirals known (the Milky Way has a diameter of ~ 130,000 light years).

M101
M101 - The Ursa Major Pinwheel Galaxy

Though just outside the constellation boundary, M51 lies close to Alkaid, the leftmost star of the Plough. Also called the Whirlpool Galaxy it is being deformed by the passage of the smaller galaxy on the left. This is now gravitationally captured by M51 and the two will eventually merge. M51 lies at a distance of about 37 million light years and was the first galaxy in which spiral arms were seen. It was discovered by Charles Messier in 1773 and the spiral structure was observed by Lord Rosse in 1845 using the 72" reflector at Birr Castle in Ireland - for many years the largest telescope in the world.

M51
M51 - The Whirlpool Galaxy

Lying close to Merak is the planetary nebula M97 which is usually called the Owl Nebula due to its resemblance to an owl's face with two large eyes. It was first called this by Lord Rosse who drew it in 1848 - as shown in the image below right. Planetary nebulae ar the remnants of stars similar in size to our Sun. When all possible nuclear fusion processes are complete, the central core collpses down into a "white dwarf" star and the the outer parts of the star are blown off to form the surrounding nebula.

Owl Nebula Owl Nebula
M97 - The Owl Planetary Nebula Lord Rosse's 1848 drawing of the Owl Nebula