Posted on by Jenny Winder

Wishing the Zooniverse a Happy 5th Birthday!

Galaxy Zoo was a project set up in July 2007 by astronomers Chris Lintott and Kevin Schawinski asking members of the public to help classify a million galaxy images produced by the Sloan Digital Sky Survey. Five years on and Galaxy Zoo has grown into an entire Zooniverse of projects allowing members to contribute to real science across a range of disciplines. Join us to celebrate the giant of citizen science, mark its achievements and look forward to the future.

Modern science can produce huge amounts of data and making sense of it all can take years and often needs a human eye to pick out the fine details. The Zooniverse unleashes an army of willing volunteers to pore over images and data sets. Galaxy Zoo members have now classified over 250 million galaxies. At the time of writing there are currently 656,773 people taking part in Zooniverse projects across the globe. Galaxy Zoo participants alone have contributed to more than 30 published scientific papers. One of the Zooniverse’s great strengths is the ability to throw up some unexpected discoveries like the now famous Hanny’s Voorwerp, named after Dutch school teacher Hanny van Arkel, the Galaxy Zoo volunteer who spotted it. Such a serendipitous discovery is possible when data is exposed to large numbers of users who are encouraged to flag up anything they think looks out of the ordinary.

To mark Galaxy Zoo’s 5th birthday there will be a relaunch of the project which will compare images using a new dataset from Hubble’s CANDELS survey of distant, early galaxies to what we see today.

The range of projects now available to members is extensive. Users of the Solar Stormwatch project analyse interactive diagrams produced by NASA’s Solar Terrestrial Relations Observatory (STEREO). Planet Hunters use data from Kepler to search for transiting exoplanets. The Milky Way Project users have access to image data from the Spitzer Space Telescope to identify infrared bubbles in the interstellar medium to help us understand how stars form. SETI Live searches for interesting signals coming from the Kepler Field. Moon Zoo participants use data from NASA’s Lunar Reconnaissance Orbiter (LRO) to catalogue features on the Moon down to the size of a wastepaper basket.

Away from space there are also projects involved in climate, nature and humanities. Old Weather is a project that models Earth’s climate using wartime shipping logs and Whale FM members listen to, and catagorize, the songs of Orcas to help understand what the whales are saying, while Ancient Lives gives participants the chance to decipher and study the Oxyrhynchus collection of papyri. The NEEMO project analyzes images of marine life and features taken from the underwater base at the National Marine Sanctuary in Key Largo, Florida. What’s the Score asks people to help describe over four thousand digitised musical scores made available by the Bodleian Libraries. With a global posse of citizen scientists eager to study real data at their disposal, the range of projects will likely grow over the coming years. So happy 5th Birthday Zooniverse and here’s to many more!

To find out more and how you can get involved visit the Zooniverse website

Lead image caption: Galaxies gone wild. Source NASA, ESA, the Hubble Heritage (STScI/AURA) ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)

Posted on by Jenny Winder

ERGO – Students Sign up to Build the World’s Largest Telescope!

Inspired by SETI Chief, Jill Tarter’s 2009 TED ‘Prize Wish’ to “Empower Earthlings everywhere to become active participants in the ultimate search for cosmic company” the Energetic Ray Global Observatory or ERGO is an exciting new to project that aims to enlist students around the world to turn our whole planet into one massive cosmic-ray telescope to detect the energetic charged particles that arrive at Earth from space. Find out how it works and how your school can get involved Continue reading “ERGO – Students Sign up to Build the World’s Largest Telescope!”

Posted on by Jenny Winder

Euclid and the Geometry of the Dark Universe

Artist’s impression of Euclid Credit: ESA/C. Carreau

Euclid, an exciting new mission to map the geometry, distribution and evolution of dark energy and dark matter has just been formally adopted by ESA as part of their Cosmic Vision 2015-2025 progamme. Named after Euclid of Alexandria, the “Father of Geometry”, it will accurately measure the accelerated expansion of the Universe, bringing together one of the largest collaborations of astronomers, engineers and scientists in an attempt to answer one of the most important questions in cosmology: why is the expansion of the Universe accelerating, instead of slowing down due to the gravitational attraction of all the matter it contains?

In 2007 the Hubble Space Telescope produced a 3D map of dark matter that covered just over 2 square degrees of sky, while in March this year the Baryon Oscillation Spectroscopic Survey (BOSS) measured the precise distance to just over a quarter of a million galaxies. Working in the visible and near-infrared wavelengths, Euclid will precisely measure around two billion galaxies and galaxy clusters in 3 dimensions in a wide extragalactic survey covering 15,000 square degrees (over a third of the sky) plus a deep survey out to redshifts of ~2, covering an area of 40 square degrees, the 3-D galaxy maps produced will trace dark energy’s influence over 10 billion years of cosmic history, covering the period when dark energy accelerated the expansion of the Universe.

The mission was selected last October but now that it has been formally adopted by ESA, invitations to tender will be released, with Astrium and Thales Alenia Space, Europe’s two main space companies expected to bid. Hoping to launch in 2020, Euclid will involve contributions from 11 European space agencies as well as NASA while nearly 1,000 scientists from 100 institutes form the Euclid Consortium building the instruments and participating in the scientific harvest of the mission. It is expected to cost around 800m euros ($1,000m £640m) to build, equip, launch and operate over its nominal 6 year mission lifetime, where it will orbit the second Sun-Earth Lagrange point (L2 in the image below) It will have a mass of around 2100 kg, and measure about 4.5 metres tall by 3.1 metres. It will carry a 1.2 m Korsch telescope, a near infrared camera/spectrometer and one of the largest optical digital cameras ever flown in space.

Sun Earth Lagrange Points Credit: Xander89 via Wikimedia Commons

Dark matter represents 20% of the universe and dark energy 76%. Euclid will use two techniques to map the dark matter and measure dark energy. Weak gravitational lensing measures the distortions of light from distant galaxies due to the mass of dark matter, this requires extremely high image quality to suppress or calibrate-out image distortions in order to measure the true distortions by gravity. Euclid’s camera will produce images 100 times larger than those produced by Hubble, minimizing the need to stitch images together. Baryonic acoustic oscillations, wiggle patterns, imprinted in the clustering of galaxies, will provide a standard ruler to measure dark energy and the expansion in the Universe. This involves the determination of the redshifts of galaxies to better than 0.1%. It is also hoped that later in the mission, supernovas may be used as markers to measure the expansion rate of the Universe.

Find out more about Euclid and other Cosmic Vision missions at ESA Science

Lead image caption: Artist’s-impression-of-Euclid-Credit-ESA-C.-Carreau

Second image caption: Sun Earth Lagrange Points Credit: Xander89 via Wikimedia Commons

Posted on by Jenny Winder

Curiosity and the Issue of Planetary Protection

Curiosity at Centre of Attention During Testing Image Credit: NASA /JPL - Caltech

Curiosity at Centre of Attention During Testing Image Credit: NASA /JPL – Caltech

There have been many reports about the possibility of NASA’s Curiosity rover contaminating Mars with microbes from Earth once it lands on the Red Planet in August. The wheels, the landing procedure and the drill bits have all come under scrutiny. But what are the concerns and what safeguards are there to prevent contamination from this or other missions?

In 1967 the United Nations drew up the ‘Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, Including the Moon and Other Bodies.’ All countries which sign up to the treaty “shall pursue studies of outer space, including the moon and other celestial bodies, and conduct exploration of them so as to avoid their harmful contamination.” Every mission is given a category (I,II,III,IV or V) depending on whether it is a flyby, orbiter, lander, or Earth return mission, whether its destination is a planet, moon, comet, or asteroid and whether the destination could provide clues about life or have the potential to support Earth life. So for instance Cassini is a catagory II mission, Curiosity is classed as a IVc mission.

Every stage of a mission is carefully monitored. From construction in a sterile clean room with laminar-air-flow systems, pressurized microbial barriers and personnel wearing hoods, masks, surgical gloves, booties and protective suits called bunny suits. Components and entire spacecraft are sterilized using dry heat microbial reduction, by being enclosed in a bioshield (like a large casserole dish) and baked them in an oven at 111.7 degrees Celsius for 30 hours. For more sensitive components a low-temperature process is used. Components are placed in a vacuum and hydrogen peroxide is injected into the sterilization chamber to establish a specified vapor concentration. Thousands of samples are taken at every stage of construction and tested for spore-forming organisms, for example the Viking mission in 1975 tested more than 6000 samples in total.

Three issues have arisen with the Curiosity rover. During the landing procedure a parachute and thrusters will slow the descent before the ‘sky crane’ lowers the rover, its wheels making direct contact with the surface. Previous rovers have waited on landing platforms for days before their wheels made contact with the surface and in tests it has been shown that even a few hours exposure to Martian levels of ultraviolet can kill between 81 and 96 per cent of bacteria that may be present. So once Curiosity lands it will probably need to remain stationary for some days to minimize the risk of contamination from its wheels.

Another issue arose last year, after launch, when it was realized that a step in the planetary protection measures wasn’t adhered to during the manufacture of the rover’s drill bits. These were meant to arrive at Mars inside a sterile box, but the box was opened and the bits tested for contamination and one of the bits was attached to the drill head. This procedure strayed from earlier agreed-to protocols. The drills have now become another cause concern as it has been found that Teflon and molybdenum disulfide from seals within the drill assembly could rub off and mix in to contaminate samples excavated during operation, making the samples more difficult to analyze. The MSL team are looking at ways to work around the problem, these could include running the drill on a slower, less percussive setting or dispensing with the drill altogether and relying on Curiosity’s scoop to take soils soil samples and using the rover’s wheels to roll over and break open rocks.

This all serves to highlight the importance of the planetary protection treaty to ensure we do everything possible to reduce the risk of contaminating other worlds and of compromising any data we return.

Find out more at NASA’s Office of Planetary Protection

Posted on by Jenny Winder

The Antikythera Time Machine

Antikythera by Marsyas via Wikimedia Commons
Antikythera by Marsyas via Wikimedia Commons

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Leonardo da Vinci may have left behind sketches of helicopters, tanks and submarines but it is rare that we find actual artifacts that seem so way ahead of their time. Almost like a science fiction tale of archaeologists finding a wristwatch buried deep in an Egyptian pyramid or motorcar under the foundations of Stonehenge, we do have an example of a scientific computer that was built between 150 and 100 BC. It was so advanced, nothing as complex would be developed again until the 14th century.

The Antikythera mechanism was lost to the world for centuries. The device was salvaged in 1900 from a ship that sank en route to Rome, in the 1st century BC, between Crete and the island of Antikythera in the Mediterranean. When one of the fragments was discovered to contain a bronze gear wheel, the idea that this was some kind of astronomical clock was dismissed as too fantastic an anachronism. It was not until 1951 that the investigation was picked up by a British science historian Derek J. de Solla Price. So far 82 fragments have been recovered of what is now considered the oldest known astronomical computer.

The device is made of bronze and contains 30 gears though it may have had as many as 72 originally. Each gear was meticulously hand cut with between 15 and 223 triangular teeth, which were the key to discovering the mechanism’s various functions. It was based on theories of astronomy and mathematics developed by Greek astronomers who may have drawn from earlier Babylonian astronomical theories and its construction could be attributed to the astronomer Hipparchus or, more likely, Archimedes the famous Greek mathematician, physicist, engineer, inventor and astronomer. Why it was built, or for whom is unknown.

Replica Antikythera Based on the research of Professor Derek de Solla Price, in collaboration with the National Scientific Research Center Demokritos and physicist CH Karakalos. image by Marsyas via Wikimedia Commons
Replica Antikythera Based on the research of Professor Derek de Solla Price, in collaboration with the National Scientific Research Center Demokritos and physicist CH Karakalos. image by Marsyas via Wikimedia Commons

The main front dial showed the 365 day Egyptian year and the Greek signs of the Zodiac and could be adjusted to compensate for the extra quarter day in the solar year. The dial probably bore three hands that marked the date and positions of the Sun and Moon, while a separate mechanism showed the Moon’s phases and it likely also displayed the 5 classically known planets, Mercury, Mars, Venus, Jupiter and Saturn.

On the back an upper dial showed 19 year Metonic cycle of Moon phases, the 76 year Callippic cycle (four Metonic cycles) and calculated the 4 year Olympic cycle (four games took place in two and four year cycles) The lower dial showed the 18 year 11 days Saros eclipse cycle and the 54 year 33 day Exeligmos or triple saros cycle. It was driven by a hand crank now sadly lost. It is small, compact and portable with full instructions engraved upon it in Greek, about 95% of which have now been deciphered.

The fragile pieces that remain have been examined and modeled using high-resolution X-ray tomography and gamma rays and various reconstructions and replicas have been built. It has even had a working model constructed out of Lego. I can’t helping thinking that Archimedes would have rather liked Lego, if only we could go back in time and give him a set…

Find out more at the  Antikythera Mechanism Research Project

Posted on by Jenny Winder

The Moon is also Partial to an Eclipse!

Partial Lunar Eclipse. Source: SockPuppetForTomruen at en.wikipedia
Partial Lunar Eclipse. Source: SockPuppetForTomruen at en.wikipedia

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What with solar flares, sunspots and an annular solar eclipse, the Sun has been the centre of everyone’s attention lately. Next week all eyes (suitably protected, I trust) will be turned Sun-ward again to watch the last transit of Venus this century. Not to be outshone, the full Moon is preparing a little show of its own on Monday June 4th as a curtain raiser for the transit on the following day. Here is the what, when and where, of the first lunar eclipse of 2012:

During the new Moon on May 20th an annular solar eclipse was produced when the Moon got between Earth and the Sun. This time a partial lunar eclipse will occur when the Earth is positioned between the full Moon and the Sun and Earth’s shadow will be cast across part of the Moon’s surface. Unlike a solar eclipse no special protective eyewear is necessary, and a pair of binoculars will show it to its best advantage. The eclipse occurs at the Moon’s ascending node in southern Ophiuchus just northeast of Antares, and will be visible at Moonrise from Australia and East Asia, with the entire eclipse visible across the Pacific, while much of North and South America will see the eclipse as the Moon sets.

Partial lunar eclipse June 4, 2012. Image Credit NASA
Partial lunar eclipse June 4, 2012. Image Credit NASA

The other nice thing about lunar eclipses is that they last much longer than a solar eclipse. The penumbral phase, when the Moon passes through the outer portion of Earth’s shadow, causing only subtle darkening of the Moon, will last four and a half hours and partial phase, when it passes through our inner, umbral shadow, will last for 2 hours and 6 minutes. Penumbral eclipse begins 08:48 UTC and partial eclipse at 09:59 UTC. Greatest eclipse is at 11:03 UTC when it will be directly over the South pacific. During greatest eclipse the maximum umbral magnitude will be 0.38 as  the southern limb of the Moon will dip 12.3 arc-minutes into the umbra and the area around the crater Tycho will be in shadow and take on a rosy glow due to the refraction of sunlight through Earth’s atmosphere. This is only appropriate as the full Moon in June is named the Strawberry Moon in native American folklore and the Rose Moon in Europe. The colour the Moon turns during greatest eclipse depends on how much dust and clouds are present in the atmosphere and can range from dark brown and red to bright orange and yellow. Partial eclipse will finish at 12:06 UTC and penumbral eclipse will end at 13:18 UTC.

If you are not lucky enough to be on the right side of the planet (like Europe, Africa,  New England and eastern Canada) then the good folks at SLOOH will be webcasting the event live. There will be another lunar eclipse in November but that will only be penumbral, which are not very noticeable, just a slight dimming, so make the most of the Moonshow on Monday as a warm up for the star attraction the next day!

Find out more about the eclipse at EarthSky
and timeanddate.com

Posted on by Jenny Winder

Ecological Concerns in Losing Our Starry Night

Earth at Night. Courtesy DMSP and NASA
Earth at Night. Courtesy DMSP and NASA

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Images of the Earth at night, taken from space are always a stunning sight, with cities, countries and whole continents glittering like jewels. But this beauty comes at a price. It used to be that anyone looking up on a clear night could see the Milky Way. As more and more of us are drawn to live in urban areas, our view of the sky is blotted out by the glare of our lights. Astronomers have known about the growing problem of light pollution for a long time. Now ecologists are becoming concerned that our artificial lights are affecting more than our view of the stars.

Researchers at the University of Exeter studying the ecological impact of artificial lighting have noted changes in distribution of invertebrate communities around artificial lighting which could affect the broader wildlife that depend on them. Simply put, it is easier for predators to find their prey, and harder for the prey to hide, in brightly lit areas. Streetlights may also be disrupting the natural rhythms of both fauna and flora, changing hibernation patterns and flowering times. It may also be affecting our own circadian rhythms as well as being a colossal waste of energy, an estimated $2.2 billion per year in the U.S. alone! On average, 30% of the light from a streetlight shines up and out.

Light pollution is a growing problem. Artificial lighting is increasing at the rate of 6% each year globally and is only going to get worse, as developing nations use more and more electric light. Since 1988 The International Dark-Sky Association has campaigned to protect and preserve the night environment and promote energy efficient options. Light what you need, when you need it, they say.

One of their projects is the International Dark Sky Places program which certify locations with exceptional nightscapes, either as communities, parks or reserves. The Kielder Forest and adjacent Northumberland national park covering 400 square miles in the UK is the latest area hoping to join the 12 dark sky reserves already recognised by the IDA worldwide. Such status can bring economic advantages too, astronomy is rapidly growing in popularity and with it astronomy based tourism, offering dark skies, observing opportunities and star parties and star camps.

Losing the stars can have a lasting impact on our culture too. Think of all the art, literature and music that have been inspired by the night sky. As we become increasingly disconnected from nature the stars are one of our most important links. There are many people today who have never been able to look up and see the Milky Way arching over their heads. Looking up at the stars allows us a vital opportunity to engage with the larger questions posed by the universe.

Find out more at The International Dark-Sky Association
and The British Astronomical Association’s Campaign for Dark Skies

Posted on by Jenny Winder

Venus Transit — There’s an App for That!

Transit of Venus by NASA's TRACE spacecraft Image credit: NASA/LMSAL
Transit of Venus in 2004 by NASA's TRACE spacecraft. Image credit: NASA/LMSAL

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There have been only six Venus transits since the invention of the telescope in the early 17th century. It was not until 1761 that the transit of Venus on June 6th was observed as part of the first ever international scientific observation project, instigated by Edmond Halley. Astronomers across the globe viewed the transit and the differences in their observations were used to triangulate the distance to Venus and, using Kepler’s laws, the distance to the Sun, the other planets and the size of the Solar System. Though the method used has not changed in the 251 years since, the equipment most certainly has.

For this transit, we have technology on our side.

In previous Venus Transits, expeditions were sent out far and wide and the 1761 transit was eventually recorded by 120 individual astronomers from 62 locations across Europe, America, Asia and Africa. They used only the simple telescopes of the day, fitted with dense filters, a pendulum clock to time the transit and quadrants to determine their exact latitude and local time. It is hardly surprising that their observations varied widely. Their calculations put the Sun’s distance between 130 and 158 million kilometres.

Transits happen in pairs. After 121 years a transit occurs followed 8 years later by another, then 105 years pass before the next pair and then the pattern repeats. Prior to the transit of 2004 the most recent transit was in 1882. There were none during the whole of the 20th century! We now approach the last chance to view a transit in our lifetime, the next will not occur until 2117.

Luckily, we’ve got some newly developed technology to help make this the most-observed transit ever!

Astronomers Without Borders are part of the Transit of Venus Project to get as many people around the world to observe the transit and to participate in a collective experiment to measure the Sun’s distance. To this end they have produced the Venus Transit phone app, available to download free for both iTunes and Android. Once downloaded you can start to practice timing the interior contacts of ingress and egress using a simulation of the transit. This is not as easy as it seems, as the black drop effect makes precise timing tricky so practice is definitely recommended. The app will tell you how far out you are so that you can perfect your timing and it will also predict times of contact based on your location together with times of sunrise and sunset.

On the day of the transit, the app will record the exact GPS time and your location, which is sent to the global database. Afterwards you can access your data on the website’s map to edit your entry, and upload descriptions, text, images, or movies and view other entries as well. This transit will be visible over most of the Earth except for parts of West Africa and most of South America, so download, get practicing and become part of a once in a lifetime, global citizen science experiment!

Find out more at Transit of Venus

Posted on by Jenny Winder

Asterisms: Signposts in the Sky

Sagittarius Teapot Asterism Author Eoghanacht via wikimedia commons

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Humans have been grouping stars into patterns since the beginning of time, each culture placing its own myths and folklore in the sky, but it wasn’t until 1930 that the International Astronomical Union (IAU) divided the sky into 88 constellations each with its own strict boundary. Though many of these are well known, and the stories they depict are well documented, few look like the figures they are supposed to represent. Asterisms are more user friendly guides and the starting point for most astronomers. These bright, simple, more prosaic shapes, stand out among the constellations and often serve as useful landmarks, pointing the way to celestial delights!

The best known and most instantly recognisable asterism is probably the Plough or Big Dipper in Ursa Major. Two of its stars, Merak and Dubhe, (The Pointers) pointing the way to the North Star Polaris in Ursa Minor. NASA’s Juno spacecraft stopped on its 5 year journey to Jupiter on March 21 to capture this image of our best known asterism:

The Big Dipper by Juno Credit: NASA/JPL-Caltech/SWRI/MSSS

In the Northern hemisphere, each season is heralded by its own asterism: The Autumn sky is dominated by the great Square of Pegasus, formed by stars from both Pegasus and Andromeda and its top left corner points the way to the Andromeda Galaxy, just a hop, skip and jump away. Most people recognise the three stars Alnitak, Alnilam and Mintaka, making up the Belt of Orion, which shines bright in the Winter sky indicating one of the richest regions in the sky, the Orion Molecular Cloud Complex. The Diamond of Virgo marking the Spring, consists of Arcturus (Boötes), Spica (Virgo), Denebola (Leo) and Cor Caroli (Canes Venatici). These encompass the constellation Coma Berenices and many of the galaxies within the Virgo Cluster. The Summer Triangle of Deneb (Cygnus), Altair (Aquila), and Vega (Lyra) currently sails over us guiding the way to the Ring Nebula in its top right corner.

Searching for Leo? Look for the Sickle, a backwards question mark that represents the lion’s head. Hunting for Hercules? The Keystone is the key and you will also find the Hercules Cluster (M13) on its right hand side. Boötes is easier to spot if you look for the Kite or Ice Cream Cone, than try to make out an ox driver! Not many people can see poor Queen Cassiopia, punished for her vanity to circle the heavens, but the W that marks the constellation is instantly recognisable. The Circlet is a lovely signpost to one of the fish of Pisces. You’d be hard pressed to recognise Sagittarius as the centaur it depicts, but there is the Teapot (Bertrand Russell was right, there is a celestial teapot!) showing the way to this rich patch of sky.

The Southern hemisphere has no shortage of asterisms either, most notably Argo Navis which represents the entire ship Argo sailed by Jason and the Argonauts and would be the largest constellation in the sky if it hadn’t been broken up in 1752 by French astronomer Nicolas Louis de Lacaille into the constellations we know today as Carina (the keel), Puppis (the poop deck) and Vela (the sails). Also in the Southern hemisphere are found The Three Patriachs in Triangulum Australe and the Fish Hook in Scorpius among others.

There are many more obscure asterisms too. Job’s Coffin graces the constellation of Delphinus, Asses and the Manger are in Cancer, Poniatowski’s bull (named for the King of Poland) is part of Ophiuchus and Aquila. There are the Lozenge, Saxophone, Coathanger, and many more.

The sky can seem a bewildering place, filled with gods, kings and mythical creatures. Asterisms like the Teapot make a more welcoming and friendly introduction, allowing a novice stargazer to easily pick their way around the sky and gain confidence and as many stars get swallowed by increasing light pollution, asterisms still shine out to show the way.

Find out more about asterisms here.

Posted on by Jenny Winder

It’s Time to Welcome Night Shining Clouds

Noctilucent clouds taken from the ISS Image Credit: NASA
Noctilucent clouds taken from the ISS Image Credit: NASA

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Astronomers usually curse and shake their fist at clouds for obscuring the sky and spoiling their observations. This month however, we enter the season when, after dark, thin veils of clouds appear to glow with an eerie blue light and are eagerly awaited and sought after.

Polar mesospheric, noctilucent or night shining clouds (NLC) form at the edge of space, between 76 and 85 kilometers up in the arid atmosphere, where there is one hundred millionth the amount of moisture found in the air at the Sahara Desert! Here temperatures can fall below -100 degrees Celsius, so what little water vapour is present freezes directly or forms on dust particles from micrometeors or volcanic eruptions.

During the Summer months, as the Sun stays close to the horizon, its rays illuminate these layers of ice crystals, producing a fine network of tenuous, incandescent filaments. They appear, in the Northern hemisphere, from mid May to mid August (mid November to mid February in the South) in latitudes between 50º and 70º, when the Sun is 6 to 16 degrees below the horizon. Look for them low in the Northwestern sky from an hour after sunset, or low in the Northeast before dawn.

They were first noted in 1885, two years after the eruption of Krakatoa when people were accustomed to looking at the spectacular sunsets and the glowing clouds were thought to be produced by the ash from the volcano in our atmosphere. Eventually the ash disappeared, but the clouds remained. In fact throughout the twentieth century noctilucent clouds have been occurring more frequently and across a wider area, as well as becoming brighter, perhaps due to climate change as increased greenhouse gases cool the mesosphere.  The clouds also vary with the solar cycle, as ultraviolet radiation from the Sun splits the water molecules and so the clouds decrease in brightness during solar maximum. Changes in brightness seem to follow fluctuations in solar radiation but about a year later, though nobody knows the reason for this time delay.

The clouds have been found to be highly reflective to radar, possibly due to sodium and iron atoms, stripped from micrometeors, forming a thin metal coating on the ice grains. In 2006 Mars Express discovered similar clouds, forming from carbon dioxide 100 kilometers up in the Martian atmosphere, that were also only observed when the Sun was below the horizon. In 2009 the Charged Aerosol Release Experiment (CARE) created artificial noctilucent clouds using rocket exhaust that were observed for several weeks. In July 2008 the crew aboard the ISS were treated to a noctilucent cloud display over Mongolia and were able to capture the image above.

So over the Summer months, keep an eye on the northern horizon after dark for a chance to catch these beautiful and unusually welcome clouds.

Find out more at NLC