Archive for the ‘Astrobites’ Category

Blue Moon

Posted: August 1, 2012 in Astrobites, Moon

It’s August 2nd and there’s a full moon in the sky!

That’s an old picture; unfortunately it’s raining so I can’t see the moon right now.  Fortunately, this month we get two full moons, instead of the usual one!

The time between two successive full moons is about 29.5 days – give or take a little.  This means that in most calendar months there can only be one full moon.  However, if a full moon falls within the first day or two of the month then it’s possible for another full moon to fall before the end of the same month.

That’s exactly what’s happening in August; there will be full moons on August 2nd and 31st.  The second full moon on the 31st is called a blue moon…

In the 19th century the Maine Farmer’s Almanac divided the year into quarters and listed the dates of the full moons.  Obviously, the full moon provided much needed illumination for farmers after sunset so they were important.  Normally there’d be three full moons in each quarter of the year, but occasionally there is an extra full moon would fall in a quarter.  Farmers always called the final moon of the quarter the “late moon” and so the third full moon was deemed to be the extra one; they called it a blue moon.  In many cultures the usual pattern of monthly full moons had names like “Grain moon” (August) or “Harvest moon” (September).  The extra full moon which infrequently appeared in the calendar was known as a “betrayer” moon or a blue moon.  The phrase “once in a blue moon”…well, perhaps it’s from this phenomenon.  They aren’t actually that rare; blue moons will occur once every two and half years on average.

One final note; the term blue moon doesn’t refer to the actual colour of the moon.  The moon usually looks white, yellow or orange-red depending on how high it is in the sky (because of how much air the moonlight has to pass through).  However, volcanic eruptions or forest fires can inject just the right size of dust particle into the atmosphere to give the moon (and Sun) a bluish cast.  These kinds of events are also comparatively rare, happening perhaps less often than a blue moon!

The Northern Crown

Posted: June 9, 2012 in Astrobites, Observing, Stars

The short nights of late spring don’t give much time to do astronomy.  Despite this there are some interesting constellations to be explored!  One of my favourites is Corona Borealis – the Northern Crown.  At this time of the year it is high in the southern sky when it gets dark.  If you’ve not seen this little constellation before, well, it’s easy to find by star hopping from The Plough to the brilliant orange star Arcturus and then onto Corona Borealis.  Like this:

It an easily recognisable constellation – just a small semi-circle of stars.  And one of the few that actually resembles the object it is meant to represent:

The brightest star in the constellation is called Alphekka (or Gemma).  It’s a second magnitude star about 75 light-years away.  Alphekka is an eclipsing binary star and it falls  *very*  slightly in brightness every 17 days as its unseen companion star eclipses it.

The Northern Crown is well known to astronomers for a couple of its odd variable stars!  The first is known as T Corona Borealis, or more informally, The Blaze Star.  It is normally invisible to the unaided eye but it erupted dramatically in 1866 and 1946, becoming the brightest star in the constellation.  The Blaze Star is a recurrent nova; a white dwarf star being fed gaseous material by a red giant companion.

A critical point is aperiodically reached and the material ignites in a thermonuclear explosion visible from Earth.  After a few months the star has returned to its usual dim self.

The second oddball star is called R Corona Borealis (it doesn’t have a more exciting name).  Binoculars or a telescope are needed to see the star because it usually hovers on the border of naked eye visibility at magnitude 6.  At irregular intervals of months or years this star dramatically fades by a factor of 30  – requiring a very big telescope to see.  The best model to explain the unusual behaviour of the star suggests that carbon dust (soot!) builds up in the star’s atmosphere preventing light getting out and causing it to fade.  On the inside of the star the radiation cannot escape and the pressure rises until the carbon dust is blown out again causing the star to return, albeit temporarily, to normal brightness.

I downloaded a more detailed finder chart from the AAVSO showing the location of R CrB:

The numbers next to the stars are actually brightness labels (so 46 means magnitude +4.6).  Naked eye limit from a dark sky are those labelled with numbers less than about 65.  Here’s a light curve showing how the light from the star has brightened and dimmed over the last 20 years:

R CrB tumbled in brightness in 2007 and has been hovering near minimum brightness since.  The star will eventually recover dramatically – will it be in the next few weeks or months?  Time will tell so watch out!

It’s noctilucent cloud season again.  In the northern hemisphere it lasts from roughly June 1st to August 1st each year.

Astronomers and clouds don’t generally get on.  However, during summer in the northern hemisphere some mysterious and beautiful clouds may appear in the sky long after sunset.  Known to astronomers as noctilucent clouds (NLC), these delicate and tenuous clouds are seen shining long after the ordinary clouds of the troposphere have darkened.  NLC are the highest clouds in the atmosphere and form within the mesosphere at heights of 85km (about 52 miles) above the ground (which means you’ll hear scientists referring to them as Polar Mesospheric Clouds).

Here’s a picture of a display seen from Northumberland in 2011.

The dark clouds are ordinary clouds in the tropisphere; they in darkness like the ground beneath them and are silhouetted against the brighter, more distant noctilucent clouds.

NLC formation is restricted to the summer months when conditions in the mesosphere near the poles are sufficiently cool enough (-120°C) to allow ice to form in the low pressure environment.  Conditions aiding the formation of NLCs tend to last from the start of June until the beginning of August.   The clouds can be seen from the ground between latitudes of 50 and 60 degrees north or south of the equator.  At higher latitudes the summer twilight is too bright for the clouds to be visible.  Northumberland (approximately 55°N) is ideally situated for NLC observers.

You can observe noctilucent clouds during the next 6-8 weeks by going out and looking towards the north after about 11pm.  Taking pictures of noctilucent clouds is also easy; for example, ISO200 for 5-10 seconds should pick them up.

NLC form when water vapour condenses onto a dusty ‘seed’ high in the atmosphere.  The precise nature of that seed has been the subject of much discussion.  The earliest reports of NLC came in 1885 just two years after the eruption of Krakatoa, an event which affected the Earth’s atmosphere and weather for a decade or more.  Could the seeds of NLCs be volcanic dust?  The association with Krakatoa is not accepted by all scientists but it is true to say that NLC have been widely observed ever since.  Aside from powerful volcanic eruptions like Krakatoa there are no plausible mechanisms for transporting dust from the lower atmosphere to the mesosphere.  Some scientists have speculated that the seeds of NLCs are the meteoric dust swept up by the Earth as it orbits the Sun.

NLCs are being seen twice as often as they were a few decades ago.  The traditional window of latitude through which the clouds can be seen is also increasing: NLC have recently been sighted from locations just 40° from the equator.  The temperature of the mesosphere is influenced by the presence of carbon dioxide and the humidity is increased by methane.  Both gases are being increasingly produced by humans and so NLCs could be regarded as a visible indication of climate change.  The validity of the connection is still controversial.  The exhaust gases of the Space Shuttle and other rockets have also been observed to contribute to the formation of NLCs.

NASA has a satellite studying NLCs from orbit: it’s called the Aeronomy of Ice in the Mesosphere (AIM) satellite.   The primary goals of AIM are to determine the processes which form NLCs, to measure the sizes of ice crystals in the clouds and to monitor the composition of the mesosphere over a period of at least two years.  AIM may eventually provide evidence to show whether or not these delicate and beautiful clouds are a manifestation of the destructive changes brought about by human production of greenhouse gases.

UFO!

Posted: June 5, 2012 in Astrobites, Imaging
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Early evening March 17th 2002 I had my telescope set up in the garden just outside the house.  It was still twilight but it was dark enough to see the constellations: the brightest stars of Gemini, Taurus and Orion were easily visible.  Jupiter and Saturn were also shining and very high in the sky.  Then I noticed a bright new star in the sky not far from Jupiter.  For a moment I thought it was a satellite but then realised that it didn’t look like it was moving.  My next thought was that it might be a new supernova!  I got the telescope – a 10 inch Dobsonian – on to the new star very quickly.

The view through the eyepiece was unusual; it was disk shaped – a bit like Jupiter – but bigger and with a brighter core at the centre.  Through the eyepiece it did actually appear to be slowly moving.  At this point I realised that I was probably watching a rocket from beneath and looking directly into the exhaust.

I watched for a few minutes and then went to get the digital camera.  I’d taken a few shots of the moon before, by holding it to the eyepiece.  No control over the exposure or flash but I was hopeful I’d get something.  Returning to the telescope I watched in amazement as the disk seemed to shatter into hundreds of pieces and disperse.  I took this picture:

Two of the pieces seemed more substantial and I watched them for a few more minutes as they slowly drifted apart.  I was always curious about what rocket I actually saw there and why it ended up in pieces like that.

So having found the picture again recently I saw that the date and time was stored in the image properties! I googled the date and found that the GRACE mission was launched that day.

The GRACE mission was designed to map the Earth’s gravitational field in exquisite detail (and it has just celebrated a decade in orbit). To achieve this objective, the mission consists of a pair of twin satellites in orbit and the instruments which can measure changes in their separation to incredible accuracy.  Over long periods of time the GRACE satellites have built an incredible map of how gravitational field strength varies across the Earth.

GRACE was launched atop a Rokot launch vehicle from Russia earlier that day so I initially thought I’d seen the separation of the individual satellites.  I tried to find a way of confirming whether GRACE was visible from the UK on the day of launch.  After a bit of digging I found a paper which seemed to confirm that various manoeuvres had to take place in the within visibility of several ground stations.  The path of the satellite (shown in the paper) would have made it visible from the UK.

So….a minor mystery solved.  I’m not sure I saw the actual separation of the satellites from the rocket – that was supposed to happen 90 minutes or so after launch (I was watching many hours later).  It was probably a burn to separate or correct the separation of the satellites.  I’m happy to have what may be one of the few pictures of the start of this incredible mission!

This is the first in a series of articles I’ll be posting about the transit of Venus on June 6th.  In this one I’ll outline what happens during a transit of Venus and why they are such rare events.

On June 5th and 6th astronomers around the world will be training binoculars, telescopes and cameras on the Sun.  They’ll be hoping to witness an event that won’t be repeated in their lifetimes.  Venus, which circles the Sun within the Earth’s orbit, will move directly between us and the Sun.  For a period of up to nearly seven hours Venus will be seen by many around the world to be silhouetted against the brilliant solar disk.  Astronomers call this kind of event a transit.

Here is a picture of the transit of Venus that I took on June 8th 2004.

It takes Venus 225 days to complete one orbit compared to the 365 (and a bit) days for the Earth.  Like the hands of a clock, it’s reasonable to think that at regular intervals Earth and Venus will be aligned with the Sun.  In fact the alignment happens every 584 days – astronomers call it an inferior conjunction.

This scenario is complicated by the fact that the orbit of Venus is tilted with respect to the Earth’s orbit by a small angle.  Most of the time the Sun-Venus-Earth alignment is not exact – Venus is seen from Earth to pass above or below the Sun rather than in front of it.

The slight tilt of Venus’s orbit to ours means that transits only occur if the planets are lined up in early June or December; at one of the places in its orbit where it is ‘level’ with the Earth’s orbit (or near a node, as astronomers call them).

Pairs of transits

How often do transits occur?  The hands of a clock align every 65 minutes and 27 seconds.  For Venus and Earth the period between alignments is about 584 days; astronomers call this the synodic period of Venus.  And like the hands of a clock, the alignment between Earth and Venus will happen in different directions on their orbits.  So if they start lined up at a transit, in June say, then the next time they line up will be January – where Venus is above the Earth’s orbit.  No transit will be seen.  Roll on another 584 days and the planets are lined up again but in the wrong place for a transit.  Fast forward again.  And again…and again.  At the fifth alignment Venus has gone round the Sun 13 times and almost exactly 8 years have passed on Earth.  Venus and Earth are just about in the same positions they were at during the previous transit.  It’s this periodicity of 13 Venus Years = 8 Earth Years that ensures that if a transit happens then another will usually follow 8 years later.

The long wait between pairs

But transits of Venus don’t happen every 8 years.  The transits of 2004 and 2012 will not be followed by another in 2020.  Although the Venus and Earth are close to being in the same position after 8 years they’re not in exactly the same place.  The transit of 2004 happened on June 8th.  The transit of 2012 will be on June 6th.  The date is slipping back.  The alignment in 2020 will occur on June 3rd by which time the planets will be too far from the node of the orbit and Venus misses the Sun.   The eight-year slippage will continue and eventually, after more than a century, the alignments will occur near the December node where a transit can occur.  Eight years later, another December transit is also likely.  Eight years later Venus misses the Sun and no transits will be seen until the alignment next occurs in early June – more than a century later.

The next picture shows the path taken by Venus as it crosses the Sun during each transit since the 14th century.  In each case Venus moves from left to right.

There are some interesting patterns going on here so let’s discuss them!  Firstly, the transit periods in June and December slip back into the previous month (May and November) because the Julian Calendar was used prior to 1582.

Starting with the 1518 transit we see the following pattern:

1518 <– 8 years –>1526 <——105.5 years ——> 1631 <– 8 years –> 1639 <——–121.5 years ——–> 1761

…and then repeat.  Add these intervals together and we see that transits follow a pattern which repeats every 243 years!  This is because Venus completes almost exactly 395 orbits of the Sun in the time it takes Earth to do 243.  If the correspondence was exact then Venus would follow exactly the same path across the Sun every 243 years.  The transits of 1518, 1761 and 2004 followed very similar paths across the Sun.  Similarly, going back in time in leaps of 243 years: the transits of 1874 and 1631 have tracks close to the northern limb of the Sun.

But 243:395 correspondence isn’t quite exact and Venus follows a slightly different path every 243 years and eventually a transit is missed as in 1388.  The cycle of double transits is broken.

We currently live in an age where Venus transits occur in pairs.  This will change in the future; from the 40th century until the 53rd century, only single transits will take place.

Transits of Venus are incredibly rare events and only the previous six have definitely been observed by astronomers.  In the next part of this series I’ll discuss what astronomers have learned from them.

Venus and Jupiter have been converging in the early evening sky for the last few months.  Venus has been climbing higher into the evening sky, whilst Jupiter has been drawing closer to the fading twilight in the western sky at dusk.  At present they shine close together in the evening sky at mid-latitudes for more than four hours after sunset.  Here’s a picture I took the other night from near Alnwick, Northumberland:

On March 13th the pair will be at their nearest to each other in the sky – a spectacular sight.  It’s hard to visualise the solar system in 3D when you see a scene like this.  Here’s a picture of the solar system as seen from above; it depicts the positions of the planets on March 13th 2012 when the planets are at their closest together in the sky.

Venus will be 121 million km from Earth and Jupiter is more than six times that distance further away at 842 million km.  Venus appears brighter not only because it is nearer to us, but also because it is enshrouded with a dense atmosphere with very reflective cloud tops.  Also, notice another alignment.  The planet Mars is almost directly on the other side of the Earth to the Sun.  I wrote about that earlier.

As seen from Earth, the planets Venus and Jupiter will be almost perfectly lined up and we observe them to be in the same direction in the night sky.  The alignment isn’t perfect and Venus will not pass directly in front of Jupiter.  At their closest there will still be a gap of sky with an angular size of three degrees.  You could fit three full moons into that gap on the sky!

On March 13th at around 7.30pm the western night sky will look like this:

Venus glides to the north of Jupiter (above).  The magenta lines show orbits that both planets follow around the Sun and they also show why the alignment isn’t perfect.  First, to explain the differing shapes of the orbits that you see above.  Jupiter’s orbit around the Sun is five times wider than the Earth’s orbit and so we are inside it looking out; it wraps completely around the sky so we’re just seeing a small section of it here.  Venus, on the other hand, has its orbit inside the Earth’s orbit.  We can see the entire orbit in the same direction as the Sun.  In the evening sky scene above, the Sun has just dipped below the western horizon and so we see that part of the orbit to the east of the Sun.

So why doesn’t the orbit of Venus look like a line in the sky?  I mean that’s what we might expect if we see a circular orbit from the side!  Although the solar system is approximately disk shaped when viewed from the side….the orbits are not perfectly aligned with each other.  In most cases the orbits are tilted very slightly to each other (just a degree or so) so that most of the time the planets appear to be strung out on a line across the sky called the ecliptic.  But when they get close together then the tiny little differences in the tilt of the orbits becomes more noticeable.  The tilt of the orbit of Venus is particularly obvious because it’s also the nearest planet to us at times.  So in the picture above we’re looking at the circular orbit of Venus – not from edgeways on – but from slightly underneath.

Date of the conjunction

When two planets are close together in the sky the event is often called a conjunction.  Depending on where you look you’ll find the date of the Jupiter-Venus conjunction listed as March 15th despite the planets being closest together in the sky on March 13th.  Why the difference?  Astronomers have different definitions of conjunction! Two planets are in conjunction when they have the same longitude on the celestial sphere.  Using equatorial coordinates, this means they must have the same Right Ascension.

Here’s a map of the sky (thanks to SkyMap Pro) showing the positions of the planets at closest approach at 10.25pm (GMT) on March 13th.

The vertical lines represent Right Ascension.  Although they are at their closest on the 13th the planets are at different Right Ascensions.  Here is the actual conjunction in Right Ascension:

This map shows the positions on March 15th at 10.37am; both planets are on the same Right Ascension line – the conjunction!  They are a little further apart by this time but the difference is tiny.

On a final note, just as the tilt of the moons orbit around the Earth doesn’t stop eclipses happening – just makes them rarer – then so the alignment of the planets can also occasionally be perfect.  On January 3rd 1818 Venus actually transited in front of the planet Jupiter.  Imagine the view through a telescope!

The next event like this will also involve Venus and Jupiter again but won’t happen until November 22nd 2065 and will much more difficult to observe (aside from the practicalities of being alive on that date!)

I’ve been sorting out the jumble of astronomy images taken over the last decade and I found some pictures of noctilucent clouds taken in 2011 from the same place that I saw the aurora a few weeks ago.  Since the pictures were taken before I learned how to stitch them seamlessly together I thought I’d do it now!  Here’s the result:

Completely out of season to be looking at the so-called night-shining clouds of the summer!  Noctilucent clouds (NLCs) are a phenomenon of the upper atmosphere – way above the normal clouds you can see in the foreground.  They occur about 50 miles above the ground and they’re still catching sunlight long after we’ve experienced sunset.  The atmospheric conditions required to form these clouds occur during June and July and their visibility is limited to between 50 and 60 degrees north (so Northumberland is ideally placed to see them).  These clouds occur so high above the ground that we can quite rightly call them spaceweather!

Another kind of spaceweather that’s been making the UK news in recent weeks is the aurora – the northern lights.  This is obviously an excuse to post an aurora image again:

I took this image from almost the same spot as the the NLC picture.  Auroras occur in the same region of the atmosphere as the NLC.  The sun is waking up – becoming more active – and this is leading to increasingly frequent sightings of the aurora from more southerly locations.  The frequency of auroral displays is strongly correlated to solar activity; more sunspots and flares cause more auroral activity.

The interesting thing is that NLC displays seem to be inversely correlated to solar activity.  When the Sun is active there are fewer displays of NLCs.  The increased action of the solar wind probably warms the upper atmosphere preventing the formation of the NLCs.

During the next couple of years we can expect the number of auroras visible from the UK to go up and the displays of noctilucent clouds to go down.