Archive

Posts Tagged ‘UK’

Geminids Meteor Shower 2015

December 11, 2015 1 comment

One of the most active and reliable meteor showers, the Geminids, happens every year in mid-December. This year’s display promises to be a good one for those meteorwatchers with clear skies.

The maximum rate of Geminids is predicted to occur around 1800 on 14 Dec 2015, but peak rates normally persist for around a day, so the nights of 13 and 14 Dec are both good for meteorwatching. In addition. you’ll see plenty of Geminids from now until a few days after the peak.

When Gemini Sends Stars to Paranal  Image Credit & Copyright: Stéphane Guisard (Los Cielos de America), TWAN

When Gemini Sends Stars to Paranal
Image Credit & Copyright: Stéphane Guisard (Los Cielos de America), TWAN

There are a few ways you can maximise your chances of seeing some Geminids (see The What, How, Where, When and Why) but the best way is to get somewhere dark, like one of the UK’s International Dark Sky Places. I’ll be heading down to Galloway Forest in SW Scotland.

The Geminids’ radiant (the point in the sky where all the meteors appear to emerge from) rises at sunset, so you can begin your meteorwatch as soon as it gets dark enough. The Moon is only 3 days old at maximum so you’ll have no interference to your dark skies.

The number of meteors that you will observe every hour depends on a number of factors:

  • the density of the cloud of dust that the Earth is moving through, that is causing the shower in the first place;
  • the height above the horizon of the radiant of the shower, the point from which the meteors appear to radiate;
  • the fraction of your sky that is obscured by cloud;
  • the naked-eye limiting magnitude of the sky, that is a measure of the faintest object you can see.

The Geminids meteor shower has a maximum zenith hourly rate (ZHR) of  around 120 (the highest of any meteor shower). This is the number of meteors that you can expect to see if the radiant is directly overhead (the point in the sky called the zenith), and you are observing under a cloudless sky with no trace of light pollution.

However conditions are rarely perfect. In the UK, for example, the radiant of the shower will not be at the zenith; it will be around 10° above the horizon at 1800h, 25° above the horizon at 2000h, 40° at 2200h, 60° at 0000h, and at its highest of 70° at 0200h.

Assuming a clear night, the other factor is the limiting magnitude of the sky, a measure of the faintest object you can see. Man-made light pollution will be an issue for most people. From suburbia the limiting magnitude of the sky is ~4.5 (around 500 stars visible), so you will only be able to see meteors that are at least this bright; the fainter ones wouldn’t be visible through the orange glow. In a big city centre your limiting magnitude might be ~3 (only around 50 stars visible); in a very dark site like Galloway Forest Dark Sky Park the limiting magnitude is ~6.5 (many thousands of stars visible), limited only by the sensitivity of your eye. So in most cases it’s best to try and get somewhere nice and dark, away from man-made light pollution.

The calculation that you need to make in order to determine your actual hourly rate is:

Actual Hourly Rate = (ZHR x sin(h))/((1/(1-k)) x 2^(6.5-m)) where

h = the height of the radiant above the horizon

k = fraction of the sky covered in cloud

m = limiting magnitude

Let’s plug the numbers in for the Geminids 2015.

ZHR = 120 (maximum)

h = 10° at 1800, 25° at 2000, 40° at 2200, 60° at 0000, 70° at 0200h

k = 0 (let’s hope!)

m = 6.5 (if you get somewhere really dark!)

So your actual hourly rate under clear dark skies is

(120 x sin(10))/((1/(1-0) x 2^(6.5-6.5) = 21 meteors per hour at 1800
(120 x sin(25))/((1/(1-0) x 2^(6.5-6.5) = 50 meteors per hour at 2000
(120 x sin(40))/((1/(1-0) x 2^(6.5-6.5) = 77 meteors per hour at 2200
(120 x sin(60))/((1/(1-0) x 2^(6.5-6.5) = 104 meteors per hour at 0000
(120 x sin(70))/((1/(1-0) x 2^(6.5-6.5) = 112 meteors per hour at 0000

If you’re observing in suburbia you need to divide these numbers by around 4, and in bright cities by 10! Nonetheless, even in a city if you’re out at midnight during peak activity you’ll see around 10 meteors per hour.

Remember though that these numbers are assuming perfectly clear skies under perfectly dark conditions, and are assuming a peak rate of 120 at each of these times. It probably won’t be quite this good, but the bottom line is: there’s never a better night to see meteors!

Solar Eclipse 20 March 2015

January 15, 2015 Leave a comment

UPDATE: Teachers! Educators! Don’t miss out on the learning opportunity of a lifetime! This partial eclipse will be the best seen in Scotland and much of the UK since 1999, and the best until 2090. And you don’t need eclipse glasses to experience it. 

Indeed, looking at the Sun is a very minor part of the experience. The most incredible thing to happen on Friday morning will be the darkening of the day, as the Sun gets covered by the Moon and much of its light gets blocked out. This will create a dusk-like atmosphere; birds will start singing, insects will come out, flowers – if you can find any – may close up! This is such an unusual and rare event that I really hope every school pupil in the country will get the opportunity to experience it. The best time for this is straight after register (0900) until around 0945.

If you have eclipse glasses you could let some of the pupils use those but I understand that you probably don’t have many pairs and fear the younger children might not use them correctly. Don’t use them then! Just get outside, tell the kids not to look at the Sun, and explore the wonderful daytime darkness. I really hope you can turn this into the learning opportunity of a lifetime.

On the morning of Friday 20 March 2015 there will be a total eclipse of the Sun. Between 0830 and 1042 the Moon will pass across the face of the Sun, blocking out part of its light. The maximum extend of the eclipse will happen at 0934 for a few minutes.

A Total Eclipse of the Sun, visible from the Faroe Islands on 20 March 2015

A Total Eclipse of the Sun, visible from the Faroe Islands on 20 March 2015

Unfortunately the “path of totality”, i.e. those parts of the world that will see a total eclipse, is in the far north Atlantic and Arctic oceans. Residents of the Faroe Islands get a ring-side seat at the total eclipse.

Path of Totality, Friday 20 March 2015

Path of Totality, Friday 20 March 2015

That said, it will still be a dramatic event in the UK, south of the path of totality, as we will see a partial solar eclipse where the Moon blocks some but not all of the Sun’s light.

A partial eclipse from Anamosa, Iowa. Credit: Steve Wendl - See more at: http://astrobob.areavoices.com/page/112/#sthash.ElDDdLAg.dpuf

A partial eclipse from Anamosa, Iowa. Credit: Steve Wendl – See more at: http://astrobob.areavoices.com/page/112/#sthash.ElDDdLAg.dpuf

The further north you are in the country the more of the Sun will be obscured, but wherever you are in the UK it’ll look quite dramatic. Here’s a handy table showing what % of the Sun’s disk will be obscured by the Moon from where you are.

Town/City % Eclipse on 20 March 2015
Lerwick, Shetland 96.8%
Kirkwall, Orkney 96.6%
Inverness 95.6%
Aberdeen 93.9%
Glasgow 93.7%
Edinburgh 93.1%
Belfast 93.0%
Newcastle 90.7%
Liverpool 89.4%
Manchester 89.1%
Birmingham 87.3%
Cardiff 86.7%
London 84.4%

Compare this with the August 1999 eclipse, where totality passed across the SW of England. During that eclipse the further south you were in the UK the better. Indeed the SW of England and the Channel Islands saw a total eclipse.  I was in Glasgow and saw an 82% eclipse. For me, this eclipse will be even better. In fact for anyone north of Liverpool, the 2015 eclipse is better than that in 1999!

Town/City % Eclipse on 11 August 1999
Lerwick, Shetland 67.9%
Kirkwall, Orkney 68.7%
Inverness 76.8%
Aberdeen 77.6%
Glasgow 82.2%
Edinburgh 81.8%
Belfast 86.8%
Newcastle 84.8%
Liverpool 90.5%
Manchester 90.1%
Birmingham 93.5%
Cardiff 97.2%
London 96.6%

Wherever you are in the UK though it’s worth watching, but BE CAREFUL. Never look at the Sun directly, even when it’s eclipsed. Here are some safety guidelines for viewing eclipses.

The Great Northern Lights Display of 27 Feb 2014

February 28, 2014 1 comment

Last night, Thursday 27 February 2014, the UK was treated to one of the best displays of Northern Lights in the past twenty years. Twitter erupted with excitement, and then pictures, which my good friend @VirtualAstro and myself @darkskyman RT-ed and commented on throughout the evening.

Aurora over Aberdeenshire, by Mark Tait @marktait78

Aurora over Aberdeenshire, by Mark Tait @marktait78

Below is just a sample of some of the best images that came in last night, but before that let’s look at why this aurora display was so good.

Two days previously a large sunspot on the surface of the Sun erupted with a huge X-class flare, rated at X4.9, the strongest of the year so far. This flare blasted off material from the Sun’s surface in what’s known as a Coronal Mass Ejection (CME). We knew that this material wasn’t aimed straight at us, but last night, two days after the eruption, it sideswiped the Earth, getting caught in our magnetic field and funnelled to the north and south poles.

It just so happened that the angle of the impact, and the timing, was perfect for evening skywatchers across the UK, and with largely clear skies across the country reports started coming in around 7pm that we might be about to see storm level activity. In the end it was rated as G2 (moderate) but the position of the auroral oval meant that even this moderate storm produced some of the best views of aurorae in the UK that I can remember.

Don’t be downhearted if you missed it; there’s a chance (55% according to NOAA) that we might see more tonight as we move through the wake of the CME. It’s unlikely to be as good as last night’s show, but still worth a look.

I tweeted the best way to see the aurora:

Then images started coming in!

This from @garethpaxton in Central Scotland (a pic of the viewfinder of his camera):

Then this beautiful one from Jim Hunter Images in East Lothian:

From @ross1772 in Newmill, Scotland

Dave @makapala uploaded a bunch of images taken from Fife to his Flickr account:

Mark Tait @marktait78 got this amazing image from Aberdeenshire, showing the verticality of the aurora:

England also got some of the action with the aurora stretching as far south as Uttoxeter, in this image by @RichardH082:

And Whitby (from @whitbyglenn)

From Ravenscar (from @andy_exton)

And NE England (via @Astro_Matt27)

Northern Ireland got in on the action too, as this amazing image from Paul Martin shows:

But of course the best of it was in the north of Scotland, such as this stunning image from Innes Mackay in Lewis:

The Fireball of 21 September 2012

September 22, 2012 4 comments

Did you see it? Plenty of UK-based stargazers did; a huge, bright, long-lasting fireball streaked through the sky last night, 21 September, at 2255BST (2155UT).


The fireball was caught on meteorlog’s meteor cam

I saw it as I was driving home from a stargazing trip to Loch Tay in the Highlands of Scotland. The first glimpse I caught was behind some trees, and I clearly saw the very bright (estimated magnitude -5), yellow fragments of a space rock that was disintegrating as it burned up in our atmosphere. It was traveling westwards from the SW, and look to be about 40° above the horizon. I lost sight of it as I drove, but once I’d turned a bend in the road, a full ten seconds later, I caught the last few fragments burning it.

Straight away I pulled over and tweeted to see if anyone else had seen in, and very soon a flood of observations came in. The team at the Kielder Observatory actually had a group of people up observing, who all saw it.

Mike Alexander, who runs the Galloway Astronomy Centre, had a group of guests out stargazing then too, and they also saw it, but even better they heard it!

By the time I got home, around 0030BST this morning, only an hour and a half after it happened, there were reports of it on BBC Radio 4, with people all over the northern UK and Ireland reporting the same thing, a disintegrating fireball burning through the night sky for around 20 or 30 seconds.

By the time I woke up this morning it had made it onto BBC News.

So what was it?

A few people suggested that it might have been man-made space debris, an old satellite burning up as it de-orbits, but this isn’t the case, for a couple of reasons. First it was traveling east to west, and satellites don’t orbit in that direction. Secondly, Mike (above) reported hearing a sonic boom approx 150 seconds after it faded. Both of these observations point to the fact that it was a large chunk of space rock, a meteor. When meteors are as bright as this one we call them fireballs.

Fireball of 03 March 2012

Last night, 03 March 2012 at around 2145GMT, my Twitter stream became flooded with reports from people saying they’d seen a giant meteor streaking across the sky.

The first I heard about it was a tweet from @VirtualAstro:

ALERT! Reports coming in of sightings of fireballs (large meteors/ Shooting stars) in the North and South of England

It turns out that these were probably all reporting the same sighting. For a brief spell the hashtag #ukcomet started to gain prominence, but it wasn’t a comet at all, rather a large chunk of space rock burning up in the Earth’s atmosphere.

On any clear dark night you can see a few meteors – which are also known as shooting stars – as the Earth hurtles round the Sun hoovering up all the bits and pieces of debris floating about in space.

Most meteors are the size of a small pebble and as they get hoovered up by the Earth they pass through the atmosphere. This generates frictional heating as the space-rock rubs past air molecules, and eventually the rock will burn up completely. This happens in part of the atmosphere called the mesosphere, which is about 75km (45 miles) above the Earth’s surface. During the brief period of frictional heating not all the energy produced is converted into heat, some of it gets converted into light, which is why we see them streaking across the sky. Usually a meteor will be moving so fast, and burn up so quickly, that is appears as a very quick flash of light, of less then a second in duration.

But there are bigger bits of rock out there too, and when something bigger than around 10cm enters the Earth’s atmosphere we might get a far more spectacular display, something called a fireball, or bolide meteor.

This is what happened last night. Rather than friction heating up the rock (there was probably a bit of that going on too) the energy seen in fireballs is generated by ram pressure. This is as a result of the large rock crashing into the atmosphere and causing all of the air in front of it to rapidly compress, forming a shock wave. The air in this shock wave heats up (did you know that compressed air heats up? Feel the tube on a bicycle pump next time you’re blowing up a tire) and flows around the rock, causing it in turn to heat up. This process starts the rock glowing, and when it’s bright enough we see it as a fireball.

Fireballs are much brighter than standard meteors – in fact the IAU defines a fireball as any meteor brighter than magnitude -4 – and last longer in the sky, and so they’re much easier to spot. Therefore even though they’re much rarer than your common or garden meteors they tend to get spotted by lots more people, and are even visible in big cities, hence the flurry of reports late last night.

Twelve hours of “daylight”

March 1, 2011 4 comments

Today, throughout the UK, civil twilight began almost exactly twelve hours before it will end this evening meaning that we have, for the first time this year, twelve hours of “daylight”. Summer is on its way!

Sunset on Sark, 11 September 2010

Sunset on Sark, 11 September 2010

City Civil Twilight Dawn Civil Twilight Dusk
Glasgow 0632 1828
Manchester 0622 1821
London 0614 1814

Of course some of this “daylight” is what we call twilight, but if you’re outside between these times you will certainly think that the sky is bright, and that the day has begun.

Technically the equinox (“equal night”) doesn’t occur this year until 20 March, with the equilux (“equal light”) occurring a few days before that (it varies around the world but in most of the UK the equilux occurs on 18 March 2011). For a detailed explanation of equinox, equilux and twilight times see my blog post from last March.

%d bloggers like this: