Partial Lunar Eclipse Tonight, 25 April

There’s a partial lunar eclipse tonight, visible from the UK, as well as from the rest of Europe, Africa, Asia and Australia.

It won’t be hugely dramatic, as it’s only a partial eclipse of the Moon, not a total one. Even total lunar eclipses are far less grand than total solar eclipses, unfolding over several hours rather than minutes, and turning the Moon a deep red rather than making it vanish altogether.

And for partial lunar eclipses, like tonight’s, all we’ll see is a slight darkening of the edge of the Moon, what we call the “limb”.

Nevertheless it’s worth watching out for if you have clear skies. And the best thing of all is that light pollution isn’t really an issue; you’ll see it just fine from a city.

Here are the timings:

Penumbral Eclipse Begins: 18:03:38 UT
Partial Eclipse Begins: 19:54:08 UT
Greatest Eclipse: 20:07:30 UT
Partial Eclipse Ends: 20:21:02 UT
Penumbral Eclipse Ends: 22:11:26 UT

Remember that these times are in universal time (UT) which is the same as GMT, so add one hour on for BST.

Best time to look is between 9pm and 9:20pm BST.

Image from NASA’s eclipse site.

Light Pollution and the Loss of Wilderness

Today marks the 175th anniversary of the birth of John Muir, the Scottish-born American naturalist, writer, and advocate for the preservation of wild land.

The protection of our wildernesses landscapes (defined as anywhere you cannot see the intrusion of human activity) is more important than ever, with the spread of suburbia and the urbanisation of more than 50% of the world’s population.

But one measure of what makes a wilderness has to-date been largely ignored: that of the darkness of the night sky. After all, if you can see the sky glowing orange at night then you are seeing the intrusion of human activity, and you can’t consider the land you’re in a true wilderness.

John Muir’s legacy as the founding father of the conservation movement lives on today, in part in the organisation The John Muir Trust.

The JMT estimates that the amount of Scotland’s landscape that is wilderness is rapidly diminishing, dropping from 31% of Scotland to 28% between 2008 and 2009, but I think if you added in the spread of man-made light pollution the situation would be decidedly worse.

I am fortunate to have been awarded the JMT’s Bill Wallace award to help fund a trip later this year (once the skies get dark again after the bright summer nights) to map light pollution in one of Scotland’s most wild landscapes, between the JMT properties of Quinag and Sandwood Bay.

Hopefully this project – the first of its kind in this remote area – will shed some light on the problem of the loss of our wilderness nightscapes.

iPhone Astrophotography: Night Modes

As a follow-up to my previous post about astrophotography with an iPhone, I spent a few minutes tonight playing around with a new app called Night Modes, which claims to allow you to have real (hardware) shutter speeds of up to one second, a substantial improvement on previous apps which have used software tricks to try and mimic long exposures. These are next to useless for capturing star-scapes, photos of the night sky overhead. Even one second exposure is rather short, and will only let you catch the very brightest stars, but still more than enough to make out the constellation patterns.

Night modes allows you to set the exposure to 1/15, 1/8, 1/4, 1/2 or 1s, lets you deactivate the autofocus (which you’ll have to do – autofocus gets confused when you try and snap a picture of the night sky). The app also allows you to set a timer delay, to avoid hand-shake blurring your image as you push the button.

Another essential item to avoid camera shake is something you put your iPhone on when the exposure is being taken – ideally a tripod, but you can rest it on anything that won’t wobble too much. In the absence of a tripod adaptor for my iPhone I simply placed it on the table in my garden, propped against a book, pointing roughly towards Jupiter.

After setting a 5s delay (enough time, I reckoned, for me to place my iPhone gently on the table, and for any wobbles to die down), disabling auto-focus and auto-exposure, and setting the exposure to the maximum 1s, I sat the iPhone down and waited. And this is what I got:

Orion, Taurus and Jupiter, taken with an iPhone 5 using Night Modes app, 1s, 3200 ISO

Not the best image ever, but you can make out Orion with those phone lines running in front, and in the top right corner you can see the bright (and slightly over-exposed) Jupiter above the V-shape of the head of Taurus. The next step will be to take some images out of the city, somewhere with less light pollution, so I don’t get that horrible orange glow to the sky.

Perihelion 2013

At around 0500 GMT on 2 January 2012 the Earth was at perihelion, its closest approach to the Sun this year.

If that sounds confusing to you, and has you wondering why it’s so cold given that the Earth is at its closest to the Sun, then this belies (a) a northern-hemisphere-centric attitude (in the Southern Hemisphere it’s summer right now), and (b) a misunderstanding of what causes the seasons.

The Earth orbits the sun in a nearly circular orbit called an ellipse. The degree by which an orbit differs from a perfect circle is called the eccentricity, e. If e = 0 then the orbit is circular; if e = 1 then the orbit is parabolic, and therefore not gravitationally bound to the Sun. The Earth’s orbital eccentricity is 0.0167, meaning that it is very nearly circular, with the short axis of the ellipse being around 96% the length of the long axis. Thus, during perihelion Earth is 0.983AU from the Sun, while during aphelion (its furthest distance from the Sun, occurring this year on 4 July) Earth is 1.017AU from the Sun. (1AU = 1 astronomical unit = the average distance between the Earth and the Sun = 150 million km).

The seasons on Earth have really nothing to do with how close the Earth is to the Sun at different times of year. Indeed how could they, given that the difference in distance between closest and furthest approach is only a few per cent? The seasonal differences we experience are of course caused by the tilt of the Earth’s axis, which is inclined by 23.5 degrees from the vertical.

This tilt means that, as Earth orbits the Sun, for six months of the year one hemisphere tips towards the Sun, so that it experiences longer days than nights, becoming most pronounced at midsummer, at which point the Sun reaches its highest in the sky at noon. Simultaneously the other hemisphere tips away from the Sun, and experiences shorter days than nights, becoming most pronounced at midwinter, on which day the Sun is at its lowest noontime altitude.

The further you are from the equator the more pronounced the seasonal effects. In fact equatorial countries don’t experience seasonal variations, while the poles experience extremes with six-month-long winters and summers. The timing of perihelion and aphelion relative to our seasons is entirely random. The fact the southern hemisphere midsummer (21 Dec) almost coincides with perihelion (2 Jan) is simply that; a coincidence. Given that fact, there is no reason to be surprised that perihelion occurs so close to northern hemisphere midwinter: it has to happen some time and it’s a coincidence that it happens to occur within two weeks of midwinter / midsummer.

New Telescope for Christmas?

Are you lucky enough to have been given a shiny new telescope for Christmas? If so you have joined the ranks of thousands of other stargazers around the world, and you’re no doubt eager to get outside and use your new toy.

But a new telescope can be quite a complicated and daunting piece of hardware, so let’s go through the basics, to help you on your way.

1. Read the instructions!
Like most complicated pieces of equipment your telescope should have a user manual or a set of instructions. If not, try looking online. These instructions will help you assemble your telescope, and become familiar with all of the various parts, including the finder scope, eyepieces, focus mechanism, and motion controls.

2. Align the finder scope in the daytime.
The small telescope that sits on the main tube of your telescope is called the finder scope. You can use this to locate an object in the sky, and if the finder- and main telescopes are aligned (facing in exactly the same direction) then that object will be in the centre of the field of view of your main scope too. Aligning the finder scope is a bit fiddly though, so do it in the daytime before you observe. Point your main telescope at a specific distant object, like a far away tree, or chimney pot, or transmitter mast. (WARNING: don’t point it anywhere near the Sun). Once that object’s in the very centre of your main scope field of view, look through your finder scope. The chances are it’s not in the crosshairs here, so adjust the position of your finder scope until it is. This is usually done using small screws that physically move the finder scope around till it’s aligned with the main scope. Now it’s ready to use tonight.

3. Pick the correct eyepieces.
Your scope probably came with a couple of eyepieces. These should be marked with their focal length, in mm. The higher this number the lower the magnification. So a 25mm eyepiece will provide smaller images than a 10mm eyepiece. But magnification isn’t everything. In most cases you should start with your least powerful eyepiece, which gives you the largest field of view. Once you’ve found your target you can substitute a more powerful eyepiece in, to get a larger image. It’ll be larger, but dimmer. After all you’re spreading the same amount of light over a larger image. You’ll also notice any wobbles in the telescope much more when you’re using a higher power eyepiece. So your low-power eyepiece will give you brighter, clearer images, even if they’re much smaller.

4. Learn how to move around the sky.
All telescopes are different, and the way that you move them from one object to the other varies. In general though they will all have hand-screws that you can tighten and loosen to lock the telescope in position, or to move it. There may also be dials or screws to give fine adjustments to a telescope’s positioning. On the other hand, some telescopes – called dobsonians – are just moved by physically nudging the telescope tube. However yours moves, you’ll be doing this in the dark, so practice, practice, practice until it’s second nature to you. Some motorised telescopes find and track the stars and planets for you, but these are a bit trickier to set up properly, so read the instructions.

5. Choose the right targets.
Don’t go hunting down very faint elusive objects on your first night out; stick to the brighter ones that are easier to find. On Christmas night and for a couple of days after, the Moon and Jupiter are up in the evening. Even a small telescope will give great views of the Moon (although you’ll get a better view when it’s not quite so full, and you can observe the line between light and dark, called the terminator line) and will let you see Jupiter’s moons, looking like four tiny specks next to the bright planet Jupiter.

Whatever you look at with your new telescope, enjoy it, and remember that patience is a virtue. You’re the proud owner of a great stargazing tool, but you need to practice to get the hang of using it properly.

Let everyone know what you’ve been observing, and how you’ve found your new telescope, in the comments below.

Winter Solstice 2012

The northern hemisphere winter solstice occurs on 21 December 2012, at 1112 GMT. At this point the Earth’s north pole will be tipped away from the Sun. As seen from Earth, the Sun will stop its slow daily decent south in our sky – over the past six months the Sun’s mid-day height above the horizon has been decreasing steadily – and once again turn north, getting higher in the sky at noon each day, until it gets to its highest point in midsummer 2013.

The actual day of the winter solstice – in this case 21 December 2012 – is commonly known as midwinter, the shortest day, and is the day when the Sun spends least time above the horizon. The further north of the equator you are, the more profound the effect. Indeed if you live within the arctic circle the Sun won’t actually rise today.

Tau Ceti, a nearby Sun-like star, MAY have an Earth-like planet

One of the closest Sun-like stars to us, Tau Ceti, in the constellation of Cetus the Sea Monster, MAY have a family of five Earth-like planets orbiting it, one of which MIGHT be in the star’s circumstellar habitable zone (CHZ), otherwise known as the “goldilocks zone” where it’s not too hot, not too cold, but just the right temperature for liquid water to exist.

Tau Ceti lies only 12 light years away from our solar system, which in astronomy terms is just next door. There are only 19 stars closer to the Sun, and only one of these is a Sun-like star, Alpha Centauri, which lies only 4.4 light years away.

Tau Ceti is a bit smaller than the Sun (0.8 times the Sun’s radius), is cooler (5350K compared to the Sun’s 5780K) and less luminous (0.5 times the Sun’s brightness), and so the CHZ in which the Earth-like planet MIGHT orbit is much closer to the star than the Sun’s CHZ, around half the Earth-Sun distance, approximately 75 million kilometres.

The five planets that MIGHT have been discovered are labelled Tau Ceti b, c, d, e, and f, and the Earth-like planet is the fourth from the star, e.

Why all the MAYBEs and MIGHTs? Well, that comes down to the method by which the planets were detected. They were discovered by observing the star Tau Ceti, and watching for wobbles caused by the gravitational pull of the orbiting planets. Now all five potential planets are similar in size to the Earth, between two and six times the mass of the Earth, but still are tiny compared to the star, and so the wobbles they cause the star to make are very small, almost indistinguishable from noise in the data. Further studies of the star’s wobble might show that some, none, or all of these potential planets might just be artifacts in the data.

How to find Tau Ceti in the sky

Tau Ceti is visible in the sky this month, lying low in the south around 8pm. To find it you have to star hop from the distinct constellations of Orion and Taurus to the much less obvious Cetus.

Find Orion, with the three stars of Orion’s belt pointing up and to the left to the bright star Aldebaran in Taurus. Aldebaran lies in a V-shape collection of stars called the Hyades making up Taurus’ head. This V-shape arrow points down and to the right to a bright-ish star called Menkar in Cetus, lying low in the south. Lower and to the right is the brightest star in Cetus, Diphda, and the fainter star to the left of this is Tau Ceti. Phew!

Morning Mercury, December 2012

Over the next few mornings you’ll be able spot the most elusive of the naked-eye planets, Mercury, low in the south-east just before sunrise.

Mercury is hard to find, and most days isn’t visible at all. Since it orbits so close to the Sun, when seen from Earth it never appears very far from the Sun in the sky. You can only catch it for a few days at a time when it’s furthest from the Sun in our sky, at a point called its maximum elongation. And even then it’s not that simple to find, as it will always be quite low on the horizon, hidden amongst twilight.

As Mercury whizzes round the Sun (it takes 88 days to make one complete orbit) sometimes we see it in the morning and sometimes in the evening. The amount of time between one morning appearance and the following evening appearance is around six or seven weeks. However Mercury isn’t very clearly visible at every maximum elongation (in some the Sun is much nearer the horizon so the sky is much brighter, making it harder to find), and even when it is clearly visible you’ll only catch sight of it on the few days before and after the date of maximum elongation.

Mercury’s next maximum elongation in of 4 Dec 2012, when it’s quite far (21°) west of the Sun, and quite bright (magnitude -0.3) making it quite easy to spot over the next few mornings.

How to find Mercury

If you have clear skies, head outside around 0630 and find somewhere with a good clear SE horizon (Mercury rises around 0630 and only gets a few degrees above the horizon by the time the Sun’s light begins to significantly brighten the sky).

Luckily there are two other planets up near Mercury right now, namely Venus and Saturn. Both of these planets are brighter than Mercury and higher in the sky, and together all three form a straight line leading diagonally down to the horizon. Find brilliant Venus, the brightest thing in the sky except for the Sun or the Moon, and then look for Saturn up and to the right, and Mercury in the opposite direction, down and to the left.

This photo, taken by the excellent Paul Sutherland, shows how the three planets lined up this morning (2 Dec) when viewed from the UK.

Mercury, Venus and Saturn in the morning sky. Image credit Paul Sutherland.

Leonids Meteor Shower 2012

One of the year’s regular meteor showers, the Leonids, happens this weekend, peaking at around 0930 on 17 November 2012. It (usually*) isn’t one of the very active showers (such as the Perseids, Geminids or Quadrantids), with the maximum rate in a normal year between 10-20 meteors per hour in perfect conditions.

The peak of the Leonids is quite broad, lasting several days, so between now and early next week it’s worth looking up to see if you can catch a glimpse of any shooting stars. The best time to view the Leonids shower is in the pre-dawn hours, but any time after 11pm on Thursday through to Tuesday night should mean you’ll see at least a few meteors.

How to see the Leonids Meteor Shower

1. Find somewhere dark with as little light pollution as possible. The countryside is best, but if you’re stuck in a city try and get away from as many lights as possible.
2. Bring a reclining deck chair. Standing outside looking up for long stretches of time gets uncomfortable.
3. Bring a blanket. It gets VERY cold outside at night in November.
4. Position yourself under your blanket on your reclining deck chair so that you take in as much of the sky as possible. Although the meteors all appear to radiate out of the constellation of Leo in the SE there’s no need to specifically face this direction as the meteors will streak across any part of the sky.
5. Wait. The rate of this shower isn’t very high, so you might only see one every five or ten minutes, maybe less often than that, so patience is a virtue.

* every 33 years the Leonids meteor shower turns into a meteor storm, in which the rates dramatically increase by a factor or 50 or more, up to perhaps several thousand meteors per hour. This regularity is due to the nature of the origin of the dust that causes these meteors. It comes from the tail of a comet, Comet Temple-Tuttle, which orbits the Sun once every 33 years. This means that the dust trail left behind by the comet – and subsequently hoovered up by the Earth to make a meteor shower at the same time every year – is refreshed every 33 years, resulting in a spike of activity for a few years afterward each pass of the comet. The comet last renewed the trail in 1998, and so the years 1999, 2001 and 2002 were all spectacular years for the Leonids, with storm rates peaking at 3000 Leonids per hour. I was lucky enough to see all of these showers, the most memorable being 2002 where in the space of just two hours under half-cloudy skies on the outskirts of Glasgow I saw over 300 shooting stars.

The Return of Darkness

With summer coming to an end in the British Isles we start the return to the dark skies of autumn and winter. Depending on where you are in the country you will have been without truly dark skies for many weeks, maybe even months, as summer evening twilight lasts throughout the night during the summer.

This all-night-long twilight is almost gone throughout the UK, indeed anywhere on the mainland UK can see astronomically dark skies around 1am at the moment. Only the furthest north outpost of the British Isles still doesn’t have that opportunity.

On the island of Unst, the furthest north of the Shetland islands, lies the UK’s furthest-north town, Skaw, at 60°49′N and 00°47′W. This tiny village will see astronomical darkness return at 0043 on 24 August, lasting only 46 minutes until at 0129 the sun’s light begins to creep into the sky again.

Midsummer sunset (at 2241) from Unst. Image Credit Brydon Thomason, Shetland Nature Ltd.

The last time that astronomical darkness was seen at Skaw was on 18 April, over four months ago! Indeed this settlement is so far north that between around 13 and 29 June each year they never get out of civil twilight, meaning that the sky’s bright all night long!

Compare this with the furthest south town in the British Isles, Saint Clement in Jersey, in the Channel Islands. Astronomical darkness returned to Saint Clement on 4 July this year, having been absent since 8 June; only four weeks without true darkness!

Such is the effect of differences in latitude that these two settlements, separated by 1299 km, have such hugely different seasonal swings between summer and winter.