2015: The Year of Dwarf Planets and Small Solar System Bodies

We’re currently living through a very exciting time in space exploration, with a small armada of robot space probes visiting previously unexplored corners of our solar system. Here’s just a few of the amazing discoveries we’ve made in the past few weeks.

New Horizons

New Horizons

This year sees us make close encounters with two of the largest dwarf planets, as New Horizons flies past Pluto for the first time, and Dawn continues to orbit the giant asteroid Ceres. All this as the Philae Lander continues to try to make contact with us from the surface of Comet 67P/Churyumov-Gerasimenko as its parent spacecraft Rosetta follows the comet around the Sun.

Each of these missions is very exciting in its own right, but to have all three happening at once is incredible.

Rosetta and Philae Latest

The Rosetta Orbiter arrived at Comet 67P/Churyumov-Gerasimenko in August last year, and the Philae lander descended onto the comet’s surface in November, carrying out its science mission for 60 hours before its batteries died. Rosetta has continued to produce great science since then; its latest scoop was the discovery of what appear to be sink-holes on the comet’s surface.

Sink Holes on Comet 67P

Sink Holes on Comet 67P

All this while Philae tries to make contact with us, and Comet 67P begins the outgassing that will eventually form its tail as the comet makes its closest approach to the Sun on 12 August 2015.

Comet 67P/Churyumov-Gerasimenko begins outgassing

Comet 67P/Churyumov-Gerasimenko begins outgassing

Dawn Latest

The Dawn spacecraft arrived at Ceres in March 2015, after having spent over a year orbiting the smaller asteroid Vesta. Ceres is the largest of the asteroids, so large in fact that it’s considered a dwarf planet, its gravity having pulled it into a spherical shape.

More and more mysteries are arising as a result of Dawn’s asteroid mission including: what are these bright patches inside craters on Ceres’ surface?

Bright spots in the surface of the Dwarf Planet Ceres

Bright spots in the surface of the Dwarf Planet Ceres

and: what’s a mountain doing on an asteroid?

A mountain on an asteroid

A mountain on an asteroid

New Horizons Latest

Stay tuned for even better images of Pluto as New Horizons speeds towards its 14 July flyby at close to 60000kph. For now the best images we have of Pluto and its moon Charon are from New Horizons’ Long-Range Reconnaissance Imager, which shows features on the surface of the distant Dwarf Planet, which we’ll see in better detail in the next couple of weeks.

Pluto and Charon, real colour

Pluto and Charon, real colour

Other Missions

This is on top of all of the other missions going on up in space right now: Cassini continues to send back breath-taking images and data from the ringed planet Saturn and its moons; no fewer than five spacecraft are currently in orbit around Mars – NASA’s 2001 Mars Odyssey, , Mars Reconnaissance Orbiter, and MAVEN, ESA’s Mars Express, and India’s Mangalyaan – while two intrepid rovers – Opportunity and Curiosity – explore Mars’ surface; and our own Moon is orbited by the Lunar Reconnaissance Orbiter.

We’ll add to this over the next few years, as the Juno probe reaches Jupiter in summer 2016, and as the Japanese mission Hayabusa 2 enters into orbit around an asteroid in 2018 and returns a sample to Earth on 2020.

Summer Solstice 2015

June 21, 2015 1 comment

The northern hemisphere summer solstice occurs today, 21 June 2015 at 1738 BST.

But surely the summer solstice is just the longest day. How can it “occur” at a specific instant?

That’s because we astronomers define the summer solstice as the instant when the Sun gets to its furthest north above the celestial equator. Or to put it another way, the instant when the north pole of the Earth is at its most tilted towards the Sun.

And this happens at exactly 1738 on 21 June 2015.

It’s important to remember though that while we are in the midst of summer, the southern hemisphere are experiencing their winter solstice, and their shortest day.

And how much longer is our “longest day”? In Glasgow, my home town, the Sun will be above the horizon for 17h35m12s today (21 June), six seconds longer than yesterday, and three seconds longer than tomorrow!

Categories: Uncategorized

How to calculate your horizon distance

May 25, 2015 2 comments

While on a recent trip to the remote South Atlantic island of St Helena (exile place of Napoleon, and location of Edmond Halley’s observatory) [blog post to follow!] I ascended the highest mountain on the island, Diana’s Peak.

At 823m above sea level it commanded splendid views of the island, but the most striking thing was the unbroken 360° view of the horizon. I did a quick calculation in my head of how far I could see, and that forms the basis of this blog post: how do you calculate your horizon distance?
It turns out it’s pretty straight forward if you know a little simple maths. It helps to start by drawing a picture, so I did:
Definitely not to scale

Definitely not to scale

For an observer of height h above sea level, the horizon distance is D. The Rs in this diagram are the radius of the planet you’re standing on, in this case the Earth. The only real assumption here is that you’re seeing a sea level horizon.As you can see you can draw a right-angled triangle where one side is D, the other is R, and the hypotenuse (the side opposite the right angle) is R + h.

Using Pythagoras’s Theorem, discovered around 2500 years ago, the square of the hypotenuse is equal to the sum of the squares of the other two sides. So we can say that:

(R + h)2 = R2 + D2

If you expand the part to the left of the bracket you get (R + h)2 = R2 + 2Rh + h2 so that:

R2 + 2Rh + h= R2 + D2

There’s an R2 term on both sides of the calculation so you can cancel them out, leaving:

2Rh + h= D2

Therefore the horizon distance, D, is:

D = √(2Rh+h2)

Here’s where you can make life much simpler for yourself. In almost every case R is much, much larger than h, which means that 2Rh is much, much larger than h2 so you can just ignore h2 and your equation simplifies to:

D ≈ √2Rh

(the ≈ sign here means “almost equals”. Honestly.)

So if you know R and h you can calculate D. To make this calculation easily you can carry round the value of √2R in your head meaning you only have to calculate √h and multiply those two numbers together.

So for the Earth, R is 6371000m, so √2R is 3569.6. Multiplying this by √h in metres would give you D in metres, so lets convert that into km to make things easier. This means dividing this number by 1000, giving an answer of 3.5696 which is ≈ 3.5.

So as a rough rule of thumb, your horizon distance on Earth,

D = 3.5 x √h

where D is measured in km and h in metres.

On Diana’s Peak, at 823m high, √h = 28.687… which multiplied by 3.5 gives a horizon distance of almost exactly 100km!

This is pretty cool, and is true of anywhere you can see the sea from a heigh of 823m.

One final calculation which sprung to mind on the mountain top was the area of sea I could see, which is easy to work out using the fact that the area of a circle is πr2, where r in this case is D, or 100km.

π is 3.14159 which means that the area of sea I could see was 31415.9 km2. Just a tad larger than Belgium, at 30528 km2.

And in that Belgium-sized circle of ocean was only one ship, the RMS St Helena that was taking me home the following day.

What about on other planets?

If you’re on Mars your horizon distance is shorter, at 2.6√h. On Mercury it’s smaller still at 2.2√h. This is due to Mars and Mercury being much smaller than the Earth, and so their surfaces curve away from you quicker. Venus is almost exactly the same size as the Earth (only a fraction smaller) so there you’d have to use the same calculation as here on Earth, 3.5√h.

Hovering above the surface of Jupiter your horizon would stretch to 11.8√h and on Saturn to 10.8√h. Uranus and Neptune are about the same size, giving a horizon distance of 7.1√h.

Mercury 2.2√h
Venus 3.5√h
Earth 3.5√h
Mars 2.6√h
Jupiter 11.8√h
Saturn 10.8√h
Uranus 7.1√h
Neptune 7.1√h

What about the dwarf planets? Being so small their surfaces will curve away from you very quickly, shortening your horizon distance. One of the smallest spherical objects in the solar system is the dwarf planet Ceres (as in cereal), which is the largest object amongst the fragments of rock in the asteroid belt. Your horizon distance on Ceres is almost exactly √h, making that a pretty simple horizon calculation!

Lyrids Meteor Shower 2015

April 18, 2015 3 comments

UPDATE 24/04/15 Now that we’re past the peak it looks like the Lyrids meteor shower performed as expected. Reports from the Society for Popular Astronomy suggest that plenty of meteors were seen over the UK.lyr2015overview

A wider survey made by volunteers submitting data to the International Meteor Organisation shows that a peak with ZHW=18 occurred more or less on cue around midnight on 22/23 April, with a possible second several hours later around 0700UT where the rate if anything was a little higher, with ZHR=22.


Over the next week one of spring’s best meteor showers will start to put on a show. The Lyrids meteor shower peaks overnight on the night of 22/23 April 2015, and should be best around midnight.


It’s quite hard to predict when exactly the peak will occur, and indeed you’ll still see some Lyrid meteors on the nights either side of the peak, so whenever you’ve got clear dark skies between now and 25 April it’s worth gazing skywards (isn’t it always?) in the hope that you’ll see a shooting star.

Why is the Lyrids Meteor Shower Happening This Week?

Meteor showers like the Lyrids happen when the Earth passes through a cloud of dust in space, These clouds are left behind by comets as they orbit the Sun, and the cometary cast-offs burn up in our atmosphere causing lots of bright streaks of light which we call meteors, or shooting stars. On any clear dark night you should see a few shooting stars, as random bits of space dust burn up overhead, but on the nights around the peak of a meteor shower, when the Earth is passing through a dense cloud of comet-dust, the rates can dramatically increase.

How Many Lyrids Will I See? There are a few ways you can maximise your chances of seeing some Lyrids (see The What, How, Where, When and Why of Meteor Showers) but the best way is to get somewhere dark, like one of the UK’s International Dark Sky Places. On the peak of the Lyrids meteor shower, under ideal conditions, you might see around 18 meteors per hour.

The peak of this particular shower doesn’t last very long, and so the rate on either side of the peak might be quite a bit less. Nonetheless it’ll still be well above the background rate of meteors. However the Lyrids occasionally surprises us and puts on a much better show. Back in 1982 there was a short-lived burst of Lyrid activity that saw the rate increase from 18 to 90. The same thing could happen this year: you never know until you look!

Ideal Conditions It’s the “ideal conditions” clause above that’ll reduce the rate from this maximum of 18. Ideal conditions are: perfectly clear skies; perfectly dark skies, free of light pollution; and the meteor shower radiant (the point where they all appear to emanate from) sitting directly overhead. The Lyrids’ radiant will be around 30° above the horizon at midnight, when the peak is meant to occur, but you can begin your meteorwatch as soon as it gets dark enough. You’ll then have until the sky brightens again pre-dawn. . 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.

Crunching the Numbers The Lyrids meteor shower has a maximum zenith hourly rate (ZHR) of  around 18. 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 20° above the horizon at 2200, 30° above the horizon at 0000, 50° at 0200, to a maximum height of 70° pre-dawn.

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 Lyrids 2015.

ZHR = 18 (maximum) h = 30° at 0000 (assuming the maximum occurs at midnight; it might not) k = 0 (let’s hope!) m = 6.5 (if you get somewhere really dark)

So your actual hourly rate under clear dark skies is (18 x sin(30))/((1/(1-0) x 2^(6.5-6.5) = 9 meteors per hour If you’re observing in suburbia you need to divide this by around 4, and in bright cities by 10! Nonetheless, even in a city you’ll see a few Lyrids over the course of the night.

International Dark Skies Week 13-19 April 2015

April 13, 2015 Leave a comment

This week is International Dark Skies Week, 13-19 April 2015.

Created in 2003 by high-school student Jennifer Barlow, International Dark Sky Week has grown to become a worldwide event and a key component of Global Astronomy Month.

I want people to be able to see the wonder of the night sky without the effects of light pollution. The universe is our view into our past and our vision into the future . . . I want to help preserve its wonder.” – Jennifer Barlow

This year’s International Dark Skies Week coincides with the International Year of Light, which makes it particularly appropriate.

Why not head outside at night and explore dark skies? You could head to any International Dark Sky Places. In the UK we have six:

Any of these sites will provide excellent views (weather permitting!) of a real dark sky.

If none of these are convenient why not visit a Dark Sky Discovery Site near you. These are sites that have been identified by local communities as being convenient for stargazing, although they don’t have the light pollution control measures that the International Dark Sky Places do.

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.

Stargazing Hotel Breaks And Cruises 2015

December 20, 2014 3 comments

In 2015 I’m delighted to be hosting a range of stargazing events, from stargazing weekend breaks under some of the UK’s darkest skies, to a cruise to one of the most remote islands in the world, steeped in astronomy history.

Steve Owens’ contribution was perfect. We liked how he joined in with the guests at meals etc and held the group together. He has a gift of being able to convey his knowledge in terms easy to understand.His lecture with slides was really informative and interesting as was the enthusiasm he put into answering our every question or listening to our accounts of minor brushes with stars!!

Here’s a list of the hotels I run stargazing breaks at:

Glenapp Castle, Ballantrae, Scotland (Galloway Forest Dark Sky Park)
Kirroughtree House Hotel, Newton Stewart, Scotland (Galloway Forest Dark Sky Park)
Selkirk Arms Hotel, Kirkcudbright, Scotland (Galloway Forest Dark Sky Park)
Yarn Market Hotel, Dunster, Exmoor (Exmoor Dark Sky Reserve)

And here’s the cruise I’m hosting:

Stargazing and astronomy cruise, 4-22 May 2015, Cape Town to St Helena. On this stargazing tour you will have the opportunity to stargaze from a truly unique place – St Helena. The island is steeped in astronomical history, and you’ll visit the sites of Halley’s observatory (he of comet fame), as well as those of the other astronomers who have visited St Helena over the centuries. Its location near the equator means that virtually every constellation in the sky is visible at one time or another from St Helena, and visitors from the UK will be amazed to see a whole new collection of stars in the southern hemisphere that simply aren’t visible from Europe: the famous Southern Cross, the Magellanic Clouds, and the galactic centre of the Milky Way.

Here’s a list of the weekends I’m running throughout the year. Click the links for the hotels above to find out more or to book!

Stargazing weekend break, 20-22 February 2015, at the Yarn Market Hotel, Dunster, in Exmoor International Dark Sky Reserve

Stargazing weekend break, 13-15 March 2015, at the Selkirk Arms Hotel, Kirkcudbright, near Galloway Forest Dark Sky Park

Stargazing weekend break and Solar Eclipse Special, 20-22 March 2015, Kirkcudbright, near Galloway Forest Dark Sky Park (this weekend break can be extended to a three night stay to watch the partial eclipse of the sun at sunrise on Friday 20 March!)

Stargazing weekend break, 9-11 October 2015, at the Selkirk Arms Hotel, Kirkcudbright, near Galloway Forest Dark Sky Park

Stargazing weekend break, 4-6 December 2015, at Kirroughtree House Hotel, Newton Stewart, in Galloway Forest Dark Sky Park

Stargazing weekend break, 11-13 December 2015, at Kirroughtree House Hotel, Newton Stewart, in Galloway Forest Dark Sky Park



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