Blue Moon Friday 31 July 2015

July 27, 2015 2 comments

This Friday 31 July 2015 there is an event which happens only “once in a blue moon”. Literally. This month there is a Blue Moon.

The occurrence of a Blue Moon doesn’t mean that the Moon will in fact turn blue. Instead a Blue Moon refers to a second Full Moon occurring within a fixed amount of time.

A full moon

A Full Moon, definitely not Blue

There are two widely accepted definitions of a Blue Moon: either it is an additional Full Moon within a season, or an additional Full Moon within a calendar month.

Moon Phases

Normally there are twelve Full Moons in a year, with one occurring every month. In fact the word “month” is derived from “Moon”. However the phases of the Moon don’t cooperate and divide the year perfectly into twelve with no left overs.

The Moon orbits the Earth every 27.32166 days, known as a sidereal month. As it does so we see different fractions of the lit half of the Moon, creating different phases. However during these 27.32166 days the Earth also orbits the Sun, and so the rate at which the phases change and repeat themselves is slowed down. Looking at the Moon from down here on Earth we see the pattern of phases repeating every 29.53059 days, known as a synodic month.

This is roughly one calendar month, but not exactly. It’s because of this “not exactly” that we don’t get a round number of Full Moons occurring every year, and don’t get exactly one occurring every calendar month.

In fact there are 12.37 Full Moons every year, and for this reason, every so often, we get 13 Full Moons in a year, which means an extra one in a season or in a calendar month.

The Maine Farmers’ Almanac Blue Moon (Type 1)

The original definition of the Blue Moon came from the Maine Farmers’ Almanac which defined a Blue Moon as the third Full Moon within a quarter-year season that has four Full Moons. Confused? You’re not alone. Normally a quarter-year season will have three Full Moons in it, as normally there are 12 Full Moons in a year. But due to that extra Full Moon that we sometimes get, every so often there are 13 Full Moons in a year. This extra Full Moon will occur in one specific season, and in that season the third of the four Full Moons is known as the Blue Moon.

Additional confusion arises due to the fact that the Maine Farmers’ Almanac uses a different definition of a season from the one astronomers use. Astronomers define the start and end points of the four seasons by the position of the Sun in the sky, or put another way the position of the Earth in its orbit. Because the Earth moves at different speeds at different points in its orbit the astronomical seasons are different lengths. Agricultural seasons in the Maine Farmers’ Almanac were all the same length.

This leads to the situation where a Blue Moon (as defined by the Maine Farmers’ Almanac) might occur in an agricultural season but not within an astronomical season. In order to avoid this additional confusion, seasonal Blue Moons are calculated with respect to the astronomical seasons these days.

For decades this definition of a Blue Moon held and was the only one. However now we have an alternative definition, thanks to a mistake in a prominent astronomy magazine.

The Sky and Telescope Blue Moon (Type 2)

In 1946 the astronomy magazine Sky and Telescope published an article by James Hugh Pruett in which he mistakenly interpreted the Maine Farmers’ Almanac. He correctly stated that due to the 12.37 Full Moons per year, we get an extra (thirteenth) Full Moon in seven years out of every 19. He then went on to state that the extra Full Moon that occurs in these seven years must occur in a specific month (correct) and that the second Full Moon in a calendar month is known as the Blue Moon (incorrect, according to the original definition).

Despite the fact that this definition of a Blue Moon was a mistake at the time, it was widely adopted, probably in large part due to its relative simplicity, and is the one that most people use these days.

This Month’s Blue Moon

This Friday’s Blue Moon is an example of a Type 2 Blue Moon, the second Full Moon within a calendar month (July 2015). The first Full Moon this month occurred on 2 July, leaving ample time for the second Full Moon to sneak in at the end of the month, on Friday 31 July 2015.

A Type 2 Blue Moon occurs on average once every 2.7 years. Most type 2 Blue Moons occur within months of 31 days, but they can occur in 30-day months. Because February is only 28 or 29 days long (shorter than the 29.53059 days of the synodic month) February can never have a Blue Moon (jn fact sometimes February has no Full Moons in it at all! The last time this happened was February 1999; the next time it will happen is February 2018).

Within any given century you can expect 37 Blue Moons, around 33 of which will occur in a 31-day month, and around seven of which will occur in a 30-day month.

Future Blue Moons

After this week’s Blue Moon the next one won’t occur until 2018, but then we get two that year! The first occurs on 31 January 2018 (Full Moons on 2 and 31 January 2018) and the second on 31 March 2018 (Full Moons on 2 and 31 March 2018).

After that we have to wait until 31 October 2020.

The next Blue Moon to occur in a 30-day month happens on 30 September 2031.

Today’s the Day We Reach Pluto

UPDATE: After nine and a half years of flying towards , is now flying away from Pluto. And here is the best image yet taken of the icy dwarf planet. Amazing!


It’s taken the New Horizons spacecraft 3462 days (nine-and-a-half years) to fly the 3 billion miles to Pluto in the outer reaches of our solar system. Today at 1250 BST it will make its closest approach, zipping past Pluto at 30,000 miles per hour, gathering data as it does so.


Everything has been building towards this moment for the thousands of scientists and engineers anxiously waiting for images and information about the tiny ice world. But for now it’s all in the hands of the automatic systems aboard New Horizons. It has turned its antenna away from Earth so that it can focus its attention on Pluto and its moons (Pluto has five known moons, Charon, Styx, Kerberos, Nix, and Hydra). This means that we currently don’t have any way of communicating with or receiving data from New Horizons. It’s on its own until the pre-programmed sequence turns its antenna back towards Earth and begins transmitting back to us. We should begin to receive signals again around 0200 tomorrow (Wednesday) morning.


And what do we hope to see? It’s almost impossible to predict what new imformation this flyby will reveal, but one thing’s for certain: the images will get a whole lot better. The picture above was taken on Sunday from a distance of 2.5 million km. That’s 100 times further than today’s closest approach. The best resolution images we’ll take of Pluto today will allow us to resolve down to 100m per pixel, far better than anything we have seen so far. The above image has a resolution of several km per pixel for example.

So will we see anything at 1250 today? While we won’t start to receive the hi-res images until tomorrow, NASA has held back the final image of Pluto taken by New Horizons before its antenna swung away from us. This is a failsafe image, just in case we don’t hear from New Horizons again*. This image will be released today at the moment of flyby, so stay tuned.

Pluto: The Largest Dwarf Planet

When Pluto was discovered in 1930 it was named the ninth planet in our solar system, but then in 2005 astronomers discovered another object out beyond Pluto, which we called Eris. That name – after the Greek goddess of discord – is apt, as it threw the definition of a planet into chaos. Eris, which at the time was thought to be a little larger than Pluto, must surely be a planet too. But what happens when we discover more such objects out beyond Neptune?

This part of our solar system is known as the Kuiper Belt, and is a little like the asteroid belt only icier. There could well be hundreds of these so-called “Plutoids” or TNOs (Trans Neptunian Objects) out there. To avoid the problems of hundreds of new planets, the International Astronomy Union created a definition of a planet in 2006 that deliberately excludes Pluto and all the other Plutoids.

So Pluto went from being the smallest planet to the second largest dwarf planet (after Eris). But recent measurements made by New Horizons have allowed us to recalculate Pluto’s size and it turns out to be larger than Eris, by a whisker.

Eris is 2326km across (give or take a few km). Measuring Pluto is tricky because of its thin atmosphere, which makes the edges of the dwarf planet fuzzy. However New Horizons is close enough that it can make better measurements than we have had before, which put Pluto’s diameter at 2370km. Pluto is now the king of the dwarf planets!

* Flying through space isn’t risk-free. There are lots of tiny pieces of dust and rock floating out there. Due to its incredible speed even a small particle could wipe out New Horizons if it impacts. As we approach Pluto the number of these particles increases, but it’s still highly unlikely that we’ll experience a catastrophic impact. We’ll know for sure when New Horizons re-establishes contact at around 0200 on Wednesday 15 July

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.


Get every new post delivered to your Inbox.

Join 3,398 other followers

%d bloggers like this: