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!

Advertisements

Lunar Eclipse 28 September 2015: The Blood Supermoon

September 22, 2015 Leave a comment

Stargazers in the UK are ideally placed to see a rare astronomical event next week, a Total Lunar Eclipse. While not as dramatic as a Total Solar Eclipse, a lunar eclipse is well worth watching for, as the Moon turns deep red at totality.

Lunar_eclipse_April_15_2014_California_Alfredo_Garcia_Jr1

A Blood-red Lunar Eclipse

Unlike a Total Solar Eclipse, where totality lasts only a few minutes, a total eclipse of the Moon last several hours. In the morning hours of Monday 28 September the lunar eclipse begins at 0111BST and ends at 0622BST as the Moon sets. During the very early and late hours of the lunar eclipse you will see part of the full Moon’s disk darken, but it’s only when the Moon enters totality that it will turn red. This dramatic event will happen between 0311 and 0423BST.

This month’s lunar eclipse is made even rarer by the fact that the full Moon on 28 Sep is what’s called a Supermoon. This means that the Moon is closer than normal to Earth, and will appear slightly larger and brighter in the sky. But don’t believe the hype: it will be only a few % closer and so your eye will not be able to detect the difference between this Supermoon and any other Full Moon – except this time it’ll be blood red due to the eclipse! (The Moon may actually look bigger to you if you catch it low on the horizon, but that’s due to the Moon Illusion).

The brilliant XKCD sums up the Supermoon hype nicely

The brilliant XKCD sums up the Supermoon hype nicely

A lunar eclipse occurs when the Moon, in its orbit around the Earth, passes into the Earth’s shadow, as cast by the Sun. You might imagine that this would happen once every lunar orbit, or once a month. That it does not is due to the fact that the Moon’s orbit around the Earth is tilted by around 5 degrees compared with the Earth’s orbit around the Sun. So in most orbits the Moon passes above or below the Earth’s shadow.

However, once in a while (there are at least two lunar eclipses each year) the orbital planes will align so that the Moon passes through the Earth’s shadow, sometimes just grazing it, in which case we get a partial lunar eclipse, and at other times passing right through the shadow, when we get a total lunar eclipse.

The Earth’s shadow has two distinct regions, forming two concentric cones: the inner, darker, part of the shadow is called the umbra, and objects within this part of the shadow receive no direct light from the Sun. The outer, lighter, part of the shadow is called the penumbra, and objects within this part of the shadow can receive direct light from the Sun, but part of the Sun’s disk will be obscured by the Earth, and so less light than normal falls on the object.

There are several distinct phases of a lunar eclipse, as the Moon travels through the penumbra and umbra. For this lunar eclipse the total time during which the Moon is at least partially in the Earth’s shadow is 5 hours 11 minutes, and 72 minutes of this is spent entirely within the umbra, i.e. in total eclipse.

lunareclipse28sep

These phases are given the names: P1, the time when the Moon’s disk enter the penumbra; U1, the time when the Moon’s disk enters the umbra; U2, the time when the entirety of the Moon’s disk is within the umbra; U3, the last time when the entirety of the Moon’s disk is within the umbra; U4, the last time when part of the Moon’s disk is within the umbra; and P4, the last time when part of the Moon’s disk is within the penumbra.

The UK is ideally placed to view this total lunar eclipse, although you will have to stay up very late, or get up very early. The Moon is in the sky for the entirety of the eclipse. Observers in western Europe, NW Africa, E North America, and South America will all see the full eclipse from beginning to end.

eclipse28sep2

A detailed information sheet for this eclipse (and others) is available (pdf) on the NASA Eclipse website.

Perseids Meteor Shower 2015

August 5, 2015 2 comments

This month sees the return of the most reliable meteor shower of the year; the Perseids. And with a New Moon occuring at the same time as the peak of this shower this is the perfect opportunity to see hundreds of shooting stars.

Read my previous blog post: Meteor Showers: The What, How, Where, When, Why for general advice on how best to observe meteor showers.

A shooting star – otherwise known as a meteor – is a tiny piece of space dust that burns up in our atmosphere, forming a bright, brief streak of light in the sky. Many people have never seen a shooting star, and think they’re rare events, but given clear dark skies you can expect to see a few every hour on a clear night. From cities, under light polluted skies, you can’t see most of the faint ones, and so only the rarer bright ones are visible.

However at regular times each year the Earth moves through thick clouds of space dust – left behind by comets – and we get a dramatically increased rate of meteors. We’re already within the diffuse outer reaches of the dust cloud that forms the Perseids, and on the night of 12/13 August we’ll be in the densest part of that cloud, and will see the rate increase by a factor of 20!

You can begin watching for Perseid meteors now, and the shower will last until late-August, but the peak of the shower occurs overnight on 12/13 August 2015, which means that the nights on either side of this will be best for meteorwatching.

Location of the Perseids Radiant at 0001 on 13 August

Location of the Perseids Radiant at 0001 on 13 August

The best time of night to watch the meteor shower is from around 2200 onwards on 12 August 2014, once the radiant, the point from where the meteors appear to originate, rises above the horizon. The later you observe the higher the radiant will be, and the more meteors you’ll see.

The number of meteors that you will observe every hour depends on a number of factors:
•the density of the dust cloud 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 darkness of your sky, measured using naked-eye limiting magnitude, a measure of the faintest object you can see.

The Perseid meteor shower has a zenith hourly rate (ZHR) of around 100. 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 that perfect. In the UK, for example, the radiant of the shower will not be at the zenith; it will be around 30° above the horizon at 2200, 40° high at midnight, and 50° high at 0200.

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 Persieds 2015.

ZHR = 100 at the peak, say.

h = 30° at 2200, 40° at 0000, 50° at 0200, 65° at 0400

k = 0 (let’s hope!)

m = 6.5 (if you’re observing under skies free from light pollution)

So your actual hourly rate under clear dark skies is

(100 x sin(30))/((1/(1-0) x 2^(6.5-6.5) = 50 meteors per hour at 2200
(100 x sin(40))/((1/(1-0) x 2^(6.5-6.5) = 64 meteors per hour at 0000
(100 x sin(50))/((1/(1-0) x 2^(6.5-6.5) = 77 meteors per hour at 0200
(100 x sin(65))/((1/(1-0) x 2^(6.5-6.5) = 90 meteors per hour at 0400

Remember though that these numbers apply only to the peak of the Perseid occuring at these times. If the peak happens at 0400 on 13 August (and we’re not sure exactly when it’ll occur) then you might see 90 meteors per hour, but “only” 20-25 per hour if the peak occurs at 2200 on 12 August.

Remember that these rates are for perfectly dark skies. If you live in surbribia then divide these numbers by 4 or 5; if you live in a bright city divide these numbers by 10. Take home message: get somewhere dark!

It is worthwhile having a look on the days leading up to the peak, when the numbers of meteors will be gradually increasing towards this rate.

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!

CJ33X3VWIAAKaJw

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.

665_pluto_charon

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.

alt

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
%d bloggers like this: