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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.
Autumn Equinox 2012
Today, 22 September 2012, marks the moment of the Autumn Equinox. At 1449 UT (1549 BST) the Sun will cross from the northern hemisphere sky to the southern, and we’ll begin the slow approach to the Winter Solstice on 21 December.
The equinoxes (one in spring and one in autumn) are the two instances every year when the Sun makes that crossing from north to south and vice versa, and they’re commonly thought to be the days when day and night are equal length, but they’re really not, for reasons I’ve outline before:
- astronomers measure the timings of equinoxes, sunrises and sunsets based on the middle point of the Sun’s disk in the sky, so when you read a sunrise time it means the time that the centre of the Sun’s disk rises above the horizon. For a few minutes before that time the top of the Sun’s disk will already have risen, giving “daylight”.
- Even before this happens the sky is lit up by the Sun below the horizon, and we experience twilight. Most people would think that the sky is bright enough to call it “daytime” long before the Sun pops above the horizon, during the phase of civil twilight.
So today, even though day and night are said to be equal on the equinox, the “daytime” (i.e the start of civil twilight) started about 0630BST in Glasgow (where I am) and will end this evening around 2000BST, giving me 13.5 hours of “daylight”. (Londoners will have from about 0615 until 1930BST, or approx. 13.25 hours of “daylight”).
The day this year where I have exactly 12 hours of “daylight” (i.e. between the morning start and the evening end of civil twilight) is 11 October and this day is called the equilux. (In London the equilux falls on 12 October).
Perseids Meteor Shower 2012
UPDATE: Here’s my easy guide to the what, how, where, when and why of the Perseid Meteor Shower.
This month sees the most reliable meteor shower of the year; the Perseids. You can begin watching for Perseid meteors now, and the shower will last until late-August, but the peak of the shower occurs around mid-day on Sunday 12 August 2012, which means that the nights on either side of this will be good for meteorwatching.
- Perseus at 0200 Monday 13 August 2012
The best time of night to watch the meteor shower is from around 2200 onwards on both 11 and 12 August 2012, once the radiant, the point from where the meteors appear to originate, rises above the horizon.
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 Perseid meteor shower has a zenith hourly rate (ZHR) of between 50 and 200. 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 midnight, and 45° above the eastern horizon at 2am.
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 2012.
ZHR = 100 at 1200 on Sunday 12 August 2012, but by 0001 on Monday 13 August, ZHR might be down to 50 say, maybe less.
h = 30° at 0001, 45° at 0200, 60° at 0400
k = 0 (let’s hope!)
m = 6.5 (assuming you can get somewhere dark).
So your actual hourly rate at 0200 under clear dark skies is
(50 x sin(30))/((1/(1-0) x 2^(6.5-6.5) = 25 meteors per hour at 0001 Monday 13 August
(50 x sin(45))/((1/(1-0) x 2^(6.5-6.5) = 35 meteors per hour at 0200 Monday 13 August
(50 x sin(60))/((1/(1-0) x 2^(6.5-6.5) = 45 meteors per hour at 0400 Monday 13 August
Remember though that these numbers might be lower if the ZHR drops off after the daytime peak.
It is of course worthwhile having a look on the days leading up to the peak, when the numbers of meteors will be gradually increasing towards this rate.
You can keep track of the increasing ZHR at the International Meteor Organisation website.
*UT = Universal Time = GMT, so for UK times (BST) add one hour to these
Curiosity
In the early morning hours (UK time) of Monday 6 August, NASA’s latest Mars rover, the Mars Science Laboratory, or Curiosity to its friends, will land on the red planet after an eight month journey from Earth.
The Mars Science Laboratory, Curiosity
Curiosity is the largest rover ever sent to Mars – it’s about the size of a Mini – and has a huge array if scientific instruments, which will enable it to complete its science missions: to determine if Mars could ever have supported life; to study Mars’ geology; to study Mars’ climate; to plan for a human mission to Mars.
Curiosity will touch down on Mars after a not-entirely-risk-free landing procedure, which uses a heat shield, parachute, engine, and sky crane, a system by which the lander separates from the sky crane, attached by a tether. The sky crane will use its engines to slow it down to almost a dead-stop, and lower the rover gently onto the surface of Mars.
If you want to watch the landing live, NASA and others are streaming it live. Landing is scheduled for 0631 BST, so you’ll have to tune in a bit before that to watch the whole process. You can also follow Curiosity on Twitter.
And if you want to see the red planet yourself, it’s visible low in the west just after sunset, forming a beautiful triangle with Saturn and Spica, the brightest star in the constellation of Virgo. Mars is the right-hand most of the three bright points of light. You’ll only just catch a glimpse of Mars after the sky darkens enough for it to appear, and before it sets around 2245 BST.
Summer Solstice 2012
The northern hemisphere summer solstice occurs today, 20 June 2012 at 2309 UT (which is actually tomorrow in the UK, 21 June 2012 at 0009 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 tilted towards the Sun as far as it can.
And this happens at exactly 2309 UT on 20 June 2012.
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 17h35m13s today and tomorrow (20 and 21 June), a full seven seconds longer than yesterday, and eight seconds longer than 22 June.
Transit of Venus
This year, on 5 and 6 June 2012, there is a very rare astronomical occurrence: a transit of Venus across the face of the Sun. There have only been six of these transits ever observed before – in 1639, 1761, 1769, 1874, 1882, and in 2004 – and this year’s transit is the last for 105 years!
So what exactly will you see, if you’re lucky enough to catch this last-chance-to-see event? If you’re able to look at the Sun safely you’ll see a tiny black dot moving slowly across the surface – that dot is the planet Venus! NASA has the exact times of the transit from major cities. Importantly, this transit is best seen from the Pacific. Observers in north and central America will see only the start of the transit before the Sun sets, while those of us in Europe will only catch the end of it if we’re up at sunrise.
UK observers: set your alarms! You’ll see the transit between sunrise and 0536 BST, at which point Venus begins leaving the Sun’s disk, taking about 18 minutes to do so.
Venus is 6000km across – just a little smaller than the Earth – and at transit it will be around 43 million km away, directly between us and the Sun. The Sun is 1.4 million km across and around 150 million km away. This means that, seen from Earth, Venus is only about 58 arcseconds in diameter, while the Sun is 1891 arcseconds across, about 33 times the apparent diameter of Venus. So: Venus small dot; Sun big bright ball.
Also, we know how far from the Sun Venus is (107 million km), and how long it takes to orbit the Sun (225 days), so we can work out how long it should take to pass across the Sun’s disk (around 6.5 hours). However the start and end times for the transit vary depending on where on Earth you’re observing, with observers in eastern Canada seeing Venus start to cross the Sun’s disk a whole thirteen minutes earlier than observers in Australia! This is because Canadians are looking at the transit from a slightly different angle than Australians.
Why transits of Venus are (were) important
If you have observations from two widely spaced points on the Earth’s surface, and if you time the start and end of transit accurately at each, you can work out the solar parallax, that is, the difference in position of the Sun when viewed from two different points on Earth, the two points being one Earth radius apart. (Hold your thumb up, close one eye, and obscure a distant object; now switch eyes, and your thumb appears to move with respect to the distant object. That’s parallax).
From the solar parallax, if you know the Earth’s radius, you can work out the Earth-Sun distance (known as the astronomical unit) using high-school trigonometry. This was important to astronomers in the 18th century, as up until then all we knew were the relative distances between all the planets in our solar system, not the actual distances. Once we had one measurement within the solar system – the astronomical unit, say – we could work out how far away everything else was.
The technique of using transits of Venus to work out the solar parallax was first suggested by Edmund Halley in 1716, after he had observed a much more common (although still only 13 times per century) transit of Mercury from the island of Saint Helena. Halley knew that Venus would give much more accurate measurements than Mercury, since it was closer to the Earth and so the angles would be easier to measure. He also knew that the next transit of Venus would happen in 1761, and urged future astronomers to make observations world-wide and thereby calculate the solar parallax, and from that the astronomical unit.
This was duly done, and a value for the astronomical unit of 153 million km was calculated. Later transits in the 19th century yielded a value of 149.59 million km. The current accepted value, calculated from telemetry from space craft is 149.60 million km, so the transit method worked pretty well.
Countdown to Conjunction: Venus and Jupiter
If you’ve been outside in the evening over the past few weeks you’ll have noticed that there are two very bright “stars” close together, following the Sun as they set one after the other in the west. Those two bright dots are not stars at all; they’re planets. The brighter of the two is Venus, which at the moment is below and to the right of the other dot, which is Jupiter.
Tonight they are around ten degrees apart in the sky, but over the next week they’ll get closer and closer, as Venus whizzes and Jupiter crawls round the Sun, until on 15 March they’ll be in conjunction, only 3 degrees apart.
Jupiter and Venus in conjunction 15 March 2012, around 1930
On the days either side of 15 March (say between 08 and 19 March) they’ll be very close too. In fact it’s worth watching this celestial merry-go-round in action every clear evening over the next few weeks as the planets move towards and then away from each other in the sky. Towards the end of March though it’ll become harder to see them both as they disappear into the glare of sunset. If you’ve got clear skies and a good western horizon it’s worth looking out for the thin crescent Moon which will appear between the two planets on the night of 25 March.
Venus
Venus, the second planet out from the Sun, is about the same size as the Earth, just a little smaller. It’s the hottest planet in the solar system, with a thick atmosphere of carbon dioxide gas (94.6% is CO2, the rest is mainly nitrogen) which traps most of the light from the Sun that shines on it, super-heating the atmosphere to around 460°C (733K). At ground level this thick, hot atmosphere creates a pressure over 90 times greater than sea-level pressure on Earth. High in Venus’ atmosphere float clouds of sulphuric acid, which is all we see when we look at Venus from the Earth.
Seen from here on Earth, the size and shape of Venus in our sky changes as we both orbit the Sun. At its closest to Earth Venus is “only” 38 million km away, and its disk is 66 arc seconds across, while at its furthest from us it’s 260 million km away, and it shrinks to around 10 arc seconds. On top of this, its phase changes from full (when it’s directly opposite the Sun as seen from Earth) to new (when it’s directly between us and the Sun) and back again. Of course when it’s in either of these positions we won’t see it, as it will be in the sky right next to the Sun. We see Venus best when it’s far to the west of the Sun (when it’s seen in the evening) or far to the east (when it’s seen in the morning). The furthest west and east points as seen from Earth are called maximum elongation, and at these points Venus presents a half phase to us.
Due to the reflectivity of its clouds, and its proximity to us, Venus is the brightest planet as seen from Earth. Venus appears brightest in our sky, at around -4.5 magnitudes, when it’s 68 million miles from us and presents a crescent phase.
During the 15 March conjunction Venus will have a brightness of -4.2 magnitudes.
Jupiter
Fireball of 03 March 2012
Last night, 03 March 2012 at around 2145GMT, my Twitter stream became flooded with reports from people saying they’d seen a giant meteor streaking across the sky.
The first I heard about it was a tweet from @VirtualAstro:
ALERT! Reports coming in of sightings of fireballs (large meteors/ Shooting stars) in the North and South of England
It turns out that these were probably all reporting the same sighting. For a brief spell the hashtag #ukcomet started to gain prominence, but it wasn’t a comet at all, rather a large chunk of space rock burning up in the Earth’s atmosphere.
On any clear dark night you can see a few meteors – which are also known as shooting stars – as the Earth hurtles round the Sun hoovering up all the bits and pieces of debris floating about in space.
Most meteors are the size of a small pebble and as they get hoovered up by the Earth they pass through the atmosphere. This generates frictional heating as the space-rock rubs past air molecules, and eventually the rock will burn up completely. This happens in part of the atmosphere called the mesosphere, which is about 75km (45 miles) above the Earth’s surface. During the brief period of frictional heating not all the energy produced is converted into heat, some of it gets converted into light, which is why we see them streaking across the sky. Usually a meteor will be moving so fast, and burn up so quickly, that is appears as a very quick flash of light, of less then a second in duration.
But there are bigger bits of rock out there too, and when something bigger than around 10cm enters the Earth’s atmosphere we might get a far more spectacular display, something called a fireball, or bolide meteor.
This is what happened last night. Rather than friction heating up the rock (there was probably a bit of that going on too) the energy seen in fireballs is generated by ram pressure. This is as a result of the large rock crashing into the atmosphere and causing all of the air in front of it to rapidly compress, forming a shock wave. The air in this shock wave heats up (did you know that compressed air heats up? Feel the tube on a bicycle pump next time you’re blowing up a tire) and flows around the rock, causing it in turn to heat up. This process starts the rock glowing, and when it’s bright enough we see it as a fireball.
Fireballs are much brighter than standard meteors – in fact the IAU defines a fireball as any meteor brighter than magnitude -4 – and last longer in the sky, and so they’re much easier to spot. Therefore even though they’re much rarer than your common or garden meteors they tend to get spotted by lots more people, and are even visible in big cities, hence the flurry of reports late last night.
Leap Year 2012
- Thirty days hath September,
- April, June, and November;
- All the rest have thirty-one,
- Save February, with twenty-eight days clear,
- And twenty-nine each leap year.
This year is a leap year, when the month of February has 29 days in it, rather than the usual 28. The rhyme above is a mnemonic to help us remember the days in each month, but it doesn’t explain why we need leap years, and why they occur once every four years.
To understand the reason for leap years we have to look to astronomy, and in particular to the orbit of the Earth around the Sun. The Earth orbits round the Sun in 365.256363 days. This is a bit awkward as it means that the year cannot easily be divided into a whole number of days. If we round the year down to the nearest whole number of days we get 365 days in a year, which is indeed what we have in most calendar years.
So why not just leave it at that? Isn’t 365 days close enough to 365.256363? After all it’s 99.93% of the actual year, which is nearly 100% right, yes?
Actually; no. In ancient Egypt, where they lived with a calendar year of 365 days, the seasons began to drift at a rate of one day every four years. If we had stuck with the Egyptian calendar of 365 days every year then the longest day, which we take to fall on 21 June in most years would fall on 20 June four years later, then 19 June four years after that, until over the course of 730 years or so the longest day would occur when our calendars said it was the middle of winter.
Obviously something needed to be done to fix this problem. Enter Julius Caesar who, in 46BC, introduced what is known as the Julian Calendar. In this calendar Caeser recognised what Greek astronomers had long known; that the year is closer to 365¼ days long. They didn’t know that the Earth went round the Sun in 365¼ days, but they knew that the seasons repeated themselves on a 365¼ day cycle, and not a 365 day cycle as the Egyptians thought.
To account for this more accurate measure of the changing seasons, and to align the calendar better with the real world, Julius Caeser announced that every fourth year would have an extra day in it, to occur at the end of February. This would allow the calendar to keep in line with the real changing seasons, so that the longest day would always fall on the same day of the calendar.
But by 46BC the seasons had already drifted a lot; in fact the Roman calendar was about 80 days behind the actual seasons, so Caesar proclaimed that 46BC would have extra days in it, and be 445 days long, so that the calendars would be aligned on 1 January 45 BC, at which point the new calendar of leap years would begin.
The Romans didn’t call these leap years though; that name came along about 1400 years later. They were called “leap years” because the occurence of them every four years caused festive days (like Christmas), which usually advanced one weekday per year, to suddenly leap forward by two days. For example, Christmas Day in 2009 fell on a Friday, in 2010 on a Saturday, in 2011 on a Sunday, but this year, in 2012, it will leap forward to a Tuesday.
Not the Whole Story
Of course things are never that simple, are they? In fact the year is not 365¼ days long either, it’s 365.256363 days long if you measure it in terms of how long it takes the Earth to go round the Sun, or 365.242189 if you measure it in terms of how long it takes the Sun to return to the same part of the zodiac (which is indeed what we need to measure if we want to track the seasons).
We no longer have a Julian Calendar of 365 days each year with 366 every fourth leap year. Instead we have adopted the Gregorian Calendar where:
Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100; the centurial years that are exactly divisible by 400 are still leap years.
So 1900CE wasn’t a leap year (nor was 1700 or 1800), event though it was due to be, but 2000CE was. This is to fine-tune our year to fit even better with the changing seasons. Without this slight tweak then even the Julian calendar would drift with the seasons, albeit not as drastically as the Egyptian fixed 365 day year.
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