Solar Storm Incoming
At 0028GMT on 7 March a giant X-class solar flare blasted material off the Sun and sent it hurtling towards the Earth. To be specific it was a X5.4-class flare, the most powerful we’ve seen in 5 years. That material is due to hit some time this morning (UK time) 8 March and could result in increased aurora activity as well as potentially disrupting communications systems and power transmission. A similar storm in 1989 knocked out the power grid in northern Canada, resulting in a black out for over 6 million people.
Here’s a graphic of the storm as it heads our way, courtesy of NASA (click on graphic for animation):
Keep up-to-date with activity via Spaceweather.com
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.
Venus, Jupiter and the Moon, February 26 2012
Tonight, just after sunset, Venus, Jupiter and a thin crescent Moon will line up in the evening sky. If you’ve got clear views towards the west it’s really worth a look.
Looking west after sunset on Sunday 26 Feb 2012
Thanks to the excellent Starwalk app for the above image.
Naked Eye Limiting Magnitude: Redux
Having just tried to assess Naked Eye Limiting Magnitude from a dark site, I realised that my previous post on the subject merited some amendments.
Rather than using the whole constellation of Ursa Minor to carry out your NELM estimate, it’s much simpler to use just part of it, that part around the “body” of UMi, roughly bounded by and immediately surrounding β, γ, ζ, and η UMi. Here’s a more detailed star chart of that part of the sky, with all 34 stars brighter than magnitude 7.2 labelled.
UMi detail down to mag 7.2
And here’s a list of the magnitudes of each of these stars:
| Star Number (Name) |
Magnitude | Star Number (Name) |
Magnitude |
| 1 (β UMi) | 2.05 | 18 | 6.55 |
| 2 (γ UMi) | 3.00 | 19 | 6.60 |
| 3 (ζ UMi) | 4.25 | 20 | 6.60 |
| 4 (5 UMi) | 4.25 | 21 | 6.65 |
| 5 (4 UMi) | 4.80 | 22 | 6.70 |
| 6 (η UMi) | 4.95 | 23 | 6.80 |
| 7 (θ UMi) | 5.00 | 24 | 6.85 |
| 8 (11 UMi) | 5.00 | 25 | 6.85 |
| 9 (19 UMi) | 5.45 | 26 | 6.85 |
| 10 | 5.55 | 27 | 6.85 |
| 11 | 5.70 | 28 | 6.85 |
| 12 | 6.00 | 29 | 6.90 |
| 13 | 6.25 | 30 | 6.95 |
| 14 | 6.30 | 31 | 7.00 |
| 15 (20 UMi) | 6.35 | 32 | 7.10 |
| 16 | 6.35 | 33 | 7.20 |
| 17 (3 UMi) | 6.40 | 34 | 7.20 |
As you can see, it’s much easier to fine-tune your NELM estimate using this chart compared to the previous one, as there are not such big jumps between brightnesses from one star to the next.
Colours in this table correspond to the Bortle Scale colour key.
Crucially, one thing I omitted to note in the previous post was that this process should be carried out when your target stars are high above the horizon. The stars of Ursa Minor, when observed from the UK, vary in altitude between 40° and 70° roughly speaking, so ideally you’d wait until they were higher than 60° above the northern horizon.
| Month | Times when Kocab (β UMi) alt > 60° |
| mid Jan | 0300 till start astronomical twilight (~0600) |
| mid Feb | 0100 till start astronomical twilight (~0530) |
| mid Mar | 2330 till start astronomical twilight (~0430) |
| mid Apr | 2230 till start astronomical twilight (~0400) |
| mid May | end astronomical twilight till start astronomical twilight |
| mid Jun | no hours of darkness |
| mid Jul | no hours of darkness |
| mid Aug | never > 60° during hours of darkness |
| mid Sep | never > 60° during hours of darkness |
| mid Oct | never > 60° during hours of darkness |
| mid Nov | never > 60° during hours of darkness |
| mid Dec | 0500 till start astronomical twilight (~0630) |
Naked Eye Limiting Magnitude: Assessing Sky Brightness
There are a variety of ways of measuring your night sky quality, and one of the most effective ways is by looking for the faintest star you can find with your naked eye, and noting its brightness, or magnitude. This provides what is known as Naked Eye Limiting Magnitude, NELM.
Of course just randomly casting about the sky for faint stars can lead you on a merry chase, and so a very useful method is to use one specific constellation – one you can always see, no matter what time of year – and look only at stars within that one constellation. This narrows the field somewhat, and makes your task that much easier.
For observers in Europe and North America the constellation of Ursa Minor, the Little Bear, provides an excellent choice for estimating NELM.
The overall shape of Ursa Minor is made up of seven bright-ish stars, but around and amongst these are many more fainter stars.
Ursa Minor
| Bright Star Name (Bayer Designation) |
Magnitude |
| Polaris (α) | 1.95 |
| Kocab (β) | 2.05 |
| Phercab (γ) | 3.00 |
| Yildun (δ) | 4.35 |
| Urodelus (ε) | 4.20 |
| Ahfa al Farkadain (ζ) | 4.25 |
| Anwar al Farkadain (η) | 4.95 |
Even some of these “brighter” stars might not be visible from city centres. For example, if you are observing from a site with Bortle Class 8 you would not see η-UMi, while those unhappy stargazers under a Bortle Class 9 sky would only be able to pick out the three brightest stars, α-, β-, and γ-UMi. Only at Bortle Class 7 and darker will you make out all seven of the main stars of Ursa Minor.
But what if you’re at a good dark sky site? Well, you’re going to need a longer list of magnitudes, and a more detailed map of Ursa Minor.
| Star Number on Above Map |
Star Name | Visual Magnitude | Bortle Class |
| 1 | α UMi | 1.95 | 9 |
| 2 | β UMi | 2.05 | 9 |
| 3 | γ UMi | 3.00 | 9 |
| 4 | ε UMi | 4.20 | 8 |
| 5 | 5 UMi | 4.25 | 8 |
| 6 | ζ UMi | 4.25 | 8 |
| 7 | δ UMi | 4.35 | 8 |
| 8 | 4 UMi | 4.85 | 7 |
| 9 | η UMi | 4.96 | 7 |
| 10 | θ UMi | 5.00 | 7 |
| 11 | 11 UMi | 5.02 | 6 |
| 12 | 19 UMi | 5.45 | 6 |
| 13 | 24 UMi | 5.75 | 5 |
| 14 | λ UMi | 6.30 | 4 |
| 15 | 20 UMi | 6.35 | 4 |
| 16 | 3 UMi | 6.40 | 4 |
| 17 | π1 UMi | 6.55 | 3 |
| 18 | HIP74818 | 6.65 | 3 |
| 19 | 14 UMi | 7.35 | 2 |
The stars in the map and table above have been numbered (by me – these aren’t official designations) from 1 to 19, with 1 (Polaris) being the brightest, and 19 (14 UMi) being the dimmest. You will only be able to see all 19 numbered stars from exceptionally dark places, virtually free of light pollution, what Bortle called “typical truly dark sky sites”. From my garden in the outskirts of a major city I can see numbers 11 and 12, but not number 13, giving me an NELM of 5.45.
