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Definition of Twilight

Published on 07/11/2012 
Question: 

As you may know the Holy month of Ramadan is approaching and as a Muslim I am going to observe the fasts in this one month. However Similar to last year there is many conflicting opinions on when the fast should close.
For this reason I have decided to make my own enquiries and get external information.

We have to close our fast when there is some light in the sky. Some Scholars are saying that is when the sun is below the horizon at 18degrees and others are saying 12 or 15degrees. This is causing great confusion
Can you please clarify this
Also During the summer months in the UK,(May-July) is it very difficult to differentiate between the two twilights? Meaning the twilight of the night, and the twilight of the morning?

The confusion I'm afraid is because unlike sunrise and sunset it is difficult to properly define 'twilight'.  There are three rough 'bands' of twilight that are generally agreed upon, civil twilight - the sun is less than 6 degrees below the horizon, nautical twilight - the sun is between 6 and 12 degrees below the horizon, and astronomical twilight - the sun is between 12 and 18 degrees below the horizon.  Civil twilight is what most people would think of as 'twilight', and at the start of which is when you would see the characteristic reflected red-orange glow from high clouds, there is generally still enough light to see by and the horizon is clearly visible.  Most people would likely think of nautical twilight as being 'darkness', in that bright stars will be visible and it would be difficult or impossible to tell which direction is East or West simply by looking for the glow of the sun below the horizon, at the start/end of nautical twilight when the sun is 12 degrees below the horizon even the horizon itself will be indistinct and for almost all intents it is completely dark.  Even after this though there will still be enough scattered sunlight around that it is not ideal for observing faint, diffuse, objects like nebulae until the sun is more than 18 degrees below the horizon.

I expect that for your purposes nautical twilight would be quite sufficient and that civil twilight is probably adequate, but that is something that you should decide for yourself.  Wikipedia has well written, detailed, articles about twilight http://en.wikipedia.org/wiki/Twilight and the brightness of the night sky http://en.wikipedia.org/wiki/Sky_brightness which have a variety of other links that you can check.

You are also quite correct that in a similar way to the fact that there are days when the sun does not rise or set within the Arctic and Antarctic circles there are concentric regions around the poles where, although the sun does rise and set, it will for a period around the summer solstice (21st June in the Northern hemisphere), never be darker than civil twilight, nautical twilight, or astronomical twilight.  It is easy to work out the lowest the sun will be below the horizon at any given location on the summer solstice from Earth's axial tilt of 23.4 degrees.  If you are at a latitude of X degrees then at the summer solstice the sun will be no more than 66.6-N degrees below the horizon, so in London (51.5 degrees North) on 21st June the sun was never more than 16.1 degrees below the horizon, so it was never darker than astronomical twilight.  In Edinburgh however at 56 degrees North the sun was never more than 10.6 degrees below the horizon and so it was never darker than nautical twilight.  It will always be darkest at midnight however (though note that since British Summer Time is one hour ahead midnight is actually 1am).

It is also worth noting that aside from the effects of the tilt of the Earth light pollution in large cities means that it may never seem completely dark.

Formation of Earth and life

Published on 07/11/2012 
Question: 

Hello, I was talking to someone the other day and we got onto the subject of Space as it fascinates me, thinking about space and how life began on Earth is the one thing i can't get my head round, thats why it fascinates me. So anyways, I was wondering, is it possible that the planet we know as home, Earth, wasn't always where we are are now? What I mean is, when I try and think how life began on Earth, maybe Microscopic life to begin with yet life doesn't just start. it has to have something to begin with, you cant put a rock into space, leave it for a few million years and then come back and there will be life on it. What I am wondering is, was life on Earth, whatever it was in the start, frozen on a drifting Asteroid, that Asteroid being Earth, kind of like Pluto is at the moment, just a huge planet of ice and rock, though this huge drifting Asteroid of Ice and Rock was glancing past the sun and got pulled into the circular gravitational pull and now rotates. Obviously of Earth was going on a head on course with the Sun it couldn't be pulled into a a gravitational circle as it would be too sharp a turn, though if it was glancing past the sun it could maybee have got pulled into the gravitational pull, then with the heat of the Sun being just right over the years the Sun defrosted this huge block of ice and rock and slowly thawed out the life, almost like the life was in Cryostasis and now its being defrosted, this can happen as theres a certain moth that does this in the Arctic, gets frozen over winter and defrosts and comes back to life in Spring, just wondered if that has every been wondered and what the answer was, that mabye our Earth and life didn't begin where we think it did, instead Earth actually drifted in from another part of space and got caught in the Suns Gravitational pull and the heat defrosted it and allowed life to begin, well not begin, but carry on now its been thawed out, then over the years the speed of the earth rotating combined with it spinning on all axis sort of moulded into a circular shape planet, kind of like sanding it down. Long question I know but it was on my mind, it also seems a bit more plausable, that life didn't begin in the Milky Way, instead the Milky Way is where our planet ended up and thawed out the life that was on it. Just wondered. Btw I don't study Astronomy, just interests me as its the only thing I can't get my head around so I tend to thing about things, so excuse me if this has already been answered. Thanks for your time, all the best

Thanks for your question.  To answer it, it is useful to split it into two parts; 'did Earth form in the solar system?' and 'was Earth the birthplace of the living organisms that now populate it?'

The answer to the first part of that question is almost certainly yes, Earth did form in the solar system.  The process of planet formation involves quite a bit of jostling about so all of the components that went on to form the young Earth did not necessarily come from near the current orbit of Earth but it would be almost impossible to place a rocky planet into the current orbit of Earth by capturing it.  While it is possible for planets to be ejected during formation, space is really very empty and the probability of an ejected planet passing near to another star is very low.  Even if it did happen and the planet was captured it would end up in a very wide very eccentric orbit (similar to a comet) very different to the current orbit of Earth and although orbits do change over time it would be very difficult to change a comet-like orbit into an Earth-like one.

The question as to whether the genesis of life on Earth was indeed on Earth is a very different one and one that has had a great deal of discussion in one form or another for centuries.  If you look up panspermia you will be able to find far more information than I can give you here, though it is good to be aware that there are some rather crazy ideas out there.  It can be something of a controversial topic but as yet no one has come up with a complete theory explaining how life can arise from non-living material either so it is difficult to really say which is the more likely origin of life on Earth.  Anyway the basic idea of panspermia is that life is spread throughout the universe by asteroids and comets, rather like your suggestion.  As well as the suggestion that life first arrived in the solar system by such a mechanism there is the related suggestion that life could be spread throughout the solar system in the same way if it did first arise within the solar system.  As the only place we know of at the moment that has life is Earth it is difficult to make deductions.  Research is being done into investigating how long terrestrial microbes can survive in space, the Russian Phobos-Grunt mission had a small capsule on board that was designed to test whether the microbes contained within it could survive in space for the 3 year length of the mission, but unfortunately, as you may have heard on the news, that ended up in the Pacific.  When we have more experiments like that, or if life is found somewhere else in the solar system, like Mars or Europa, we can begin to answer the question of where life on Earth arose, and how widespread life is in the universe, in more detail.  All we have to go on at the moment is that pretty much anywhere you look on Earth, no matter how extreme, you find life.

End of the Universe

Published on 07/11/2012 
Question: 

Many astronomers predict that the universe will continue expanding till eventually all life will end the stars will go out and the universe will be cold and lifeless.
If gravity is the strongest (as well as the weakest, we can all pick up a pen) force known, why does everything have to end.
The astronauts showed, with the particles in the bag, that even in a vacuum objects with mass attract, so with this in mind, why won't the universe eventually slow to a stop and then contract to its starting point and a new big bang happen.
Is this possibly what has been happening since before this time began?
Matter cannot be unmade apparantly, only changed into something else, so even with infinite distances even the minutest amount of gravity would attract a smaller object to a larger one even at a molecular level, surely.
I would be interested in your comments, although I am not an astronomer or otherwise involved in space science, I am interested to find out if this has been put forward by greater minds than mine and if so who, when and where can I read about it?

If the Universe only contained matter then the expansion of the Universe would indeed gradually slow down due to the gravitational attraction and eventually start to re-collapse down to a 'Big Crunch', the opposite of the Big Bang.

The fly in the ointment is what we currently call Dark Energy.  Astronomers and physicists are sometimes not the most imaginative of people, so rather like with Dark Matter we tend to just call something 'Dark' if we can't see it and don't know what it is.  Dark Matter is essentially just matter that we can't see, it still interacts through gravity in just the same way as normal matter.  Dark Energy on the other hand is different, rather than attract things together Dark Energy drives them apart and accelerates the expansion of the Universe.  There are an extremely large number of ideas about what Dark Energy could be but none of them are obviously any more likely than the others and indeed some physicists are unconvinced whether it really exists at all, though a lot more think it does than doesn't.

Anyway, the eventual fate of the Universe is down to the balance between Dark Energy and matter (including Dark Matter), and to what exactly the Dark Energy is, so there are various different scenarios:

  • One option is the Big Crunch I mentioned earlier, depending what the Dark Energy is and how it behaves, this could still happen, though it currently seems less likely.
  • The one that you were asking about is commonly referred to as 'Heat Death', which is essentially that if the Universe continues expanding forever then eventually all sources of stellar fuel will be exhausted, so all stars will eventually go out, and after long enough the Universe reaches a uniform temperature near absolute zero.  The subtly here that makes this a problem for life is that any form of life requires temperature gradients in the background, so if there are none that is a problem.
  • Another option is what is known as a Big Rip, which happens if the rate of expansion accelerates indefinitely.  In this scenario the Universe is eventually expanding so fast that it rips apart, first galaxies and stars, and then eventually molecules and atoms.

There are variations on these three broad scenarios as well depending on the flavour of Dark Energy used.

As there is currently no clear idea of Dark Energy might be there is also no clear idea of exactly what the eventual fate of the Universe will be, however a Big Crunch scenario does seem less likely.

How many solar systems are there?

Published on 24/09/2012 
Question: 

How many solar systems are present in the Universe and what is the possibility of finding a solar system similar to ours?

Those are very good questions, and ones to which we don't quite know the answer.  There are a number of projects running at the moment that hope to try and provide some answers, such as NASA's Kepler mission and the European CoRoT, as well as various ground-based efforts.  So far they are suggesting that a reasonable fraction of stars (around 10 per cent or even more) have planets, but so far we have not really able to find systems quite like our own.

In terms of the number of solar systems present in the universe, there are something like 300 billion stars in the Milky Way, so if 10 per cent of them have planets there are around 30 billion planets in our galaxy alone, and there are over 100 billion galaxies in the observable Universe for a total of something in the order of 10^21 (that's 1 then 21 zeros) planets in the observable Universe.  There is still quite a bit of uncertainty in that number however, and we don't yet know how many of them would look like our solar system.

Meteor sighting

Published on 24/09/2012 
Question: 

Hi Again. It was not yet completely dark. Just dark enough for cars to have their headlights switched on at about 7:15 pm on 19 Sept 2012. We were travelling north on Airport Way, near Teversham heading towards Quy Waters roundabout. A bright light caught my eye. It was bluey green in colour and it was travelling at the speed of a meteorite but burned longer, maybe 3 seconds. It seemed to disappear perhaps behind a cloud or possibly burned itself out. It looked as if it were heading towards earth at about a 30 degree angle. It seemed significant enough to mention. I was wondering if other people saw it and what it might have been.

What you saw was most likely a relatively large meteor, as you can probably guess the larger a meteor is the longer it will last, and 3 seconds of visibility is not particularly unusual.  Meteors with a blue-green colour are less common, but not by no means unheard of, especially amongst brighter meteors since for fainter ones you won't be able to see the colour (the same reason almost all stars appear white).  The blue-green colour can be an indicator of a higher copper content in the meteor.

Addendum to original reply:
The BBC also picked up on this and Cambridge is far from the only place in the UK the meteors were seen.  One of the astronomers the BBC asked suggested the meteors might have been man-made space junk such as an old satellite burning up on re-entry.  This would otherwise look much the same as a natural rocky meteor, but could certainly explain a high copper content!