July 2008 Archives

Total Solar Eclipse, 01 August

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This Friday we'll once again have the amazing spectacle of a total solar eclipse. This usually happens about once a year somewhere in the world (although the last one was in 2006), but for any given point on the globe it's more like once a century.

The event of this year will be visible in the far north of Canada, the northern tip of Greenland and on a large swatch of Russia, Mongolia and China (yes, it does look like everything happens in China nowadays); also, almost all of Europe (except Portugal and Spain), most of Russia and China, all of the Middle East and India will see a partial eclipse. Totality will start in Nunavut, in Canada, at 19:21 Melbourne time (09:21 UTC), and end in northern China at 21:21 Melbourne Time (11:21 UTC).

As usual, NASA has a very detailed web page dedicated to the eclipse, including an embedded Google Map showing the path of totality and technical information for any given point. Plus, for those of us outside the path of the eclipse, NASA will also be broadcasting the event live from China starting at 20:00 Melbourne time (10:00 UTC) on Friday.

The next total eclipse will happen in July of 2009, but before that we'll have a partial lunar eclipse on 16 August that will be partially visible from Australia. More details closer to the event.

Astronomy 101 - Lesson 2 - Units

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After speaking for a while about numbers, it's time to see what we can do with them; for this, let's take a look at the units of measurement commonly used in astronomy (which are not that different from the ones used in day-to-day life in most of the world, in fact). Basically, most of the time we will speak of measuring length in metres - which expands to square metres for area and cubic metres for volume -, mass in grams and time in seconds (or multiples thereof); these are the basic units of the International System of Units, or SI.

Before we go into any detail about them, let's take some time to look at the multiples that are commonly used. In general, these multiples follow, once again, powers of ten, and some specific powers of ten can be "abbreviated" by prefixes added to the name of the unit being used. The prefixes you are more likely to see are:


  • large: kilo (103), mega (106), giga (109), tera (1012), peta (1015)

  • small: centi (10-2), mili (10-3), micro (10-6), nano (10-9), femto (10-12), pico (10-15)

Do not confuse these with the prefixes commonly used in the computer world, where they have a different meaning associated to powers of 2 rather than 10; a kilobyte is 210 (or 1024) bytes, not 103 (or 1000) bytes, no matter what the manufacturer of your hard-drive tries to tell you.

Time The original definition of a second made it to be 1/86,400 of a mean solar day (which is the period of time between local noon in two consecutive days). This suffers from the problem that the mean solar day increases by about 1.4 milliseconds a century; the current definition, therefore, is based on properties of the radioactive decay of atoms of a Cesium isotope, which is, once again, thought to be the same anywhere in the universe.

Note that time units don't always follow powers of ten: one rarely refers to kiloseconds or megaseconds (other than in some sci-fi novels), but rather one talks of hours, minutes, days etc. We do use milliseconds, microseconds and other smaller units, though, and it's not uncommon to hear of kilo-, mega- or gigayears.

Length As mentioned above, the basic unit of length is the metre. This unit was originally defined by the French, soon after their revolution, to be 1/10,000,000 of the distance from the North Pole to the equator. This is clearly a less than useful definition (and they were slightly off in their measurement, anyway), so the current definition is actually based on the speed of light; specifically, it is the length travelled by light in a vacuum in a period of 1/299,792,458 of a second. The speed of light in the vacuum is believed to be a global invariant in the universe so, should we ever move out of this planet, our units will still make some sense.

As a side note, there are a few other units of length that are very common, and very useful, in astronomy, and you should get yourself acquainted with them:


  • Astronomical Unit, or AU: this is the average distance between the Earth and the Sun, and equals approximately 1.496 x 108 km (or, more commonly, 150 million km); this is normally used to refer to distances in planetary systems

  • light year: the distance travelled by light in a vacuum in the period of one year, it equals just under 10 trillion km, or 9.46 x 1012 km (and one year, in this context, is exactly 365.25 days of 86,400 seconds each, and yes, this is an important data point; there are several ways of measuring a year and even a day, and we'll come back to this at a later date), or about 63,240 AUs

  • parsec: this unit is a bit more complicated, and we'll discuss it later; for now, just remember that it's equal to about 3.26 light years, and that despite the fact that this seems like an arbitrary number there is a good reason for it

Mass Once again, we have an original definition that is different to the modern one. Originally, one kilogram was defined as the mass of 1 litre of pure water (and 1 litre is 1 cubic decimetre), while the current definition says that 1kg is the mass of the standard kilogram prototype, a block of a platinum-iridium alloy stored in France (and one gram is, of course, 1/1,000 of that). This is still not a very satisfying definition (especially considering new reports that claim this block may be slowly losing mass; this definition is therefore likely to change in the coming decades.

Very important: mass is not weight. Mass measures the amount of material in an object, while weight measures the effects of gravity on that object (and, being a measure of force, its SI unit is the newton). In other words, mass is an intrinsic and (mostly) unchanging property of an object, while weight is a local property dependent on the local gravitational field. To make this a bit more clear, objects aboard the International Space Station may well have no weight, but their mass remains the same as on the ground.

That said, the way mass is usually measured is by measuring weight and assuming a constant gravitational field, which works very well in the surface of the Earth (this is not true if a balance-beam scale is used, as it actually compares the mass of the object being measured with a known mass in the scale; this type of scale can be used wherever there is some gravity and will give the same reading anywhere).

And this is it for today. Next week, we finally start looking upward and talk about what we can see on the night sky.

Carnival of Space #63

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The 63rd edition of the Carnival of Space is up at the Angry Astronomer website. Despite his anger, he seems to be a very welcoming guy, and the Carnival contains an excellent collection of recent articles from all over the astrosphere.

Much of astronomy has to deal with large numbers; astronomical numbers, in more senses than one. For example, the average distance from the Earth to the Sun is 149,597,870.691 kilometres — or, more commonly, 150 million kilometres. The mass of the Sun is approximately 1,989,100,000,000,000,000,000,000,000,000 kg, and I'm not even going to try to write that in words. The approximate age of the Earth is 4.6 billion years, while the age of the universe is closer to 15 billion years.

It is probably very clear from the paragraph above that these large numbers are very hard to work with — or even to write. It's even hard to get an instinctive grasp of how large they really are; at some point, really, all you have are way too many zeroes. That's the reason why scientists use a more compact notation for very large — or very small — numbers. This notation (not surprisingly known as "scientific notation") divides the number in two parts and uses the magic of the powers of ten to get rid of all those zeros. Those two parts are the mantissa and the exponent.

In short, the mantissa is the part of the number that we are reasonably sure of. Look again at that number describing the mass of the Sun; it should be pretty obvious that those zeroes are just "filler"; we don't know what all those digits are supposed to be, but we are fairly confident about the first five non-zero digits. Those five digits will form the mantissa.

The exponent indicates how large the number is. It tells us how many zeroes go after the mantissa or, more generally, how much we need to move the decimal point of the mantissa to get to the real number. In other words, it tells us which power of ten must be multiplied by the mantissa to get the number we're trying to express.

Let's get back to the examples above. As a general rule, the mantissa will always be kept between 1 and 10; so, the average distance from the Earth to the Sun would be written as 1.49597870691 x 108 km — or, if want to be a bit more compact, 1.5 x 108 km — meaning that, starting from 1.5, we move the decimal point 8 positions to the right to arrive at the intended number. The mass of the Sun, in the same notation, is merely 1.9891 x 1030 kg, which is certainly much more readable than the other version.

What the scientific notation does is to allow us to write and, more importantly, read numbers much more efficiently. It keeps all those zeroes out of the way and allows us to concentrate on what we actually know about the quantity we're talking about. It also makes it much easier to compare large numbers; the exponent will tell you at a glance the magnitude of the numbers in question.

And, as I mentioned, it also works the other way: a negative exponent will tell you how many positions to the left you need to move the decimal point, allowing you to easily represent very small numbers. The diameter of a hydrogen atom, for example, is 0.0000000000106 metres — or 1.06 x 10-11 metres.

The next lesson will be about units, and it will focus on what we can measure with these numbers, with special attention to the astronomically relevant units of measurement.

Astronomy 101

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I was listening to the latest episode of The Skeptics Guide to the Universe podcast last night, and there was a discussion that was basically about whether it's worth trying to educate the public about science and trying to fight the seemingly endless influx of pseudo-science all around us. The discussion (started by this post) was mostly centred on "alternative medicine" claims (not surprising, since Steven Novella is a doctor); one of the pieces of information mentioned was that negative studies - that is, studies proving that a particular therapy does not work - have never convinced any "true believer" to abandon anything. But this applies to other areas as well; I mean, the fact that astrology has been repeatedly proven to be bunk has not caused it to be abandoned. This has many people in "our" camp feeling somewhat depressed at the apparent futility of trying to educate people who don't want to be educated.

This is an interesting problem. After all, why is it that we have so much trouble getting the general population to accept and believe in scientific information (properly researched and reviewed), while the purveyors of "alternative" information find such a receptive public? I don't have an answer to that; I wish I did. Part of it is probably the tendency of people not to trust information coming from those perceived to be in a position of authority; the "alternative" guys usually try to align themselves with "normal" people, which is why, for example, saying that a particular therapy was "invented by a school teacher" is seen as a good thing. This comes from a lack of understanding about how science operates and where scientific information comes from: ideas survive not based on the authority of their proponents, but on their merits - at least in the long term.

On a vaguely related note: not too long ago, I also read (or heard) a criticism by someone - sorry, I honestly can't remember who, when and where this was - saying that, despite the incredible number of scientific blogs available nowadays, there aren't many that act as "gentle introductions" to scientific subjects. To be fair, that's not entirely true - at least if you extend the word "blog" to also include podcasts. The Evolution 101 series was a great introduction to basic and advanced concepts about evolution and natural selection; Astronomy Cast is a great resource for astronomy-related concepts, as is Phil Plait's blog (especially the videos he publishes occasionally). And I'm sure there are others out there.

Still, this combination of factors is why I've decided to start writing regularly about the "basics of astronomy"; this will be my "astronomy 101" series. I realise that there may be a few of those around, on- and offline; still, I think I can add something to the "astrosphere", and this will have the added benefit of helping me focus on, and review, things I should - and probably do - already know; making me "structure" my knowledge better, in a way. Also, I have a feeling that scientific-literate people have some responsibility for putting information out there and for trying to get more people to our side.

The structure I'll follow will be loosely based on university-level introductory astronomy courses and on a few books (most prominently the famous Universe, by Freedman and Kaufmann); when relevant, I will mention bibliography or add links to more detailed material.

My intention is to post new material once a week; I originally planned on writing longer articles once a fortnight, but I guess shorter, more frequent ones will work out better. If not, I can always change the style; it's not like I on a contract, or anything...

I hope everything works out well and that someone out finds my texts interesting. The series starts later this week.

Articles already published:

  1. Astronomical Numbers (18/07/2008)
  2. Units of Measurement (25/07/2008)
  3. The Night Sky (02/08/2008)
  4. Spherical Earth (09/08/2008)
  5. The Size of the Earth (16/08/2008)
  6. Mapping Our World (23/08/2008)
  7. Mapping the Sky (30/08/2008)
  8. The Sky In Motion (1) (13/09/2008)
  9. The Sky In Motion (2) (25/11/2008)

Back from holidays

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What a month for me to go on holidays. The Shuttle was launched, had a perfect mission and landed with no flaws (other than severely damaging the launch pad); the Phoenix lander found ice right under itself and sent lots of data and pictures; New Horizons crossed the orbit of Saturn; I met Phil Plait and Neil deGrasse Tyson (hmm, that may be important only to me); and so on. It was a very busy month in space exploration, and I'll assume that everyone reading this knows about that already, so I won't rehash anything.

Now I'm back and will try to keep an eye (and comment) on future developments. Like the upcoming total solar eclipse, on 01 August, about which I'll write next time.

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This page is an archive of entries from July 2008 listed from newest to oldest.

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