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.

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from   Astronomy 101 by Astronomy Down Under on August 4, 2008 3:15 PM

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Just a minor correction, you have the prefixes femto and pico flipped. Pico is actually 10-12 and femto is 10-15.

I just recently started reading your Astronomy 101 series and it's a great gentle introduction.

Thank you for writing it.

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This page contains a single entry by Wilson published on July 25, 2008 10:53 PM.

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