Vuurwerk, champagne, oliebollen, nieuwjaarsresoluties, geweld. Elk jaar gaat Nederland collectief uit z’n dak als de klok op 31 december 12 uur slaat. Het einde van 2011 en het begin van 2012. Maar wat is een jaar eigenlijk? Ik vroeg me de afgelopen weken af waarom we een jaar niet meten van 21 december tot 21 december (de kortste dag) of van perihelium tot perihelium (dat woord heb ik net geleerd! het is wanneer de afstand van de aarde tot de zon het kortst is; onze baan om de zon is nl. niet rond, maar elliptisch), in plaats van het nogal arbitraire 31 december. Ik weet er het fijne allemaal niet van, maar gelukkig Phil Plait van mijn favoriete astronomie weblog wel:
Let’s take a look at the Earth from a distance. From our imaginary point in space, we look down and see the Earth and the Sun. The Earth is moving, orbiting the Sun. Of course it is, you think to yourself. But how do you measure that? For something to be moving, it has to be moving relative to something else. What can we use as a yardstick against which to measure the Earth’s motion?
Well, we might notice as we float in space that we are surrounded by zillions of pretty stars. We can use them! So we mark the position of the Earth and Sun using the stars as benchmarks, and then watch and wait. Some time later, the Earth has moved in a big circle and is back to where it started in reference to those stars. That’s called a “sidereal year” (sidus is the Latin word for star). How long did that take?
Let’s say we used a stopwatch to measure the elapsed time. We’ll see that it took the Earth 31,558,149 seconds (some people like to approximate that as pi x 10 million (31,415,926) seconds, which is an easy way to be pretty close). But how many days is that?
Well, that’s a second complication. A “day” is how long it takes the Earth to rotate once, but we’re back to that measurement problem again. But hey, we used the stars once, let’s do it again! You stand on the Earth, and define a day as the time it takes for a star to go from directly overhead to directly overhead again: a sidereal day. That takes 23 hours 56 minutes 4 seconds = 86,164 seconds. But wait a second (a sidereal second?) — why isn’t that exactly equal to 24 hours?
I was afraid you’d ask that — but this turns out to be important.
En zo gaat hij nog even verder. Een aanrader!
Foto: Zonsondergang boven Zuid-Amerika, NASA.
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