How do you know?

When I’m out stargazing I often have someone ask me ‘What’s that bright thing I’ve been seeing after sunset every evening?’ This month the answer to that question is either ‘Venus’ or ‘Jupiter’, and then they come back with ‘How do you know?’

I usually come up with some inane answer like ‘I looked it up’, but I’m really fumbling for more details. Here are all the answers I would give them if I had more time.

The sky is predictable. To people who don’t pay much attention to the sky beyond what the weather is doing, it probably  just looks like a mess of twinkly lights. But there are patterns up there that we can become familiar with. The motions of the stars are like the motions of the continents on the Earth — a map that you look at today is not going to be randomly jumbled up into different shapes tomorrow.

When you know the constellations, you will also notice if a planet has moved into a particular area of sky, because it will look like an extra star that wasn’t there before.

The planets themselves are predictable. Since the planets orbit the Sun in roughly circular paths that are all more-or-less on the same flat plane, we know that they will be confined to certain areas of the sky as seen from the Earth.  They also each have their own regular orbital period, just like the Earth takes one year to go around the Sun, so we know how fast they will move in the sky when we see them.

The planets are recognizable. There are only five planets we can see without a telescope from Earth, and most of them are unmistakable due to their position, brightness, and colour. Venus is by far the brightest, Mars is red, Mercury is the closest to the Sun and moves the most quickly against the background stars. Saturn and Jupiter can be tougher to distinguish with the naked eye, but turn a telescope or binoculars on them and you’ll see Saturn’s ring system or Jupiter’s inner moons and caramel-coloured stripes.

You can keep track of changes. If you go out and notice a bright object one night, and by one method or another you find out that it’s Jupiter, you can look again the next night and find Jupiter in almost the same place. From one night to the next the change in any planet’s position will be pretty small, although as mentioned previously, planets closer to the Sun will move faster.

You can look these things up. If Jupiter and Saturn seem too similar without a telescope, or if you’ve gone weeks without a clear sky and haven’t been able to keep track of things, there are plenty of resources to help you out. There are books that will tell you where to look for different planets during the year, websites where you can download free full-sky charts for your location, and even computer programs that will simulate real-time views of the sky on your desktop.

People who aren’t interested in the natural world at all tend to think it’s mysterious and disorganized and random. But the same can be said for anything made by humans, if the person looking at it is looking for the first time and doesn’t understand it. (Personally, I think knitting looks like some kind of arcane ritual whereby you wad up a ball of string, stick pins in it, and magically turn it into a sock.)

I wonder if that perceived unfathomability scares people away from science. Maybe if more people knew that there are things in the natural world that can be understood by the average person without a formal science education, they’d be interested in learning more about it.



  1. Ann said,

    8 December 2008 at 1:13 pm

    Good post, btw.

    … Maybe you could answer their question “how do you know?” by asking it back to them, on a terrestrial level:

    “Who lives in the house next to yours?” When they answer, ask: “But how do you know?” the answer will be: “Well, they lived there yesterday, and I haven’t seen any signs that they’ve moved, so…”

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