And You Thought Size Mattered …

A few weeks ago I posted an entry, called Feeling insignificant yet?, on the relative size of the planets in our solar system, and the relative sizes of neighbouring stars. The purpose of the post was to show how insignificant the size of Earth is, in comparison to other bodies, and to remind people that not everything in the universe operates on a human scale. The universe itself is so far beyond the human scale, its damn close to incomprehensible, and the size comparisons in that post were a good way to show some relative differences in size.

But it leaves people with the impression that size actually matters when we talk about these things. And despite the mind-boggling sizes of the bodies described by “Feeling Insignificant Yet?” they are not, in fact, terribly relevant. They are impressive numbers to the human scale, but in universal terms, even the super-giant stars like Betelgeuse and Antares are barely perceptible specks of dust in a vast ocean of nothing.

I’ve tried to put together some pictures to illustrate this, and while they aren’t as colourful as the pictures of the planets, I’ve come up with the idea of concentric spheres in different scales to illustrate the vast distances between various heavenly bodies.


The above picture shows the inner solar system, to scale. The Sun, at the centre, is the only body in this image who’s diameter is visible at this scale … the tiny spec at the centre here illustrates the size of the sun. Next out, we see the orbit of Mercury, and while popular science tends to show Mercury as orbiting essentially on top of the sun’s surface, we can see that based on the size of the sun, the orbital distance of Mercury is quite large … the Earth is only 3 times farther from the sun than Mercury, so seen from the surface of Mercury, our sun would likely look only 3 times bigger than when viewed from the surface of the Earth. Its worth noting that the scale here doesn’t allow me to show any of the planets, just their orbital path around the sun … remembering how much bigger the sun was than the rest of the planets, the distances between planets dwarfs even that huge size.

For comparison, the size of the sun compared to other planets … remember, on the obital diagram above, the big ball of gas in this picture is but a tiny dot.


As indicated, this diagram of the outer planets is at a scale of about 1:20 against the inner solar system model I drew above. The tiny circles in the middle, inside Jupiter’s orbit, represent a scaled down version of the inner solar system model … as you can see, at this scale, the sun is no longer visible. Pluto, for the record, is about 5 billion km from the sun, on average, and while most text books show Pluto with a hugely eccentric orbit (and in fact it IS eccentric in comparison to other planets), the ACTUAL eccentricity works out to a minor-major axis ratio of 97%, meaning at this scale, even Pluto’s highly eccentric orbit shows as roughly circular to the naked eye. Its worth noting that on this diagram (the Outer Solar System), super-giant Antares would be quite visible, having a diameter roughly 3/4 the size of Jupiter’s orbit … it would show up here as somewhat larger than the inner solar system.


This is our local stellar neighbourhood, ranging from Sirius 2, our virtual neighbour at 8.6 light years away, to Aldebaran, some 66 light years away. One light year is defined as the distance light will travel in a year, and comes in at roughly 9 trillion km … as a way of comparison, the orbital distance of Pluto is roughly 5 billion km, so the distance from the Sun to Pluto (the entire expanse of the last graphic) is about 1/2000th of a light year. On the diagram above, the scale is too large for any stars to be visible for size, and given that the smallest circle is over 16 light years across, our entire solar system is too small to be visible.


And so we get to the final picture in our series, with many of the same stars as the end of “Feeling Insignificant Yet?” We see

Ultimately, the size of the planets IS impressive, but its nothing in comparison to the distances between them. The T-shirt I ended “Feeling Insignificant Yet?” with showed the Milky Way galaxy at about 100000 light years across, and its worth remembering that the distance to Rigel is about 775 light years, less than 1/100th the total size of our galaxy.

Space, says the introduction to the guide, is big. Really big. You just won’t believe how hugely, mind bogglingly big it is. And so on.
Douglas Adams, Hitchiker’s Guide to the Galaxy

Doug got it bang on with that one … and you thought size mattered.


One Response

  1. great article. thanks for sharing.

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