It can be hard to grasp exactly how empty space is.  The astronomer David Morrison once got the question:

What keeps the planets in their orbits?

His answer begun with the sentence:

Well, there is nothing to stop them.

Once the protoplanetary disc had disappeared there were nothing to make any resistance.  Within a few hundred million years the Solar system had stabilised.  The eight planets could continue in virtually unchanged orbits for several thousand million years.  There is still some variation in how elliptical the Earth’s orbit is.  However, we then talk about a cycle of about a hundred thousand years.

Even within the Solar system the distances are far from comfortable.  The problem with sending out space probes is not avoiding hitting something.  It is hitting anything at all or getting close enough to place it in orbit.  Here is a digital illustration showing distances between the planets’ orbits.  The starting point is the size of the Moon equalling one pixel.  It took me about an hour to scroll through the entire image.  Then I read the text found between the planetary orbits too.  This give a bit of perspective on how large the distance in fact are.  It takes the Sun’s light 4 hours to reach to Neptune.  The Earth it reaches in 8 minutes.

To me such distances appear completely sensible.  They explain everything looking so small despite their real sizes.  The Moon is 27% of the Earth’s size but covers half a degree of the sky.  Other heavenly bodies we can see at night are not seen as more than dots.  Mercury, Venus, Mars, Jupiter and Saturn can be seen by an unaided eye.  Uranus is barely visible and shines so weakly it may be mistaken for a star.  Moreover, it moves too slowly against its background of stars.  It was not until 1781 it was discovered to be a planet.  Neptune was discovered by its gravity affecting how Uranus moves.  People calculated were it was after which others found it there.

The distances within the Solar system are still entirely insignificant compared to the distances to the stars.  The distance to stars are for this reason counted in light-years.  This is as far as light moves in 365¼ days.  At least this is the most common definition.  The distance to the Sun’s closest neighbour is 4.24 light-years.  However, it shines too weakly to be seen by an unaided eye.  More famous is the two stars it is thought to orbit.  The sky’s brightest shining star is more than twice as far away.  Stars’ shininess is not only about how far they are.  It also matters how much light they produce.  Most stars people know about are for this reason relatively large.

The distance to stars up to 30.000 light-years can be measured by triangulating.  This way one can measure the distance to regularly varying stars.  This means the amount of light produced varies cyclically.  How quick the cycle is directly proportional to how much they produce at their peak.  By finding such in other galaxies the distances to them can be judged.  One can also use explosions arising from double-stars’ interactions.  This type of explosions always emit the same amount of light.  So how bright they look reveal how far away they are.

On the other hand the distances to the stars should probably not be overestimated either.  The ones we can see by an unaided eye are within 16,000 light-years.  Even the most short-lived stars live for 1 – 2 million years.  Someone calculated the probability of any single one of them being dead as at most about one percent.  If one can kept track of orders of magnitude this appears completely understandable.


Uploaded on the 24th of April 2024.