The universe is a huge and largely empty place. Not only is it that the very atoms that make up ordinary matter concentrate almost all of their mass in the tiny central region that is their nucleus, but also that the spaces between planets, stars and galaxies are eminently empty. The energy density of the universe can be useful to us. This is nothing more than the amount of total energy (whether in the form of dark energy, dark matter, or ordinary matter) divided by the volume of the entire universe and equals about 6 protons per cubic meter . But 95% of that energy is in the form of dark matter and dark energy, so the actual number of protons per cubic meter is about 20 times less.
The distance between planets is measured in hundreds of millions of kilometers. The distance between stars is measured in light years, because talking about tens of trillions of kilometers is not particularly comfortable. The distance between galaxies is measured in hundreds of thousands or millions of light years. And in the space that separates them, both one and the other, there is nothing. Well, let’s see, there are some things, but they involve such a ridiculously small amount of matter that we can ignore them. The cosmos is so empty that the light of the most distant galaxies reaches us without further impediment. Of course, it does so after having been traveling for billions of years at the maximum possible speed, at the speed of light.
All this, of course, is difficult to capture graphically, in an image. I’m not telling you anything new, I know. The Internet is full of images that are deceptive in their simplicity. Take for example the Hubble Ultra Deep Field. In this image we can see almost 10,000 different galaxies. Collected in this way, they would all seem to belong to the same cluster of galaxies, which are tied to each other by their own gravity, but nothing is further from reality. Between each of these galaxies there are, at a minimum, millions and millions of light years.
In addition, the internet is full of images of the solar system that respect the scale in one aspect and not in others. Some show the size of the planets to scale, others show their distances from the sun to scale, but none of these images show the size and distances simultaneously to scale. It is not that it is impossible, but rather that the distances between planets are so incredibly large that our brain has a hard time understanding them and that image would require occupying thousands of times more than the objects it intends to show.
Let’s take an example that we are most familiar with, something that we can begin to visualize. Let us take as an example the system formed by the Earth and the Moon. Our planet is about 12,700 km in diameter while the Moon is less than 3,500 km .
Therefore this would be the size of both bodies to scale. Well, maintaining the proportions, which of these 3 possibilities would correspond to the average distance between the planet and the satellite?
The correct answer is… none of the 3! The average distance between the Earth and the Moon would be that of the following photo.
In real life this corresponds to about 384,000 km of distance, little more than the distance that light travels in one second or a quarter of the diameter of the Sun. In the space that separates both bodies, the rest of the planets of the Solar System would fit, placed one next to the other, and we would even still have space to include Pluto, to please the most nostalgic.
That is the average distance that separates us from the Moon, although at some points in its orbit it can be found at a greater or lesser distance. During perigee, the closest point to Earth in the lunar orbit, we have the satellite only 363,000 km away, while during apogee it moves away up to 405,000 km. This incredible distance, about 60 times the size of the Earth itself , is the distance traveled by astronauts who flew over the Moon or landed on its surface during NASA’s Apollo missions . They used to take about 3 days to cover it, while light takes just over 1 second to cover the same distance.
You may be wondering how we are able to see the Moon so large in the sky, being so far away. And I will tell you two things. The first is that the Moon actually appears quite small in the sky. A building or a mountain a few hundred meters high looks much larger at distances of about 10 km. The second is that the Moon is surprisingly far away, but it is also huge. It has a diameter of 3500 km , that is, as the distance that separates Madrid from Moscow.
Of course this distance is nothing compared to that between the planets of the solar system . If we wanted to stretch out the last image until it included Venus, the closest planet to Earth, we would have to make it about 120 times larger . At the end of countless black pixels we would find another little ball, slightly smaller than the Earth. Let us therefore forget to include Mars, Mercury and the rest of the planets.
