# HOW BIG IS BIG?

A lightyear is the distance light travels in a year. The numbers get so big that they lose meaning, so let’s set up a mental picture we can understand. In one second, a beam of light would travel around the Earth about seven and a half times, and since the circumference of the Earth is about 24,900 miles, this means that light travels approximately 7½ x 24,900 = 186,000 miles in a second (in round figures). That calculates to just under 670 million miles in an hour; which converts to just under 16.1 billion (thousand million) miles in a day; which converts to somewhat over 5.8 trillion (million million) miles in a year, so a lightyear is equal to 5.8 million million miles.

That didn’t help much did it?

Look at it this way. Light takes about nine minutes to arrive at the Earth from the sun. It takes about 9 years to arrive at the Earth from Sirius, so Sirius is about 526 million times further from the Earth than is our own sun. That probably helps a bit, but it still is insufficient to get a clear mental picture.

Let’s try one final analogy. Place a basketball at the goal posts of a football field. The Earth would be a pea a foot away. Jupiter would be an orange at about the two yard line, Pluto—the “edge” of our solar system—would be a speck near the thirteen yard line, and Sirius would appear as two balls 106  miles away, one about twenty-five percent larger than our sun and a pea about the size of the Earth.

These are relationships we can actually understand.

While we’re at it, let’s place more of our universe into this picture. The galaxy in which our sun and all its planet’s resides is about 100,000 lightyears across. In the picture we have developed with the basketball and the football field, our galaxy is one million miles across, or about four times the actual distance between the Earth and the Moon. And the Andromeda Nebula, which is the closest large galaxy to our own, and about two million lightyears from our galaxy, would be 20 million miles away from our basketball, or about one fifth the actual distance to our sun.

The distances get so large so fast, that it is nearly impossible to keep a relative picture in mind of how all this relates together. If we collapse our example so that the entire galaxy, which in our example extended to four times the distance to the Moon, shrinks to the size of a Frisbee, then Andromeda will be another Frisbee at about the seven yard line. We are part of about 13 galaxies in a sphere ten million lightyears across—in our collapsed example about forty yards across.

Let’s transfer our base of operations to the fifty-yard line so we can better visualize what we have: In a twenty yard sphere centered at the middle of the fifty-yard line, we have some thirteen Frisbees of various sizes, in various orientations. Now imagine a sphere that extends outward four and a half football fields in all directions from center field, a sphere nearly 900 yards across—approaching a mile in diameter. This sphere in our collapsed example represents the Virgo Supercluster containing our Local Group of thirteen galaxies plus 160 or so other clusters of galaxies.

Time to collapse again; this is still too big to contemplate. So we collapse our mile-wide supercluster down to basketball size again, which represents a distance of about 200 million lightyears. Within a sphere extending three yards out, that is a sphere six yards across, we will find about eighty other superclusters similar to our own Virgo Supercluster. This sphere that is now six yards across represents a billion (thousand million) lightyears.

Now, if you extend the current sphere so that it takes in the entire football field and the bleachers, this represents on our scale, the size of the known universe, about twenty-eight billion lightyears across.

These numbers get so large so quickly, that it is nearly impossible not to get lost contemplating them. We started out with a basketball representing our sun, and discovered that the Sirius star system was over a hundred miles away, and our galaxy extended four times the distance to the Moon. The nearest other galaxy was one fifth the distance to the sun. So we collapsed our galaxy to a Frisbee and located us inside a forty-yard sphere; this Local Group was but a part of the Virgo Supercluster inside a mile-wide sphere. So we collapsed again to basketball size, and found our supercluster part of 160 other superclusters in a six-yard sphere. Finally, we extended our sphere to include the bleachers and beyond, to contain the entire known universe.

So how big is big?