Our Cosmic Address


The current way of thinking about Earth's place in the Universe is not to put it on a two- or three-dimensional map of some kind, but rather to envision its place in a nested hierarchy of structures.This way of thinking about "where we are" is directly analogous to an address on an envelope: each line is a larger structure which contains the item in the previous line. For example,

Barack H. Obama, the President of the United States
lives within the
White House
which is on
Pennsylvania Avenue
which is one of many streets in the city of
Washington
in the
District of Columbia
in the
United States of America,

 

So his address is:

President B. H. Obama
White House
Pennsylvania Avenue
Washington
D.C.
U.S.A.

The "nested hierarchy" paradigm of envisioning our place in the universe takes that address as far out as we can take it. Here are the steps:

Planet Earth, a ball of rock and metal on whose surface all "geography" (including countries) exists. Earth's diameter is roughly 8,000 miles.

The Earth-Moon system, consisting of the Earth and the Moon, is easy to visualize as seen from somewhere else in space. The Moon is about 1/4 the Earth's diameter and is about 30 times the Earth's diameter away.

The Solar System, which contains the Earth-Moon system and many other planets and their moons, consists of the Sun (a star) and all things in orbit around it, including but not limited to:
Eight major planets (Pluto isn't considered "major" any more),
Dozens of satellites (moons),
Hundreds of thousands of asteroids, and
Uncountable numbers of smaller bits, including comet nuclei.
The scale of the solar system is such that if the Sun (in actuality about 800,000 miles in diameter) were represented by a soccer ball in the middle of the Planetarium, then the Earth would be an apple seed just outside the Planetarium building and the most distant "major" planet, Neptune, would be about the size of a marble at the corner of Stevens Creek and Stelling at the NE corner of campus. On this scale, the nearest other star is about 10,000 miles away! The distance unit that is most commonly used to express distances within our solar system - between planets, say - is the "astronomical unit", defined to be the average distance between Earth and the Sun, or about 93 million miles.

The Milky Way Galaxy contains the Sun (and its collection of planets) and an unimaginably large number of other stars: roughly 400 billion of them. 400 billion is about the number of seconds in 10,000 years, so don't bother asking why we haven't counted. Its extent is so vast that distances between its member stars are generally expressed in light years. One light year is defined to be the distance light travels in one year (several trillion miles), and the distance between our Sun and its nearest neighbor star, Alpha Centauri, is a little more than four light years. The most prominent part of this great assemblage of stars is a huge disk, about a hundred thousand light years across.

The Local Group is a cluster of galaxies which includes the Milky Way and about a dozen other galaxies, most of which are rather smaller than the Milky Way. There is one other large, spiral galaxy in the group: the Andromeda Galaxy, which is about two million light years away. The Local Group represents a "clump" of galaxies, separated from each other by a few hundred thousand to a few million light years. The nearest other "clumps" (or clusters) of galaxies are several times that far away.

The Virgo Supercluster contains thousands of clusters of galaxies, including the Local Group as one of its smaller members. It is called the "Virgo" Supercluster because its central cluster is one containing thousands of individual galaxies which we see in the direction of the constellation Virgo roughly a hundred million light years away.

The Observable Universe contains a very large number of superclusters of clusters of galaxies, including "our" Virgo Supercluster, which are scattered across space in a vaguely foam-like structure, with sheets and filaments of superclusters surrounding large, apparently empty regions called "voids". Our current view of the Observable Universe extends more than ten billion light years in all directions.

So... here is the President's full Cosmic Address:

President B. H. Obama
White House
Pennsylvania Avenue
Washington
D.C.
U.S.A.
Planet Earth
Earth-Moon System
Solar System
Milky Way Galaxy
Local Group
Virgo Supercluster
Observable Universe