Your Cosmic Address
Where you are in the universe, written out as a nested coordinate from Earth's surface to the boundary of the observable universe. Ten rungs, each a measurable fact.
When Carl Sagan opened the first episode of Cosmos in 1980, he gave the audience an address. Not a street and a city. Not a country. A cosmic address: Earth, Solar System, Milky Way, Local Group, Virgo Supercluster, Universe. Six rungs. The framing landed hard because the rungs were physical, not metaphorical. Each one named a real structure whose size, mass, and motion had been measured.
The address has grown since 1980, and tightened in several places. We now know the Sun is moving through a local interstellar cloud whose extent has been mapped from absorption-line spectroscopy. We now know the local cavity in the interstellar medium is shaped by ancient supernovae whose dates we can recover. We now know that what we used to call the Virgo Supercluster is a substructure of a much larger gravitational watershed called Laniakea, defined as recently as 2014. And we know the observable universe's comoving radius to roughly half a percent.
This note walks the current ten-rung address and gives the sources for each.
A barred spiral galaxy with about 100 to 400 billion stars and a total mass of approximately 1.5 trillion solar masses (including the dark-matter halo). The Sun completes one orbit every 225 million years.
≈ 100,000 light-years across
What "cosmic address" actually means
There are two related ways to write down where you are:
- Geometric position, expressed as coordinates in some reference frame. Latitude and longitude on Earth. Right ascension and declination on the celestial sphere. Galactic longitude and latitude in galactic coordinates. Comoving coordinates in cosmology.
- Nested containment, expressed as a chain of progressively larger structures that contain you. Your country is in a continent is on a planet is in a solar system is in a galaxy.
Sagan's framing uses the second form, which is more memorable because each step is something you can picture. This note uses the second form too, with one improvement: every rung is paired with the actual measurement that pins it down.
The rungs, in numbers
| Level | Structure | Scale | Pinned by |
|---|---|---|---|
| 1 | Earth | 12,742 km diameter | Geodesy; WGS84 reference ellipsoid |
| 2 | Solar System | ≈ 100 AU (heliopause) | Voyager 1 boundary crossing, 2012 |
| 3 | Local Interstellar Cloud | ≈ 30 ly across | Spectroscopy of nearby stars |
| 4 | Local Bubble | ≈ 1,000 ly across | Soft X-ray background, neutral H mapping |
| 5 | Orion Arm | ≈ 10,000 ly long | Spiral-arm mapping via maser parallaxes (Reid et al.) |
| 6 | Milky Way | ≈ 100,000 ly across | Star counts, Gaia kinematics |
| 7 | Local Group | ≈ 10 Mly across | Galaxy velocity dispersion measurements |
| 8 | Virgo Supercluster | ≈ 110 Mly across | Tully-Fisher distance survey, redshift mapping |
| 9 | Laniakea Supercluster | ≈ 520 Mly across | Cosmicflows-3 (Tully et al., 2014) |
| 10 | Observable Universe | ≈ 93 Gly across | CMB age × particle horizon |
The rungs added since 1980
Sagan's original address had six rungs. The modern version has four more. Each addition was a measurement that revealed structure between what we already knew.
The Local Interstellar Cloud (added c. 1980s–90s). The Sun is currently inside a wisp of partially-ionised gas roughly 30 light-years across, drifting through the larger Local Bubble. It was identified through absorption lines from neutral hydrogen along the lines of sight to nearby stars. The Sun has been inside this cloud for about 60,000 years and is expected to exit it within the next few tens of thousands of years, on standard kinematic models.
The Local Bubble (mapped through the 2000s; refined by Gaia and Planck). A cavity in the interstellar medium roughly 1,000 light-years across, hot, diffuse, and underdense compared to the surrounding interstellar gas. The leading explanation is that one or more supernova explosions about 14 million years ago carved the cavity. A 2022 paper (Zucker et al., Nature) used Gaia stellar kinematics to argue that young stars on the Bubble's surface are still moving away from those ancient supernova sites at velocities consistent with the shock-wave hypothesis.
The Orion Arm (the Sun's small home arm, distinct from the four major spiral arms). The arm structure of the Milky Way was debated for decades. By the 2010s, masing water and methanol sources in star-forming regions, parallax-measured by the Very Long Baseline Array, gave precise distances to spiral-arm tracers and confirmed that the Sun sits in a smaller spur between the major Sagittarius and Perseus arms. We now know the Orion Arm's curvature, its length (roughly 10,000 light-years), and our position on its inner edge.
Laniakea (defined 2014). For decades, the largest structure we knew we belonged to was called the "Virgo Supercluster" - a roughly 110-million-light-year region centred on the Virgo Cluster. In 2014, Brent Tully and colleagues at the University of Hawaii redefined supercluster boundaries kinematically, using galaxy peculiar-velocity flows to identify watersheds. By that definition, our home supercluster is much larger than Virgo. They named it Laniakea - Hawaiian for "immeasurable heaven" - and the Virgo Supercluster became one of its lobes. Laniakea contains roughly 100,000 galaxies and the entire region is flowing toward the so-called Great Attractor.
The address may not stop at Laniakea. There are larger structures (the Shapley Supercluster, the Hercules-Corona Borealis Great Wall) but whether these are gravitationally bound systems or transient overdensities is unresolved. Laniakea is the largest structure we know is our home.
The boundary case: the observable universe
The tenth rung is fundamentally different from the others. Rungs 1 through 9 are gravitationally-bound or kinematically-coherent structures. Rung 10 is a causal boundary, not a structure: it is the volume from which light has had time to reach us since the Big Bang. The radius of the observable universe is set by:
where a(t) is the cosmic scale factor. Numerically, the comoving radius is approximately 46.5 billion light-years, giving a diameter of about 93 billion light-years. This is much larger than (age of universe × c) because the universe has been expanding while light has been travelling.
Beyond rung 10, more universe almost certainly exists. We just cannot see it, and (under the standard cosmological model) we never will. The boundary is fixed by physics, not by the limits of our instruments.
Why this matters
A cosmic address is a small thing. It is just a list. But it does something specific: it stops the universe from being either too small (the scale of personal life) or too abstract (the scale of "deep space"). You are at a measurable, named position on a chain of nested physical systems. Most of them have only been characterised in the last hundred years. Two of the rungs only have names since the 2010s.
This is the framing the lab uses throughout its work. The sky is not an undifferentiated dome. It has structure, and the structure can be written down.
References
The numbers above are drawn from primary literature:
- Heliopause distance: NASA Voyager mission archives, particularly the 2012/2013 boundary-crossing papers.
- Local Interstellar Cloud structure: Frisch et al., several reviews including ARA&A 2011.
- Local Bubble shape and origin: Zucker et al., Nature 2022.
- Orion Arm geometry: Reid et al., ApJ, multiple papers 2009-2019, masering-source parallax surveys.
- Galactic Center distance: GRAVITY Collaboration, 2019 (8.178 ± 0.026 kpc).
- Local Group dynamics: van der Marel et al., ApJ, 2012 (Andromeda proper motion).
- Laniakea: Tully, Courtois, Hoffman, Pomarède, Nature, 2014.
- Observable universe radius and age: Planck Collaboration, 2018 final results (13.787 ± 0.020 Gyr).
A consolidated bibliography lives at /bibliography.
All values are well-established public measurements. The CosmicAddressMap component above is original schematic art; data points are widely-published values listed in the references.