John C. Menzies
By John C. Menzies and Eugene J. Lynch
April 4, 2025
On a clear, dark night, far from city lights, the Milky Way Galaxy stretches across the sky like a luminous river of stars, gas, and dust. This glowing band, a partial view of our galaxy’s spiral structure, is a breathtaking sight that has captivated humanity for millennia. Beyond its beauty, the Milky Way’s familiar presence in the night sky offers a powerful way to grasp the immense scale of the universe—by serving as a cosmic ruler, with its full diameter as the measuring unit.
The Milky Way is approximately 100,000 light years in diameter, a figure established through extensive astronomical research. When we observe its shimmering arc from horizon to horizon, we’re seeing only a slice of this vast structure, which contains an estimated 100 to 400 billion stars. Human eyes can discern just a tiny fraction of these—roughly a few thousand stars, blended into a smooth, milky glow, with dust clouds obscuring much of the view. Infrared telescopes are needed to reveal the galaxy’s hidden depths, but the visible band remains a familiar starting point for our analogy, anchoring the cosmic scale to something we can see.
Introducing the Galactic: A New Cosmic Unit
To make this cosmic ruler more intuitive, let’s formalize it as a new unit of distance: the Galactic, defined as 100,000 light years, the approximate diameter of the Milky Way’s stellar disk. Now, let’s measure the universe with this ruler. The observable universe—the region we can theoretically see, given the speed of light and cosmic expansion—spans about 93 billion light years in diameter. The universe’s expansion stretches light from distant galaxies, making this diameter far larger than its 13.8-billion-year age might suggest. Using the Galactic as our unit, the observable universe’s diameter is roughly 930,000 Galactics. The distance to the edge of the observable universe, or its radius, is about 46.5 billion light years—equivalent to 465,000 Galactics. Imagine lining up 465,000 Milky Ways to reach the cosmic horizon, a vivid way to transform an abstract number into a concept rooted in the galaxy we call home.
The Incomprehensible Complexity of Traditional Cosmic Measurements
Traditional units like light-years and parsecs, while precise, often make cosmic distances feel abstract and overwhelming. A light-year, the distance light travels in one year (~5.88 trillion miles or 9.46 billion kilometres), is intuitive for stellar distances but becomes unwieldy for galactic scales. For example, the Andromeda Galaxy is 2.5 million light years away, a number that’s hard to visualize. The parsec, defined as the distance at which a star’s position shifts by one arcsecond due to Earth’s orbit (~3.26 light years), is even less accessible. Derived from trigonometric parallax, parsecs are favored in astronomy for their mathematical precision—e.g., the Milky Way’s diameter is ~30 kiloparsecs—but they lack a tangible connection to everyday experience (International Astronomical Union). Converting between light-years and parsecs, or scaling to megaparsecs for cosmological distances (e.g., the observable universe is ~28.5 megaparsecs across), adds layers of complexity that can alienate non-experts. These units, rooted in abstract physics and geometry, make it challenging to feel the scale of the cosmos. The Galactic, by contrast, grounds distances in the familiar image of our galaxy’s glowing band, offering a more intuitive bridge to the universe.
A Unique Perspective
What makes the Galactic analogy unique is its use of the Milky Way, a sight familiar to anyone who’s gazed at a starry sky, as a bridge to the cosmos. Unlike the abstract light-years or parsecs, this approach leverages our galaxy’s visual splendor to make the universe’s scale feel more accessible, a perspective rarely emphasized in traditional astronomy. By defining the Galactic as 100,000 light years, we create a unit that simplifies galactic-scale distances—Andromeda is just ~25 Galactics away—while vividly illustrating the universe’s vastness at 930,000 Galactics across.
The Milky Way doesn’t just help us measure distance—it connects us to the cosmos. As Carl Sagan noted, we are “made of star-stuff,” forged from elements born in ancient stars. Next time you’re under a clear night sky, find the Milky Way’s glowing path and let it guide your imagination across the universe, from nearby galaxies to the cosmic edge 465,000 Galactics away.
With the Milky Way as your guide, the universe’s vastness feels just a little closer to home.
What a brilliant idea,John to bring the stars closer to us!The stars…!
The Galactic is a great way to make the universe feel closer to home. It takes incomprehensible cosmic distances and provides a metric of which we are all aware – no PhD required. Light-years and parsec, way to hard for most people. The Galactic makes the cosmos approachable. Instead of humans feeling small within the vastness – the Galactic allows humans to stand back and visualize their place in the Universe and what is that – the most complex thing in the known universe.
We perceive the Galactic as an educational and outreach tool, not a scientific standard. In that role it has the potenital to change the understanding and conceptualization of the universe entirely and of our place within it!