How fast is our Galaxy cluster moving?

The cosmos is a place defined by motion. For us living on Earth, accustomed to the comforting stability of our surroundings, grasping the sheer velocity at which our home galaxy—the Milky Way—is traversing the universe can be genuinely mind-bending. Even before addressing the speed of our entire galaxy cluster, we have to account for the speeds of the smaller structures nested within it, starting right here at home.

# Earth's Orbit

Our immediate experience of motion is the Earth circling the Sun. This orbit dictates our calendar and seasons, moving us along at approximately 30 kilometers per second ( $\text{km/s}$ ). That equates to a staggering speed of about $67,000 \text{ miles per hour}$ as we sweep around our star. This is the familiar, almost constant whiz we are always making, but this local motion is just one component in a much larger cosmic calculation.

# Galactic Spin

The Sun, carrying our entire solar system with it, is not stationary within the Milky Way. We are whipping around the center of our own galaxy in a grand, galactic orbit. Measurements suggest the Sun travels at roughly $220 \text{ kilometers per second}$ ( $220 \text{ km/s}$ ) relative to the Milky Way's core. At this rate, it takes our solar system about $225 \text{ million years}$ to complete one full circuit around the galactic center. To put that into perspective, the last time the Sun was in this particular spot in the galaxy, the dinosaurs were only just beginning to dominate the terrestrial landscape on Earth.

The Milky Way itself is an open spiral galaxy, a structure that exhibits differential rotation, meaning its inner and outer parts spin at different rates. These internal dynamics—the spinning and orbiting of stars within the disk—are essential, but they describe motion within the galaxy, not the movement of the galaxy through space.

How fast do spiral galaxies spin?

# Relative Velocity

When we talk about how fast something is moving in space, the answer always depends on what we are measuring its speed against. This is the concept of relative motion. If we only considered the motion of the Sun around the galactic center, we might think that $220 \text{ km/s}$ is the number we seek. However, the Milky Way, along with its satellite galaxies, is moving through the universe relative to other cosmic benchmarks.

The most stable reference frame we have for measuring the bulk motion of our local patch of the universe is the Cosmic Microwave Background (CMB) radiation. The CMB represents the faint afterglow of the Big Bang, an ancient light bath that permeates all of space, offering a relatively universal "rest frame" against which galactic movement can be measured.

When scientists calculate the Milky Way's velocity relative to the CMB, the numbers increase considerably. Our galaxy, the Milky Way, is surging through space at an estimated $627 \text{ kilometers per second}$ ( $\text{km/s}$ ). If you prefer miles, this translates to roughly $370 \text{ miles per second}$ . This motion isn't random; it has a direction, currently pointing toward a region known as the Great Attractor, a massive gravitational anomaly.

This difference between the solar system's speed around the galactic center ( $220 \text{ km/s}$ ) and the Milky Way's speed relative to the CMB ( $627 \text{ km/s}$ ) shows us that the local motion around the center is only a fraction of our total velocity vector through the cosmos. Imagine driving a car ( $220 \text{ km/s}$ relative speed) down a highway while the entire highway is being pulled toward a city at a faster rate ( $627 \text{ km/s}$ total speed). The motions add up, resulting in our overall cosmic drift.

# Zooming to Clusters

The prompt specifically asks about the galaxy cluster our Milky Way belongs to. The Milky Way is part of the Local Group, which is, in turn, a member of the much larger Virgo Supercluster. This supercluster is gravitationally bound, meaning the galaxies within it are influencing each other's motion, pulling them toward a common center of mass.

The motion of the Local Group, and thus the Milky Way, is influenced by the combined gravitational pull of everything around it, including the massive Virgo Cluster at the center of our supercluster, and even larger structures beyond, like the Great Attractor. The speed of the Local Group as a whole, relative to the CMB, is often cited near the Milky Way's individual value, as the Local Group is gravitationally bound and moving together in that direction.

# El Gordo's Exception

While our Local Group moves generally toward the Great Attractor, other massive galaxy clusters exhibit their own intense motions, sometimes exceeding what standard models predict for their mass. One remarkable example is the El Gordo galaxy cluster. This structure, located about $3 \text{ billion light-years}$ away, is one of the most massive galaxy clusters known to exist in the observable universe.

What makes El Gordo scientifically interesting is its speed. It is moving remarkably fast, with a velocity that suggests it is not simply following the expected large-scale flow of cosmic expansion or local gravitational tides. Its observed speed is higher than models based on its mass would typically predict, indicating it might be currently moving away from us due to the universe's expansion, but its specific velocity vector implies it is being influenced by an extreme gravitational source or that it has only recently experienced a major interaction.

To compare the speeds effectively, consider placing the motions on a chart scaled by velocity:

Motion Reference Frame	Approximate Speed ( $\text{km/s}$ )	Notes
Earth around Sun	$\sim 30$	Daily terrestrial motion.
Sun around Galactic Center	$\sim 220$	Galactic rotation.
Milky Way relative to CMB	$\sim 627$	Total velocity vector through the universe.
El Gordo Cluster	Specific velocity not easily generalized	Exhibits unusually high, complex motion relative to distant background.

This table clearly shows that the speed we measure when looking at the CMB—our velocity relative to the oldest light in the universe—is nearly 20 times faster than the speed at which we orbit our own star.

# Cosmic Inertia

The fact that our galaxy cluster is moving at hundreds of kilometers per second leads to a conceptual point about cosmic inertia. Because the scale of the universe is so immense, even at these colossal speeds, the distances covered in human timescales are almost imperceptible. The Local Group, as part of the larger cosmic web, is moving, but the gravitational forces shaping that movement operate over timescales vastly exceeding the age of our own planet. Think about the time it takes for the Milky Way to travel a distance equal to its own diameter (about $100,000 \text{ light-years}$ ) at its CMB velocity of $627 \text{ km/s}$ . While the calculation is complex due to the structure's rotation, at a steady $627 \text{ km/s}$ , it would take approximately 150 million years to cross its own width. This enormous time frame illustrates why the local motions, like Earth's orbit, feel so constant and unchanging to us, even though we are fundamentally propelled through space at an extraordinary pace.

The speed of our Local Group within the Virgo Supercluster, and the combined velocity toward features like the Great Attractor, dictates the eventual fate of our local neighborhood in the far future. We are not just moving; we are flowing downhill gravitationally toward the largest concentrations of mass we can observe, with El Gordo being another massive object experiencing its own, perhaps more turbulent, gravitational path. The velocity of our cluster, the Local Group, is thus the aggregate of these gravitational attractions, currently measured against the CMB as the $627 \text{ km/s}$ vector pointing in a specific direction.