What is Galilean relativity?

Galilean relativity is the principle that there is no absolute frame of reference for stillness. All non-accelerating, or inertial, frames are equally valid for describing the laws of physics.

What is the Barry Center of the solar system?

The Barry Center is the center of mass of all objects in the solar system. It's the true point around which everything, including the Sun, orbits, mainly influenced by the large planets like Jupiter and Saturn.

How fast is the Sun moving through the Milky Way?

The Sun moves at approximately 230 kilometers per second relative to the center of the Milky Way, completing a full orbit in about 230 million years.

What is the Local Standard of Rest (LSR)?

The LSR is a hypothetical reference frame representing a perfectly circular orbit at the Sun's current position. It helps astronomers measure the Sun's deviations from a simple circular path within the galaxy.

Why is the vertical oscillation of stars important?

The vertical oscillation of stars, including our Sun, as they move in and out of the galactic disk, could be linked to mass extinctions on Earth and is used to test models of dark matter.

How do we measure our absolute motion in the universe?

Our absolute motion can be measured against the Cosmic Microwave Background radiation, the afterglow of the Big Bang, which provides a universal rest frame.

Why is carbon better for life than silicon?

Carbon is superior because it forms stable, complex molecular chains and its compounds like carbon dioxide are easily cycled in biological processes, unlike the very stable silicon dioxide.

How Earth REALLY Moves Through the Galaxy

PBS Space Time

Education5 min read21 min video

Feb 8, 2023|3,198,107 views|65,420|5,239

Space Outer Space Physics Astrophysics Quantum Mechanics Space Physics PBS Space Time Time PBS Space Time Matt O’Dowd Einstein

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Key Moments

TL;DR

Earth's motion through the galaxy is complex, involving solar system movements, galactic orbits, and cosmic background reference.

Key Insights

The visual of planets in a helix is an oversimplification and implies a preferred reference frame, which Galilean relativity disproves.

Earth's motion is best described by choosing the most useful reference frame for the specific question, not a single 'absolute' motion.

Within the solar system, the barycenter, not the Sun, is the true center of orbit, causing slight variations in Earth's path.

The solar system orbits the Milky Way's center at ~230 km/s, taking ~230 million years for one orbit.

Earth's vertical oscillation relative to the galactic disk (~60 million year cycle) may influence mass extinctions and helps test dark matter models.

The Earth's motion relative to the Cosmic Microwave Background defines a universal rest frame, showing the Earth moves at ~368 km/s.

DEBUNKING THE HELIX AND GALILEAN RELATIVITY

Popular depictions of planets moving in a helix through space are misleading because they suggest a single, superior frame of reference. Galilean relativity, as described by Galileo, posits that there is no absolute rest frame; all non-accelerating (inertial) frames are equally valid for describing the laws of physics. Therefore, while the heliocentric model with orbiting planets is useful, it's not inherently 'truer' than viewing the solar system as tracing a helix. The choice of reference frame depends entirely on the question being asked and the scale of observation.

CHOOSING USEFUL REFERENCE FRAMES

While no single absolute reference frame exists, choosing the most useful one is crucial for understanding motion. For everyday tasks like catching a ball, treating the Earth's surface as stationary is effective. For space missions, NASA uses the solar system's barycenter as a reference. These choices allow for practical calculations and understanding within specific contexts, acknowledging that larger cosmic motions are often ignored for simplicity.

SOLAR SYSTEM DYNAMICS AND THE BARYCENTER

Within our solar system, the planets do not orbit a fixed Sun. Instead, they orbit the system's barycenter, its center of mass, which is influenced by the gravitational pull of all its objects, especially Jupiter and Saturn. This means the Sun itself moves in a complex pirouette. The barycenter's motion causes slight stretching and squishing of Earth's elliptical orbit over long timescales and also causes the orbit's orientation to rotate, a phenomenon linked to Milankovitch cycles and ice ages.

EARTH'S ORBIT AROUND THE MILKY WAY

The solar system, including Earth, orbits the center of the Milky Way galaxy at an estimated speed of 230 kilometers per second, completing one orbit in approximately 230 million years. This motion is incredibly complex due to the distributed gravitational field of the galaxy, unlike the Sun's dominance in our solar system. This orbital path isn't a simple ellipse; it's influenced by the gravitational tugs of other stars and galactic structures.

THE LOCAL STANDARD OF REST AND PECULIAR MOTION

To better understand the Sun's galactic motion, astronomers use the Local Standard of Rest (LSR) as a hypothetical circular orbit from the Sun's current position. Measurements reveal the Sun has a 'peculiar motion' relative to the LSR: it moves slightly faster than a circular orbit, drifts inward towards the galactic center, and moves upward out of the galactic disk. This deviation from a perfect circular orbit is due to gravitational kicks from other galactic objects over billions of years.

VERTICAL OSCILLATIONS AND GALACTIC STRUCTURE

The Sun's upward motion out of the galactic disk is part of a roughly 60-million-year oscillation. As it rises hundreds of light-years above the disk, gravity eventually pulls it back down. This vertical motion may be linked to mass extinctions on Earth, as the denser galactic center poses greater risks. Studying these stellar oscillations also helps test dark matter models by assessing the mass of the galactic disk and whether dark matter is concentrated there or more spread out.

EARTH'S VARYING GALACTIC SPEED

The planet's orbital plane, the ecliptic, is tilted relative to the solar system's galactic motion. This tilt causes Earth to spend half its orbit moving in the same direction as the Sun through the galaxy, increasing their combined speed, and the other half moving in the opposite direction, decreasing speed. This variation in Earth's galactic speed is a potential way to test dark matter models, as more dark matter particles should theoretically interact with detectors when Earth is moving faster.

THE COSMIC MICROWAVE BACKGROUND AS A UNIVERSAL REST FRAME

The ultimate reference frame is provided by the Cosmic Microwave Background (CMB). This ancient light, emitted shortly after the Big Bang, reveals the universe's state of expansion and average motion. By measuring the Doppler shift in the CMB light, scientists have determined that our local group of galaxies is moving at approximately 368 km/s relative to this universal rest frame, influenced by vast structures like the Great Attractor and other galaxy overdensities.

EPICYCLIC MOTION AND THE FLOWER PATTERN

The combined effects of the solar system's inward drift and its orbital path around the galaxy result in a complex trajectory. Instead of a simple ellipse, the solar system traces a flower-like pattern, known as epicyclic motion. This occurs because the Milky Way's mass is spread out, preventing perfectly elliptical orbits and creating this intricate, multi-dimensional path as the solar system moves through and around the galactic disk over vast periods.

THE ECLIPTIC PLANE'S ROTATION AND THE ROLLING WHEEL

The ecliptic plane, where the planets orbit, does not remain fixed relative to the solar system's galactic motion due to the conservation of angular momentum. Over hundreds of millions of years, this plane will rotate. This transforms the visual representation of the solar system's motion from a corkscrew to that of a wheel rolling through space, with the ecliptic plane becoming edge-on relative to the sun's orbital direction. This motion is crucial for understanding our complete path.

Mentioned in This Episode

●Organizations

●Books

●Concepts

●People Referenced

Common Questions

Earth's motion is complex, involving its orbit around the Sun (which itself orbits the galaxy's center), and the Milky Way's movement through the universe, influenced by other galaxies and cosmic structures.

Topics

Galilean Relativity Frames Of Reference Barycenter Solar System Dynamics Milky Way Galaxy Local Standard Of Rest Galactic Motion Universal Motion Silicon-based Life Carbon-based Life

Mentioned in this video

conceptGreat Attractor

A mysterious over-density in the clustering of galaxies that is pulling our local group, including the Milky Way, at high velocity.

conceptcarbon dioxide

A molecule that is relatively easily broken down, allowing carbon to be recirculated, making carbon a more favorable element for life.

concepttrilobites

Ancient marine arthropods mentioned to illustrate the ancient past when Earth was on the other side of the galaxy, just after their extinction.

conceptsilicon dioxide

A stable molecule that is difficult to break down, making silicon a less plausible basis for life compared to carbon.

bookDialogue Concerning the Two Chief World Systems

This 1632 book by Galileo Galilei is referenced as the source of the thought experiment on Galilean relativity.

conceptnitrogen

An element considered as a potential basis for life, but its limitation is that it does not form long molecular chains with itself.

bookDragon's Egg

A science fiction work by Robert Forward, referenced for its concept of nuclear life on neutron stars as an alternative to chemical life.

conceptsilicon

An element discussed as a potential basis for life, but deemed less suitable than carbon due to the stability of silicon dioxide and its inability to form complex molecular chains.

conceptAndromeda galaxy