• For 100s of years – millenia, really – Earth was

thought to be motionless at the center of the universe

• Everything else up there seemed to move around it

• It makes perfect sense that it doesn’t move…

• But it does move!

• It moves through space, in different ways and in

different directions all at the same time

• And we move right along with it, just as though

we were on a spaceship

Spaceship Earth

In fact, we are on…

Rotation around its axis

Speed depends on distance from axis of rotation

Revolution around the Sun

So we’re spinning around and revolving around the Sun

Motion relative to nearby stars

• Stars in our part of the Milky Way are moving in ~random directions relative to us (as are we relative to them)

• The average speed is ~70,000 km/hr (~40,000 mph)

• We don't perceive this motion because of distance

• This is why constellations don’t change much over time

• But, given enough time…

They do move

Take for example the Big Dipper

100,000 BCE – 100,000 CE

http://www.astronomy.ohio-state.edu/~pogge/Ast162/Movies/proper.html

https://www.halcyonmaps.com/constellations-throughout-the-ages/

This is “proper motion”,

the apparent

movement of stars

through the sky

It is a combination of

the stars’ motions and

the motion of our solar

system

Motion relative to nearby stars

• So we’re spinning around, revolving around, and moving through the local solar neighborhood along with other stars

Motion around the center of the galaxy

• Our local solar neighborhood moves around the center of the Milky Way

• The Very Long Baseline Array of radio telescopes was used to find the speed

• The VLBA uses a technique called “interferometry”

Motion around the center of the galaxy

• The VLBA measured Doppler shifts in the radio waves coming from a

collection of “radio hotspots” in the galaxy

• The hotspots are the blue and green dots on the map of the Milky Way

(based on infrared measurements by the Spitzer Space Telescope)

Motion around the center of the galaxy

• The result: We are moving at about 914,000 km/hr ≈ 600,000 mi/hr around

the center of the Milky Way

• So we’re spinning around, orbiting the Sun, moving through the local

neighborhood, and revolving around the center of the Milky Way

Motion within the Local Group of galaxies

• In the Local Group, the Andromeda Galaxy is moving toward the Milky Way

at more than 300,000 km/hr

Motion within the Local Group of galaxies

©John DubinskiUniversity of Toronto

Department of Astronomy & Astrophysics

Motion within the Local Group of galaxies

©Matthias SteinmetzAstrophysikalisches Institut Potsdam

• What do you think will happen when the galaxies collide?

a) Many collisions will occur between Andromeda stars and Milky Way

stars.

b) Very few, if any, collisions will occur between Andromeda stars and

Milky Way stars.

• What do you think will happen when the galaxies collide?

a) Many collisions will occur between Andromeda stars and Milky Way

stars.

b) Very few, if any, collisions will occur between Andromeda stars and

Milky Way stars.

Motion within the Local Group of galaxies

We can see real galaxies in the act of colliding

Motion due to the expansion of the universe

Motion due to the expansion of the universe

• Earth is moving relative to other galaxies because the universe itself is expanding

Motion due to the expansion of the universe

• Earth is moving relative to other galaxies because the universe itself is expanding

• But it's not expanding into space

Motion due to the expansion of the universe

• Earth is moving relative to other galaxies because the universe itself is expanding

• But it's not expanding into space

• Space itself is expanding!

Motion due to the expansion of the universe

Motion due to the expansion of the universe