# Measuring the Movement of the Stars

• ScientificMind
In summary, in astronomy, the speed of celestial bodies is always measured relative to a reference frame. Typically, the reference frame is either the Earth or the "local standard of rest" which is the Sun and its closest neighborhood. The rotation of the Milky Way can then be deducted to get the galactic reference frame, which is useful for describing interactions with other galaxies. The reference frame of the Local Group of galaxies or the cosmic microwave background radiation can also be used. To measure the velocity of Earth's motion, the contributions of its rotation, revolution, and the velocity of the Sun around the galactic center are deducted from the velocity relative to the CMBR. Similarly, to measure the velocity of a celestial body, its rotation and the

#### ScientificMind

I was wondering, since the Earth is constantly moving around the sun, the sun is moving around the milky way, and the milky way is moving through space (and for all I know the milky way might also be rotating around our local group, cluster, super cluster, etc.), how do we know how fast anything is moving? For example, it seems like if two objects are moving away from one another at say 2 m/s, from the point of view of either one it would look like it was standing still while the other was moving away at 4 m/s.

Yes, by definition, speed is measured between any two objects and either can self-label as the one which is "standing still" (unless there is acceleration). You may be struggling under the misconception that there is a universal rest frame against which a "true" speed can be measured.

russ_watters said:
Yes, by definition, speed is measured between any two objects and either can self-label as the one which is "standing still" (unless there is acceleration). You may be struggling under the misconception that there is a universal rest frame against which a "true" speed can be measured.
I suppose that I didn't think of it that way, thank you. Though that raises another question for me, what do we do instead to measure the motion of celestial bodies? That is, do we measure them using the Earth as "standing still," do we simply measure them in a general sense of motion without bothering to give them exact speeds, or is it something completely different?

ScientificMind said:
I suppose that I didn't think of it that way, thank you. Though that raises another question for me, what do we do instead to measure the motion of celestial bodies? That is, do we measure them using the Earth as "standing still," do we simply measure them in a general sense of motion without bothering to give them exact speeds, or is it something completely different?
There's always a reference frame specified or implied when talking about velocities in astronomy. All observations are by necessity conducted from the reference frame of Earth, so you'll always get a speed w/r to Earth as the basis.
When the daily and yearly motions of our planet are deducted from it, we get motion w/r to the so called "local standard of rest" (LSR), that is w/r to the Sun and its closest neighbourhood. This is a good reference frame to describe motion of nearby stars.
You can then deduct the rotation of Milky Way to get the galactic reference frame. This one is useful for describing interactions with other galaxies.
You can talk about the reference frame of the centre of mass of the Local Group of galaxies if you deduct the relative motion of MW with respect to it.
You can also talk about the frame of reference of the cosmic microwave background radiation (CMBR) if you deduct all the other contributions. This frame of reference is used by most cosmological models to describe the global behaviour of the universe.

So, let's say you measure the divergence from homogenity of the CMBR - you look at the sky, notice that CMBR is blue-shifted in one direction and redshifted in the opposite one, and using Doppler shift you calculate the velocity of Earth relative to CMBR.

Then you may deduct the velocity of Earth's rotation, which will amount to some 0.5km/s (fluctuating daily), the velocity of Earth's revolution (30km/s fluctuating yearly), the velocity of Sun around the galactic centre (iirc ~240km/s fluctuating every 250 million years, so practically constant from our human perspective), the velocity of MW moving towards Andromeda Galaxy (110km/s) and you end up with some ~650km/s velocity of the Local Group w/r to the CMBR.

Another example, you look at the Andromeda galaxy, use doppler shift to find its approach velocity as seen from Earth, deduct the quickly changing velocity of rotation and revolution of the planet, and you end up with the velocity w/r to the Sun of about 300km/s (the "helio-radial velocity" in the wikipedia article on Andromeda). Then you may deduct the galactic orbital motion of the Sun to conclude that the actual speed of approach between the two galaxies is the 110km/s mentioned earlier.

## What is the purpose of measuring the movement of stars?

The purpose of measuring the movement of stars is to understand the dynamics and behavior of celestial objects. This can provide valuable information about the formation and evolution of the universe.

## How do scientists measure the movement of stars?

Scientists use a variety of techniques to measure the movement of stars, including spectroscopy, astrometry, and interferometry. Each method involves analyzing the light emitted by stars to determine their velocity and direction of movement.

## Why is it important to measure the movement of stars?

Measuring the movement of stars can help scientists map out the structure of the universe, track the motion of galaxies, and identify potential hazards such as near-Earth objects. It also allows for a deeper understanding of the laws of physics and the fundamental forces that govern the universe.

## What tools are used to measure the movement of stars?

Scientists use a variety of tools to measure the movement of stars, including telescopes, spectrographs, and specialized software for data analysis. In addition, space-based observatories such as the Hubble Space Telescope and the Gaia satellite have greatly advanced our ability to measure the movement of stars.

## Can the movement of stars be accurately predicted?

The movement of stars can be predicted to a certain degree of accuracy using mathematical models and computer simulations. However, due to the complex nature of the universe, there will always be uncertainties and unexpected variations in the movement of stars.