# Naked Science Forum

## Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: JennyGracie on 06/02/2019 14:59:38

Title: How are locations in space described?
Post by: JennyGracie on 06/02/2019 14:59:38
Ron wants to know,

On Earth it is simple to describe a location, but with our planet rotating, and spinning around the sun, how are locations in ”space” described?

Title: Re: How are locations in space described?
Post by: Halc on 06/02/2019 16:16:00
Quote
On Earth it is simple to describe a location, but with our planet rotating, and spinning around the sun, how are locations in ”space” described?
One requires a reference frame and an origin to specify a location.
A reference frame can be inertial or accelerating, and either can be rotating or not.
So a paper map of Paris assumes a rotating (and accelerating) reference frame, without which the map would be meaningless.
In that frame, from the origin of say Paris, Jupiter might be located at direction A (two coordinates) in the sky and at distance X (one more coordinate).  This defines Jupiter's location in 3D space.
One could give the position in non-rotating terms relative to the sun or to the center of the galaxy, with three similar coordinates, and with a couple references to identify where we assign direction '0'.  One of them is probably relative to the solar system's (or galaxy's) plane of the ecliptic.

All of these are examples of accelerating reference frames since they're relative to objects, and I cannot think of any object that is not accelerating.  I can think of frames which are not accelerating, but absent any obvious objects that are stationary in that frame, there seems to be no useful place to put the origin of the coordinate system.  Hence absolute coordinates are almost never used when describing the positions of things.
Title: Re: How are locations in space described?
Post by: Janus on 06/02/2019 16:57:20
There are a number of coordinate systems involved.     For visual reference from the Earth we use the Celestial sphere, which gives the visual position as seen from the Earth.
We use Ecliptic system to describe objects' positions in reference to the Earth's orbit.  This is referenced to the Vernal equinox( where earth is in its orbit at that day of the year) and the plane of the ecliptic ( Earth's orbit)
We can use the galactic coordinate system to locate other stars positions with respect to our Sun.  This uses the galactic plane and center of the galaxy as reference. (We don't worry about the Earth motion around the Sun because, compared to the distances between stars, it is insignificant.)

A planet's heliocentric (sun-centered) position is based on its orbital elements.  This also uses the Earth's orbit and the vernal equinox.  The true anomaly is the angle the planet in its orbit it has traveled from perihelion (its closest approach to the sun). This, and the Longitude of the perihelion ( the position its perihelion relative to the vernal equinox).  Its radial distance is its distance from the sun. It will also be a certain distance above or below the ecliptic, which can be determined from the inclination of its orbit (how much the planet's orbit is tipped relative to the Earth's) and the longitude of the ascending node ( one of the two points in its orbit where it orbital plane crosses that of the Earth.)  True anomaly is constantly changing as the planet travels in its orbit, so it is time sensitive.   Longitude of perihelion and ascending node also slowly drift over time. How much you have to take this into account depends on how far into the future or past you are trying to determine the planet's position.

Which system you use depends on your needs.  If you want to just find a star or planet in the night sky,  you use the celestial sphere.

If you want to know a planet's position in the Solar system for any given time, you use orbital elements.

If you want to know a star's relative position in the galaxy, you use galactic coordinates.