system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do astronomers describe where things are in space when the Earth, Sun and whatever object they are observing are moving?
gpt
Yes, things are moving, but they are very, very, very far apart.  that means that there relative motion is only a small percentage of the distance.

Most measurements are made in the equatorial coordinate system called ascensio recta or right ascension. With declination, it makes a system like longitude and latitude.

The "equator" is the plane of the Earth's orbit around the Sun.  The Earth's north pole is inclined (giving the Earth seasons) but celestial North is the direction that's 90˚ from the Earth-Sun plane using the same right hand rules as we use on the planet.  Declination is the equivalent of "latitude", how far an object is above the Earth-Sun plane.

The "prime meridian" points toward the Sun at the vernal equinox.  Measured from there, right ascension is the "longitude".  It's usually measured in hours, where 00h = 24h.  Stars move up and down as they rise and fall, so this "time-like" angle measurement reflects when they are at the highest part in their arc using the Earth as a giant protractor.