[jfengel]

They use a Galactic Coordinate System, which is actually centered on the Sun, in the plane of the galaxy. (The sun does revolve around the galaxy, but with a period of millions of years it doesn't make a measurable difference.)

https://en.m.wikipedia.org/wiki/Galactic_coordinate_system

It can easily be transformed into telescope pointing directions with the input of location on the planet, time of year and time of day.

[euroderf]

IIRC the Earth has made 2.5 orbits about the galactic center since its [the Earth's] formation.

[dwaltrip]

The period is ~250 million years. Quite the journey! :)