Imagine you are in a place where everything around you is moving. If somebody asks you to locate yourself, how would you define your position? This will not be easy; you will need a stationary point (a reference), so that you can define your position with respect to it. Thankfully, we have the ability to do this. Today, with modern techniques, we can detect even the millimeter per-year motions of mountains. But it begs the question: in a universe where nothing is stationary how can this accuracy be possible? While developing radio astronomy, scientists decided that quasars are the best, most stationary reference points in the universe. Today, as we use our hand-held GPS devices, our position is ultimately connected to the position of the quasars located at the far edges of the universe.
Positioning is a branch of geodesy which deals with the figure and gravity field of Earth, as well as the accurate positing of an object on the surface of the Earth. In geodesy, two different reference systems are used: the terrestrial reference system (TRS) and the celestial reference system (CRS). TRS uses the center of the Earth as the origin, rotation axis, and latitudes/ longitudes. TRS rotates with the Earth, so the position of a point changes over time.
The celestial reference system uses the barycenter (center of the solar system) as the origin. The axis of the celestial reference frame is defined by the north celestial pole (the direction of the Earth’s rotational axis) and the intersection of the equator with the ecliptic (vernal equinox). CRS is the ultimate reference frame used to define the position of any location on the surface of the Earth. By its definition, CRS is an inertial reference system, and has no depth parameter (i.e., all objects in the sky are considered on a single sphere called the celestial sphere). Eventually, all points in TRS have to be tied to CRS. In order to make the transition from TRS to CRS, the dynamics of the Earth’s motion, including precession, nutation, and polar motion, must be determined very accurately. Such determinations are within the scope of modern geodesy.
Each reference system uses certain benchmarks. For TRS, before the era of satellites, certain locations on the surface of the Earth were used as benchmarks, and they had to be assumed stationary. Until 1983, the benchmark in the US was located in Kansas. Not surprisingly, modern geodesy has experienced a boost with the use of satellites. Today, the center of the Earth can be pinpointed due to the motions of satellites.
The realization of CRS has historically been made by stars. For this purpose, star catalogs (a collection of references for the positions of the stars), which were initially compiled by Hipparchus (135 BC) and later revisited by Ptolemy (and later by Al-Biruni), are used. Historically, astrolabes were used to define the positions of stars. Thanks to such research, it has been long known that stars in our galaxy have motions along a certain direction, called “proper motion.” The magnitude of this motion is in the order of hundreds of mili-arc-seconds (mas), which is about a millionth of a degree per second. If stars are used as reference points, these proper motions have to be corrected for accurate positioning.
Astronomers use the principle of redshift to estimate the distance of faraway objects, including stars. In the 1950s, quasars attracted the attention of scientists due to their huge redshifts, indicating that they come from very far-off parts of the universe. Despite being billions of light years away, their incredible luminosity (brightness) allows their light to reach Earth. Quasars are the brightest objects in space. They form around massive black holes at the centers of galaxies. These black holes cause an “accretion” disc around the quasars; this accreted matter is diverted at both ends of the black hole, like jets of matter. It is this matter that we observe here on Earth.
Despite the great amount of matter being produced, how can quasars be seen from backyard telescopes? One quasar is a trillion times brighter than our sun. Quasars are so big, in fact, they can be the size of our entire solar system. This is why their light reaches us, even though some quasars are 10-15 billion light years away (“closer” quasars are “only” 2 billion light years away). To imagine this distance, think that light from sun comes to us in only 8 minutes!
Why are quasars so effective for determining location? Well, stars within our galaxy move relatively quickly. Determining our location using these stars would be like trying to locate your ship in the sea using other ships. Instead, you must use a fixed point on the shoreline. Quasars are like these stationary objects on shore – and, thankfully, they’re scattered in all directions of the celestial sphere.
With developments in astronomy, scientists have discovered quasars that do not have any detectable proper motion. As a result of this, since 1993, quasars have been used as the ultimate reference frame (Celestial Reference Frame) for positioning on Earth. Their proper motion is 0.01 mas/a, which is limited by the Hubble’s constant.
Our position with respect to quasars is made by continuously observing them from the surface of the Earth, using a network of radio satellite dishes. These satellite dishes are scattered around the planet and they use a method called “very long baseline interferometry” (VLBI). VLBI satellites are eyes with very long baselines, and use triangulation (just as our eyes do) to locate a point in the sky with exceptional accuracy. Using these points, scientists can determine changes in the Earth’s orientation.
It might seem strange that stars billions of light years away enable scientists to properly position any point on Earth. But this is just another example of the universe’s perfect order, and the fact that everything has a purpose.
- M. Feissel & F. Mignard, The adoption of ICRS on 1 Januray 1998: meaning and consequences, Astronomy and Astrophysics, 331, L33-36 (1998).