For most of human history, we relied on the stars to navigate. By knowing the time and day and looking to the polar regions where some stars seemed to be stationary, we could determine cardinal directions. Today, we navigate using satellite data, which is used not only for maps and traffic apps, but also for countless other applications, from photography to banking. We still rely on celestial navigation—but the stars aren’t enough. We need supermassive black holes.
Astronomers believe that there is a supermassive black hole at the center of every galaxy. Most of the time these black holes are quiet, but when they start actively feeding, such as when a large amount of gas falls on them, they can become extremely bright because the gas is subjected to incredible forces. In some cases they enter a quasar phase – and this is useful.
On Earth, there is the Greenwich Meridian, where longitude begins. In space, we use quasars (…) and that is how we determine our coordinates.
Dr Christopher S. Jacobs
Quasars are extremely bright and have a point-like light source. They are also far enough away that their proper motions do not affect measurements. They appear to be stationary in the sky. They also emit light in different wavelength ranges, so they can be tracked using radio waves, visible light, and more. The position of these quasars is used in the celestial reference frame.
“What is the importance of the celestial reference frame? It is how we know where we are in space. On Earth, we have the Greenwich meridian, where longitude begins. In space, we use quasars, these galaxies that emit a beam of radiostatic radiation, and that is how we determine our coordinates,” Dr. Christopher S. Jacobs of NASA’s Jet Propulsion Laboratory told IFLScience.
The celestial reference system is important for the Global Positioning System (GPS) because the Earth is not a perfect sphere that rotates on its axis and around the Sun in perfect periodic motion. To compensate for the small deviations caused by the Earth’s imperfections, the GPS is calibrated using the celestial reference system. This is ideally done daily.
It is estimated that it takes weeks for accuracy to drop to a level that is noticeable in everyday life. If you are just trying to find your way around a new city, this might be fine, but if you need higher precision, such as with ships or spacecraft, this can be a big problem.
“The whole GPS system can drift around in the long term if it doesn’t have a fixed point to stabilize at. So when you navigate using GPS, your phone is really linked to the quasars. Banks use it to measure time. If they want to send a large sum of money, they want to know where the money is at a certain point in time. If you’re navigating at sea, you need to know where you are,” Dr Jacobs told IFLScience.
“I work with interplanetary probes that fly to Mars and other planets. And beyond Earth there is no GPS. And that’s where quasars and the structure of the sky come into play.”
Jacobs led astrometry sessions in the 32nd.and General Assembly of the International Astronomical Union in Cape Town. The work presented includes research led by Professor Patrick Charlot to improve the celestial reference system.
“We can improve (the celestial reference system) in several ways. Improving it means measuring the position of the sources more accurately. Improving it also means having more sources and distributing them better in the sky. We also need a better coverage comparison, for example with the optical frame that Gaia has built,” Professor Charlot told IFLScience.
Some of the challenges are technical. For example, the southern hemisphere is not as evenly covered as the northern one, or there are many more sources in the optical wavelength range, so deeper radio observations are needed to have radio counterparts. The GaiaNIR mission, a planned follow-on mission to the Gaia Observatory, would reveal quasars in the plane of the Milky Way that are currently obscured by the dust of our galaxy.
There are also scientific considerations. Assumptions about the movement of the solar system around the Milky Way and the Milky Way itself may become factors influencing the high precision that this reference system is intended to achieve.
In addition, quasars are thought to be point sources, although they are not actually point sources. As our telescopes improve, these sources appear more extended. Therefore, it is important to find out where the radio emission is coming from, and this is one of the uncertainties in the physics of supermassive black holes.
If the celestial frame of reference not only helps us navigate our modern world and study the complexity of the cosmos, it also helps us study the inner workings of our planets. The movement of tectonic plates and other causes of our planet’s erratic movements are brought to light thanks to the light of supermassive black holes, which shine 25 trillion times brighter than the Sun from billions of light years away.
