Since the discovery of James Bradley in 1727 it is well known that the apparent position of a star viewed from the Earth depends on the Earth's velocity. In astronomy, this is called aberration (also referred to as astronomical aberration or stellar aberration). Aberration is a phenomenon which produces apparent shifts of the directions of celestial objects dependent on the velocity of the observer. It causes objects to appear to be displaced towards the direction of motion of the observer compared to when the observer is stationary.
If we define our reference frame as being at rest relative to about 1.2 million quasars, many of them billions of lightyears away from the Milky Way, the objects seen by Gaia experience aberration due to Gaia’s motion around the Sun (or more exactly the barycentre of our Solar System), by the motion of the Solar System around the Galactic centre and by the motion of our Milky Way relative to the quasar reference frame.
After taking into account the aberration effect from the Gaia motion around the Sun, the residual aberration for each source is a large shift that changes linearly with time due to the acceleration of the Solar System relative to the rest-frame of the quasars. This effect was first discussed by John Pond in 1833. The slow linear change of the residual aberration produces a systematic pattern in the measured proper motion of quasars on the sky.
With the high quality of the astrometric data of the Early Data Release 3 (EDR3), the systematic pattern was successfully detected with the data gathered within three years. In the EDR3 science demonstration paper "Gaia Early Data Release 3: Acceleration of the solar system from Gaia astrometry" it was determined that the amplitude of the pattern is (5.05+-0.35) microarcsecond per year which corresponds to the acceleration of (2.32+-0:16) 10-10 m/s2 (or 7.33+-0.51 km/s/Myr) towards the point with coordinates (alpha = 269.1+-5.4 deg, delta = -31.6+-4.1 deg), within a few degrees from the Galactic Centre. This is in general agreement with the theoretical expectations from the supposed nearly expected nearly circular orbit of the Solar System with the Galaxy. The apparent proper motions thus show that the acceleration of our Solar System is pointing roughly towards the center our Milky Way, as expected.
The systematic proper motion field of Quasi-Stellar Objects (QSOs) induced by the measured acceleration is also visualised with the Gaia Sky videos. The videos start showing the positions of 3000 quasars (not the real ones observed by Gaia but quasars with positions randomly distributed over the sky). Then the proper motion vectors are shown and finally the motion is shown 2.5e16 times faster than in reality. The quasars appear to move closer towards the direction of acceleration, close to the Galactic centre.
The accumulation of the quasars towards the Galactic center which is visible in the video is purely due to the extreme exaggeration of the effect and the fact that the direction to the Galactic center (which in reality moves around the sky during 250 million years) is kept constant in the movies. This is an unavoidable consequence of the desire to make the aberration effect easily visible.
Released under CC BY-SA 3.0 IGO
Acknowledgement: Made by Stefan Jordan and Toni Sagristà with Gaia Sky (http://www.zah.uni-heidelberg.de/gaia/outreach/gaiasky)
Based on paper by Gaia Collaboration, Klioner et al., 2020, "Gaia Early Data Release 3 Acceleration of the solar system from Gaia astrometry", Astronomy and Astrophysics, in press.
Page last updated: 03 December 2020