From Gerry Gilmore (2018) Gaia: 3-dimensional census of the Milky Way Galaxy

4.4 Fundamental physics

Relativistic effects are highly significant for Gaia measurement accuracy, with tests of General Relativity being a significant driver from the very start of the project. This established tight constraints on the mission. For example, sufficient modelling of Newtonian aberration requires that the spacecraft orbit (Lissajous orbit around L2) is quantified with a velocity accuracy of 1 mm/s. Finite light velocity effects lead to position-dependent propagation delays in the field of view which must be accounted for. Monopole light deflection (the famous 1.75arcsec solar limb effect first verified by Eddington & Dyson in 1919) exceed the microarcsec level all-sky for the Sun, and up to 90 deg from Jupiter, significantly complicating the computational effort. Quadrupole light bending is 240µas at the Jupiter limb, and is 1µas at 8 Jupiter radii. This allows a special Gaia experiment – to quantify light bending by Jupiter, this test involving an oblate rotating mass moving in a deeper (Solar) potential.

The Gaia mission (also here):

Another possible experiment is to explore light bending of star images close to the limb of Jupiter to measure the quadrupole moment of the gravitational field of the giant planet.


  1. Is looking so close to something as bright as Jupiter and still doing precision astrometry using GAIA possible? Has this been tried? Did it work?
  2. Did they see "quadrupole light bending due to an "oblate rotating mass moving in a deeper (Solar) potential"?
  3. GR tells us that the light will appear deflected near Jupiter and Jupiter's mass distribution is oblate because it rotates fast, so the deflection will be expected to have a quadrupole moment. But is any of that deflection due directly to the rotation, or would it be about the same for a static, non-rotating but oblate Jupiter? How far down the GR rabbit hole does this go?

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