Jupiter Deflections


After the success of the total eclipse gravitational deflection measurements in 2017 and the failure of the daytime measurements ended in 2019, the next idea was to measure the deflections caused by Jupiter. The deflections should be only about 18 milliarcseconds, 100 times less than for the Sun. The advantage, however, is that the measurements can be made over a few hours, at night, from any location. This should allow the measurement noise to be averaged down to a few milliarcseconds, with a high probability of success. The gravitational deflections have been measured with radio telescopes, and the Gaia star catalog has been corrected for this effect, but the deflections have never been directly measured with an optical telescope.


However, Jupiter’s disk is bright enough to cause an apparent sloping background near the edges, due to optical scattering in Earth’s atmosphere and the telescope optics. This normally causes an error in the optical center measurement that needs careful corrections. This also means that a bright star is needed in the experiment, and this occurs only once every few years. In November 2021, a magnitude 6.0 star passes within 5 Jovian radii from Jupiter, and a magnitude 5.5 star passes even closer to Jupiter in August of 2023 (but not visible from San Diego). The next best event might be a magnitude 7.3 star pass in October of 2023.


If a narrow-band methane filter (at 890 nm) is used, this can darken Jupiter by a factor of ~20, compared to a star. This makes it possible to see much dimmer stars near Jupiter, perhaps allowing monthly measurements. The drawback is that the stellar signal is attenuated so much that exposures need to be more than 1 second long, reducing the number of measurements per minute. This tradeoff is still being investigated.


The other option might be to use the Galilean moons to measure the deflections. A second and third nearby moon is often in a good position about once per month to provide a good plate scale, so measurements might be relatively frequent. There are a few drawbacks, however; as the moon slowly rotates over the few hour measurement period, the surface features might shift the optical centroid measurement, corrupting the data. One way around this is to measure the moon’s position as it travels in front of the planet, and use that to estimate the albedo effect. Another complication is the phase of the moon will affect its optical position, but this should be pretty constant over a few hours. Another minor complication is that even NASA does not know the orbits of the moons better than a few milliarcseconds, so the relative positions need a small offset.


The biggest uncertainty, however, when the moons are used, is if a deflection is even theoretically possible.  Some theorists claim the moons are too close to Jupiter’s gravity well, and the deflections are miniscule.  Other theorists claim the deflections should occur at nearly the standard amount, and will be measureable. The primary driver of completing this difficult measurement is to see which theory is correct. I don’t expect to make much progress until the end of 2021, and expect better data in 2022, when Jupiter will be higher on the ecliptic and less subject to atmospheric turbulence as seen from San Diego. I’ve already selected a telescope with essentially zero optical distortion over a few hundred arcseconds, so that will simplify the analysis.  I am working with another advanced amateur astronomer, too, so I am not totally independent. After a summer of measurements in 2020, when Jupiter was always pretty low in the sky, it looks like the moon positions are measureable to better than 5 milliarcseconds, so the next few seasons look good. Additions and corrections to this experiment will be posted later in 2021.



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Web page last updated March 24, 2021.