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 8 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. This event has several
very nice features that make it the best, and maybe only, chance to measure
deflections. Click on this link
for a detailed proposal.
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.
Thank you for your interest in Stellar Products!
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Web page last updated January 17, 2022.