Since other planets have their own gravitational pull, I'm wondering if their effect on Earth's orbit has ever been measured? I know that in comparison with the Sun's pull the effect might be rather small, but I'm wondering if it's measurable...
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1$\begingroup$ To predict accurately the positions of any of the planets, including Earth (something NASA has to do quite well, in order to get space probes there) you have to account for the perturbations caused by the gravity of the other planets. $\endgroup$– antlersoftCommented Jun 24, 2019 at 19:30
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$\begingroup$ Sure! See, for example The role of Jupiter in driving Earth's orbital evolution: an update. We can even measure / calculate the effects of the planets on the Sun's position relative to the centre of the mass of the solar system, see the diagram at the top of this Wikipedia page. $\endgroup$– PM 2RingCommented Jun 24, 2019 at 19:32
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1$\begingroup$ astronomy.stackexchange.com/a/13491/21 shows the data/equations NASA uses to compute planetary positions, including that of the Earth and Moon. TLDR: they consider the gravitational attraction of thousands of objects on the Earth (and each other), including all of the planets and several asteroids. $\endgroup$– user21Commented Jun 25, 2019 at 2:03
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$\begingroup$ @PM2Ring I think the most interesting aspect to this question is that it asks if the effect has been measured, which is very different from the comments and answers about the effect being calculated. $\endgroup$– Chappo Hasn't ForgottenCommented Jun 25, 2019 at 23:24
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$\begingroup$ @Chappo Fair point. But measurement and calculation are intertwined to a degree. You can't do pure calculations, since you need measurements of orbital parameters. And you can't do pure measurements because all your reference points are moving. :) The geological evidence of long-term cycles mentioned in TopCat's answer are in one sense measurements, but we need to do calculations to extract the orbital data from them. Etc. $\endgroup$– PM 2RingCommented Jun 25, 2019 at 23:39
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2 Answers
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For decades, geologists have understood that climate changes are due in part to gradual shifts in the Earth’s orbit, which are caused primarily by Venus for its proximity and Jupiter for its mass, and repeat regularly every 405,000 years. In 2018 a team of geologists and Earth scientists unearthed the first evidence of these changes – sediments and rock core samples that provide a geological record of how and when these changes took place.
https://www.pnas.org/content/115/24/6153 "Empirical evidence for stability of the 405-kiloyear Jupiter–Venus eccentricity cycle over hundreds of millions of years"
So given the timescales involved, this is as close to measuring this effect as one could expect to get.
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Has the effect of other planets on Earth's orbit been ever measured?
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...but I'm wondering if it's measurable...
set up the question in an interesting way because it calls into question (intentionally or not) what kind of measurement would satisfy the question.
The motion of the Earth and other planets can be numerically reproduced with incredible accuracy, and this can only be done when taking into account a large number of planet-planet gravitational effects. Here's an example of a python script doing an approximate calculation and being compared to data from the Horizons web interface which uses data from the JPL Development ephemerides or DE's which can reproduce the motion of the inner planets to kilometers and outer planets to tens of kilometers.
After the Sun, Jupiter's effect is huge, but interaction between the inner planets is quite big as well, with Venus's and Earth's effects upon each other being quite important. Venus also has a big effect on Mercury as well.
In addition to affecting the inner planets directly, the massive outer planets Jupiter through Neptune move the Sun around by a million kilometers as well, and the inner planets will track the Sun's motion slow movements on the scale of decades or centuries.
But besides showing that simulations match measurements, I don't think there has been any way to perform an actual measurement of the effect of one Planet on another. So the answer to your question could be yes or no depending on how you would like to think of a measurement.
