The short answer: very careful observation and math.
The mathematical equations for planetary motion were worked out through careful plotting of planetary positions over many years (check out Tycho Brahe). Galielo, Kepler, Copernicus all had a hand in clarifying how things were supposed to move, and how the sun, not the earth, was the thing we call revolve around. It took some time for that basic knowledge to settle down. Then, in 1687, Newton comes along, and describes how planets move in regular ellipses under the influence of gravity and all heck breaks loose.
Out of this math we discover something pretty powerful: if you can capture a handful of careful observations about where a planet is (positions and very precise timings), you can completely predict where a planet is going to be at any time in the future and, this is pretty crazy, you can run the math backward to figure out where it was at any date in the past that you like. The system is very predictable.
So predictable in fact, that in 1846, when we saw the position of Uranus not quite matching our math, we didn’t question gravity, but instead we asked “how big of a thing would it take to pull Uranus out of it’s predicted path, and where would that thing need to be in order to have that influence?” We used our same gravity math to figure out that there needed to be a new planet at a specific place in the sky, which we found pretty much right away and thats how we found Neptune. Neptune was literally discovered on paper before we ever ID’d it as a planet through a telescope.
There’s no way you can get the Voyager spacecraft out to the gas giants without knowing this math beforehand.
We have been watching the position of both jupiter and saturn in the sky since antiquity, with neptune being the last discovered in the mid 1800’s using telescopes. This was more than enough to know their location and orbit path.
Optics, combined with careful angle measurements, gave us a very good idea of what we were working with.
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You can track their movement relative to the earth, sun, each other and then model their orbits from there
The short answer: very careful observation and math.
The mathematical equations for planetary motion were worked out through careful plotting of planetary positions over many years (check out Tycho Brahe). Galielo, Kepler, Copernicus all had a hand in clarifying how things were supposed to move, and how the sun, not the earth, was the thing we call revolve around. It took some time for that basic knowledge to settle down. Then, in 1687, Newton comes along, and describes how planets move in regular ellipses under the influence of gravity and all heck breaks loose.
Out of this math we discover something pretty powerful: if you can capture a handful of careful observations about where a planet is (positions and very precise timings), you can completely predict where a planet is going to be at any time in the future and, this is pretty crazy, you can run the math backward to figure out where it was at any date in the past that you like. The system is very predictable.
So predictable in fact, that in 1846, when we saw the position of Uranus not quite matching our math, we didn’t question gravity, but instead we asked “how big of a thing would it take to pull Uranus out of it’s predicted path, and where would that thing need to be in order to have that influence?” We used our same gravity math to figure out that there needed to be a new planet at a specific place in the sky, which we found pretty much right away and thats how we found Neptune. Neptune was literally discovered on paper before we ever ID’d it as a planet through a telescope.
There’s no way you can get the Voyager spacecraft out to the gas giants without knowing this math beforehand.
Does that help?
We have been watching the position of both jupiter and saturn in the sky since antiquity, with neptune being the last discovered in the mid 1800’s using telescopes. This was more than enough to know their location and orbit path.
Optics, combined with careful angle measurements, gave us a very good idea of what we were working with.