Here's the rest of my response to James Phillips, from
his comment:
“Is it true that N.A.S.A. uses the geocentric model rather than the heliocentric model and if so what is their rationale for doing so.“
This is a very poorly worded question. The short answer is, “Of course they do - where appropriate.”
They use longitude, latitude, and altitude when it is the convenient unit of measure - and they use these quantities on the Earth, Moon, Mars, etc. Why would they not? Why would you reference everything with heliocentric coordinates if you are in orbit around the Earth? Or in orbit around Mars? The heliocentric coordinates are just a coordinate transformation away from any other coordinate system you choose to use.
You use the model appropriate to the scale of the problem you are solving. A geocentric model can be sufficiently accurate near the Earth, but deviates as one moves further away from the Earth. When traveling between planets, NASA routinely transitions between the frame of the Earth, to the heliocentric frame, and to the frame of the target planet when the spacecraft gets near.
NASA also has datasets where the Earth is treated as FLAT. I'm sure the Flat Earthers will regard that as vindication for their interpretation of Scripture (
Wikipedia,
Flat Earth Society).
Near Earth
For near Earth trajectories, the coordinate system of choice is GEI (Geocentric Earth Inertial) which is fixed with respect to the distant stars. This is the one you use to compute your trajectories as Newton's laws and gravity apply in their simplest form.
If you want to know where your satellite is visible from the surface of the Earth, you use GEO (Geocentric body-fixed) which rotates with respect to GEI. It is simple to convert between these two systems (unless you want to include nutation), just a rotation around the z-axis that completes one rotation in a sidereal day. This coordinate system is often used for transferring data to and from tracking stations. Other coordinate systems I've used in my day job are described here:
Coordinate systems and transformations,
GEOPHYSICAL COORDINATE TRANSFORMATIONS.
There are similar planetocentric coordinate systems used for close flybys and orbits by spacecraft.
Since there are a number of spacecraft at Mars, which coordinate system do you think they use for tracking spacecraft? See
MSL Update to Mars Coordinate Frame Definitions (2006), pg 6:
“When a spacecraft is in the vicinity of Mars, it is convenient to utilize Mars-centered coordinate systems. These are systems that are centered at the center of the planet itself, as opposed to the system barycenter or on the planet surface. The systems described here are utilized regularly by the flight operations and mission planning teams for JPL Mars missions.”
Beyond the Earth
If you want to compute trajectories farther from the Earth, say to go to Venus, or Mars, or beyond, you compute the trajectories in a heliocentric system (or more accurately a heliocentric barycenter system), again because the laws of motion and gravity apply in their simplest form, which means you can compute future (or past) positions more accurately. There are two methods for doing this.
- Compute planetary positions as a full N-body simulation (Scholarpedia).
- There are algorithms which start with a reference elliptical orbit (heliocentric barycenter) and then compute how the gravitational forces from the other planets perturb that orbit. These perturbations show up as slow variations in the orbital parameters (called Secular variations of the planetary orbits, or VSOP). From that, you install your spacecraft position and velocity, subject to the same laws of motion and gravity. Once you know the position of your spacecraft in the heliocentric frame, you can compute where the object would appear from any other location by using a coordinate transformation, such as those described above. You would convert to the GEO system if you wanted to know where to point an Earth-based antenna to send commands to your spacecraft or receive data.
Check out
JPL: Basics of Space Flight: Interplanetary Trajectories. The web tool
JPL: Solar System Dynamics: HORIZONS Web-Interface computes trajectories of many solar system objects and can transform the positions as they would appear at other locations in the solar system. The FAQ on the Horizons system also has info on computing planetary positions which I have utilized.
Orbital dynamics is so precise, we can compute trajectories decades before an actual launch. We can compute if existing boosters have the capability to send a spacecraft onto a given trajectory. If we need a new booster to handle more distance, higher speed, or more payload, we can compute those requirements before we cut a single piece of metal to build it. We don't build the biggest rocket we can, fuel it up, and hope it makes it to the destination. How is that done? (For those who want to bring up the Pioneer Anomaly (
Wikipedia), it is looking more and more like this is not new physics, but a very tiny thrust created by
emission of heat from the spacecraft).
Strange Way to Run a Cover-up...
This is not just a NASA thing. ESA, Japan, India, China and other countries are sending spacecraft to other planets. Are they part of the coverup as well? All the data and mathematics for computing interplanetary trajectories are a matter of PUBLIC record. Many of these techniques were developed over 100 years before NASA even existed. Amateur astronomers who understand the math can do these computations on their desktop computers to far higher accuracy than those researchers from the 1700s to the 1950s who developed the techniques via hand calculation, slide-rule, and adding machine. Today, this is a project of college undergraduates (see
Interplanetary Trajectory Development). I ran simple solar system models on an N-body code I wrote on an Apple II(
Wikipedia) back in 1980 while an undergraduate.
With so many people who have the knowledge of how to do this, it's a strange way to run a 'conspiracy' against Geocentrism (
Moving-World DECEPTION).
The Real Conspiracy?
Perhaps the more interesting question would be what computations are the supporters of Geocentrism
Galileo was Wrong using when they do their graphics? How are they computing the position of, say Jupiter, in the sky on a given date and to what accuracy? Can they compute when the
ISS will pass over my location? Are their graphics constructed using software where the computations are in a heliocentric system or are they doing the computations themselves in a geocentric system?
If they're doing the computations themselves, why don't they show their work so that others can use (and test) them as well?
Here's the NASA info on trajectory and navigation for spacecraft:
Here's some work by people OUTSIDE of NASA
Some months ago,
Rick DeLano claimed that Geocentrism could explain the Lagrange points, five points of stability in the restricted 3-body problem (
Wikipedia), and one of the predictions of Newtonian gravity and laws of motion.
I challenged him to demonstrate it, considering that we make use of these locations in a number of operating space missions.
The Geocentrists have been strangely silent on this.
If you taught Geocentrism in a physics class, how would you use this knowledge to plan spacecraft missions?
Here's a
syllabus of an astrodynamics class at Georgia Tech. This is training for people to really do this work. I wonder if any of the Geocentrists could do the homework problems posted here. How would they answer the practical problems of interplanetary navigation in a Geocentric model? As yet, Geocentrists have not demonstrated any competence in this field where they claim so much knowledge. Unless you believe all spaceflight is a hoax (or you chicken out and just claim everything beyond Earth orbit is a hoax), your only other choice with Geocentrism is to terminate all space flight, leaving space travel to other countries less entrenched in dogma.
These are not idle questions of only philosophical interest. Billions of dollars in space assets, the lives of astronauts, even national security, rely on doing this stuff right. Would you trust these things to those who have not demonstrated any competence in the topic?
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