Astronomy: Orbit Terminology

[Pixelworks]

I am looking for the terms that describe, or rather differentiates the difference between the direction of orbit and the 'anti-orbit' direction.

If my understanding is correct, the terms retrograde and prograde motion both refer to the motion but only in regard to the body being orbited. E.g. the Earth's moon has a retrograde orbit around the Earth, but only because it orbits opposite the direction of the Earth's rotation. If for example the Earth would rotate in the opposite direction the same direction of orbit would be considered prograde. Correct?

If this is the case, what is the determining factor for satellites orbiting a body that does not rotate (i.e. a satellite orbiting the Earth's moon)?

The use-case that I am looking for is to describe the direction a spaceship is taking if it achieves escape velocity to leave a lunar orbit either in the moon's orbit direction or in the anti-orbit direction. If the terms retrograde and prograde are established and applied to the ship's heading (based on the body it is orbiting) then this terminology is still variable and could change depending on the direction of rotation. That is why if I am wondering if there are two other terms that are used specifically to indicate the direction of orbit and the anti-orbit direction.

Thank you for any insights.

 

[.Scott]

In this article, NASA refers to the leading part of ISS as the "forward end". So, I would guess the other end would be the "rearward" end.

 

[DaveC426913]

In this article, NASA refers to the leading part of ISS as the "forward end". So, I would guess the other end would be the "rearward" end.

Aft-end or stern.

 

[DaveC426913]

E.g. the Earth's moon has a retrograde orbit around the Earth, but only because it orbits opposite the direction of the Earth's rotation.

Moon revolves and rotates in the same direction as the Earth's rotation.

It just happens that the Earth rotates faster than the Moon revolves.

 

[.Scott]

I didn't address the term "retrograde" as it pertains to celestial motion.
The strictly scientific definition is provided in this wiki article.

However, the original astronomical meaning (from the 14th century) and still the more common meaning, is used to describe an apparent westward motion of a planet.

 

[.Scott]

Aft-end or stern.

I wouldn't use "stern" for this. "Stern" describes something that functions as a stern - not just its position. If you put a boat into orbit, its stern could be either the forward end or the aft end.

 

[Pixelworks]

Moon revolves and rotates in the same direction as the Earth's rotation.

Sorry. Yes, you are absolutely correct... that example was a Brain-fart on my side. :) In a moment of 'unconcentration', I forgot that a moon's zero rotation relative to the host planet (aka tidal-locked) does not mean that the body doesn't rotate.

But the question remains the same, how is the prograde or retrograde direction determined if the body in question is not rotating?

 

[Pixelworks]

I think maybe I explained my question incorrectly or perhaps the terms that I am looking for simply do not exist in the form that I am looking for. Let me try a different formulation...

First off, I do not mean the front or back of a spaceship itself, but rather the terms for the ship's heading when it leaves orbit. Regardless whether a ship is orbiting a body in a prograde or retrograde direction, it can leave orbit either in the direction of the planet's orbit or against the planet's orbit (i.e. leaving orbit in the backwards direction opposite the direction that the planet is traveling).

Of the eight planets in our Sol system, all except two (Venus & Uranus) rotate counterclockwise. That means if we retain the usage of prograde and retrograde for this definition of direction, then leaving the Earth's orbit in a prograde direction would be the retrograde direction for the same direction when leaving a Venus orbit (as seen from above the Sol system (zenith) looking down.)

Which is why I am wondering if there is a term for leaving a celestial body in its direction of its orbit or the anti-orbit direction. Where the term is based on the celestial body's orbit direction and not on its rotation direction. Similar to a solar system, the galaxy is also spinning. At least at this level, I have seen references that traveling in the direction that the galaxy is turning is called 'spinward' and the opposite direction aptly classified as 'anti-spinward'.

Aside from that, there is also a polar orbits (and orbit from north to south, or vice-versa) that I would assume are classified as neither prograde nor retrograde. I do not know if it has ever been done or even if there is a practical purpose for doing so, but I would assume that this would be called either the zenith or nadir direction, depending on if the ship leaves the planet in a northward or southward direction.

 

[Bandersnatch]

You probably want the term 'posigrade', but you'd likely need to add some qualifiers to make it clear which orbit you mean. The opposite is also retrograde.

 

[Pixelworks]

posigrade

Had to look it up to be sure, but YES, that is the term that fits. It is a shame that retrograde is used for both cases though, so yes, that direction would need clarification when used.

Thank you!

 

[DaveC426913]

I realize this is not what you are asking but there are some pretty strong constraints on orbits and leaving them.

Assuming it lifted off from Earth in the first place, it will almost surely have a prograde orbit, since that takes advantage of the 1000mph boost of Earth's orbit. To then leave orbit retrograde, it will have to kill its 18,000mph prograde velocity and reverse it.

Likewise, leaving the galaxy anti-spinward would require killing all of its prograde 230km/s velocity and reversing it for a total delta-v of ~460km/s.

(And if you have that much fuel available to make that produce kind of delta-v then you essentially don't need to worry about orbits at all, you can just go in any direction you desire.)


So with realistic fuel and delta-v constraints, the only time it's practical or functional is if you have arrive at the planet in a retrograde orbit (say, Venus). Then you might as well leave in the same direction.

 

[Bandersnatch]

Assuming it lifted off from Earth in the first place, it will almost surely have a prograde orbit, since that takes advantage of the 1000mph boost of Earth's orbit. To then leave orbit retrograde, it will have to kill its 18,000mph prograde velocity and reverse it.

As I read it, the OP doesn't want to reverse the orbit, just to identify a direction. As in e.g. a spaceship making a burn along or against the instantaneous orbital velocity vector when performing a Hohmann transfer. It would burn posigrade when raising the orbit, or retrograde to lower it.

I realize this is not what you are asking

but then again you may well know that, and are just musing.

 

[DaveC426913]

As I read it, the OP doesn't want to reverse the orbit, just to identify a direction.

Well, he sure seems to say so, if I read it corectly:

it can leave orbit either in the direction of the planet's orbit or against the planet's orbit

 

[Filip Larsen]

For ISS and similar orbiting targets that a chaser might want to approach and rendezvous with, there is also a fairly well-established terminology using R-bar, V-bar and H-bar to designate, relative to the target, the direction towards Earth, the forward velocity direction (perpendicular to R-bar) and angular momentum axis (perpendicular to R- and V-bar). Due to the quirkiness of (relative) orbital dynamics these directions are important for approach and departure procedures.