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When I need to talk about an orbital element that is not osculating and that it is the true/actual element at a given instant, I use the term "actual": actual perigee, actual eccentricity, actual semi-major axis, ... I mean that that perigee is not the osculating perigee, but it is the smallest radius vector reached by a given satellite at a given time.

Is there any official terminology for what I call "actual"?

Cristiano
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  • Actual? What does that actually mean to you? – David Hammen Dec 29 '19 at 17:21
  • @David Hammen "I mean that that perigee is not the osculating perigee, but it is the smallest radius vector reached by a given satellite at a given time." – Cristiano Dec 29 '19 at 17:23
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    And how are you going to determine that, along with eccentricity, semi-major axis, ...? You need to realize that orbital elements are a fiction, and fiction and "actual" do not go hand in hand. – David Hammen Dec 29 '19 at 17:28
  • After I found the actual apogee Rmax and the actual perigee Rmin, actual eccentricity= (Rmax - Rmin) / (Rmax + Rmin). The actual semi-major axis is the average radius vector in 1 orbit. – Cristiano Dec 29 '19 at 17:34
  • The argument of periapsis, for example, tells where in an orbit a body is closest to whatever it's orbiting. However, unless the body actually IS at perigee, you can't compute this value directly. See https://astronomy.stackexchange.com/questions/28691/why-is-neptune-true-anomaly-decreasing for possible help –  Dec 29 '19 at 18:35
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    The only way you can truly find the actual periapsis is to observe it at the time it happens. At that time, you can predict (i.e., make an educated guess regarding) when / where apoapsis will occur. At any point in time, you can predict (i.e., make an educated guess regarding) when / where the next periapsis and apoapsis will occur. But they are ultimately educated guesses -- i.e., not "actual". – David Hammen Dec 29 '19 at 18:44
  • @DavidHammen Good! My question is: is there any official term for the periapsis that it is observed at the time it happens? That's what I call "actual periapsis". I'm not talking about prediction. – Cristiano Dec 30 '19 at 10:18
  • @barrycarter Here's the procedure that I use to calculate what I call "actual argument of perigee" to plot the graphs in my site: "The calculation starts from the ascending node found as explained in the previous graph, then the satellite is propagated forward until the perigee is found. The [actual] AoP is calculated as the angular distance between the ascending node and the perigee in the direction of motion on the orbital plane."; Is there any official term for that AoP? The problem is that I need to be sure that that AoP should not be confused with the osculating AoP. – Cristiano Dec 30 '19 at 10:51
  • The angle between the ascending node and periapsis point is the longitude of perigee rather than the argument of perigee. And unless you're using some unknown magic, the value you are calculating is not "actual". It's an estimate. – David Hammen Dec 30 '19 at 14:46
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    @Cristiano The problem: suppose the position of the perigee changes for the next orbit? Do you just increment it suddenly when that happens? Perhaps you could say "orbital elements for a specific orbit" or something, meaning you've observed them for one specific orbit. –  Dec 30 '19 at 16:15
  • @DavidHammen did you post that link just to show that you don't know what the argument of the periapsis is? https://en.wikipedia.org/wiki/Argument_of_periapsis – Cristiano Dec 30 '19 at 17:03
  • @barrycarter I totally agree with your "orbital elements for a specific orbit", for example, an orbit calculated with a specific TLE + SGP4. It seems that it's a good practice to always say what an orbital element is. Thank you. – Cristiano Dec 30 '19 at 17:07
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    @Cristiano - It was a brain fart. I know full well what argument of perigee is. Dealing with low Earth orbit and all of the weird idiosyncrasies that result has been part of my job for 30+ years. – David Hammen Dec 30 '19 at 19:49
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    And I would never consider calling anything that results from using TLE+SGP4 "actual". – David Hammen Dec 30 '19 at 19:50

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