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Imperial Interstellar Scout Service Details of the worlds of the Imperium (and beyond).

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Old September 10th, 2017, 03:03 AM
sudnadja sudnadja is offline
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Default Scout Service Basics: Equatorial, Ecliptic and Galactic Planes

In the "Real Stars" and other cartography threads, I have noted that there is occasional discussion about three fundamental planes used in (Terran) astronomy, and therefore is relevant to understanding the discussion. I've made a plot here to help illustrate that:




The plane that the TravellerMap is sitting on is the galactic plane. It is aligned with the spiral arms and the earth and the galactic core and the majority of the arms are coplanar with each other on this plane.

The purple circle represents the ecliptic plane. This is defined by the earth's orbit around the sun. The large purple point represents the position of earth in it's orbit at the time of this post (september 9) (keep in mind this is all scaled out by ~3 parsec for reference, just use the purple point as the relative place that earth is in it's orbit, not the cartesian position).

The orange circle represents the equatorial plane. If you take the earth's equator and project it into the sky that's the plane that you will get. The orange arrow in this case is the celestial pole - polaris is very close to the direction of that arrow.

The purple dot (earth) is right now very close to the point at which the ecliptic (purple) and equatorial (orange) planes meet. This defines the start of fall for those of us in the northern hemisphere. When the earth reaches the point where the two circles are furthest apart it will define the start of winter (~december 21 or 22) and then the start of spring is when the earth reaches the point where the planes intersect on the opposite side of the sun from where the earth is now.

Note that there is quite significant difference between the galactic plane and the equatorial and ecliptic planes.

This sort of thing is also important for those that play a somewhat more hard scifi game and the relative position of the planet to its parent star for computation of jump shadow or jump masking effects. Also, if you plot a sky map from other planets, for example if players are curious where their home star is in the night sky of a planet they're visiting, you can start to approach those tasks if you've properly prepared 6-element orbit parameters and state vectors (which I'll talk about later).

Last edited by aramis; September 11th, 2017 at 06:20 AM.. Reason: Img to IMGW
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