# Understanding Longitude of ascending node (orbit orientation)

I'm trying to understand how the longitude of the ascending node "works" for the planets in our solar system.

The NASA fact sheet easily gives you the angles for the "longitude of the ascending node". Let's take Saturn for example, with an angle of 113 degrees.

From what I understand from the book "Fundamentals of Astrodynamics" is that this is 113 degrees counter-clockwise from the point of reference (the vernal equinox direction).

Then I looked at this page, which has a really good explanation for the ascending node: http://astronomy.swin.edu.au/cosmos/A/Ascending+Node

Then I tried to apply that knowledge in the image of Saturn using a "top view" and a "side view" (http://calgary.rasc.ca/orbits.htm):

In this image, they have highlighted that part of the orbit of Saturn that is above the ecliptic plane, using a bright white line.

Now, correct me if I'm wrong, but isn't the vernal equinox represented by the horizontal line pointed towards the east? When I then take that 113 degrees counter-clockwise in the "top view", I get to the point where the planet reaches above the ecliptic. So far so good.

BUT, if I look at the "side view", that point seems to be the point of the descending node, not the point of the ascending node...

## 1 Answer

You are probably confusing front and back in the side view. The way the orbital plane of Saturn is oriented in the side view we are looking at it from below. So the bottom line is in the back and the top line is in the front, that is, Saturn is going clockwise in the side view. Ascending node on the left (back), descending node on the right (front). See it now?

• Yes, you are right. I also built a simulation and in that one, the planet is going the other way in the side view. This probably caused my confusion. Mar 26, 2017 at 7:14
• Can you verify that the east is the "first point of aries"? Thanks! Mar 26, 2017 at 7:14