Longitude of Ascending Node (LAN) Reference Direction?

[Fadamor]

In Career Mode I received a contract for a satellite to orbit Mun with a specific apoapsis (no problem), periapsis (no problem), inclination (no problem) and longitude of ascending node (no... wait, what?!). The name is descriptive enough that I had an idea about what it was, but I hit Wiki to make sure. Basically, if you overlaid a 360° compass card on the plane of reference (usually the equatorial plane of the body being orbited) but reversed the numbers so that they run COUNTER-clockwise, and referenced 000° to a specified reference direction, you can determine the LAN by reading where on the imaginary compass card your orbit crosses from below the plane of reference to above it. Easy peezy, lemon squeezy, right? Except, what is the direction I should be using for 000°? In our real-world celestial navigation, 000 for the purposes of measuring a LAN is the ray created by drawing a line from the Sun out through the point in Earth's orbit at the vernal equinox. In KSP... where is the 000° reference for LAN?

[Snark]

22 minutes ago, Fadamor said:

In KSP... where is the 000° reference for LAN?

Couldn't tell you off the top of my head, but the real answer is that it doesn't matter.

The reason that it doesn't matter is that it's a graphical thing.  You don't actually need to look at the numbers displayed on the contract at all, unless you particularly like to as a reference.

That's because the target orbit is graphically displayed in the map view.  Go into map view.  If you focus on the Mun-- or on any spacecraft that's within the Mun's SOI-- then you'll see the target orbit displayed in some bright color, with a little animated bright point moving round-and-round to show you which direction you need to be orbiting (pay attention to that-- a very common cause of "hey, how come my contract's not completing" questions from new players is that they got to the correct orbit but are going in the wrong direction).

So all you have to do is use the map view to guide you as you set up your encounter and you're all set.

[OhioBob]

As Snark said, it really doesn't matter as far as completing the contract goes.  I wouldn't even worry about when it comes to contracts.  But if you really want to know where zero longitude is just to satisfy your curiosity, it seems to be just some random direction.  Kerbin doesn't have a vernal equinox, so we can't use that, or anything else like it, for reference.  However, we can located 0 longitude using the positions of Kerbin and the Sun at specific times of year.  Kerbin is located at 180 degrees longitude on Day 1, 00:38:53 of every year, and is located at 0 longitude on Day 214, 00:55:05 of every year.  So whenever the clock reads "Day 1 - 0h39m", a line drawn from Kerbin to the Sun points in the direction of 0 longitude.  And when the clock reads "Day 214 - 0h55m", a line drawn from the Sun to the Kerbin points to 0 longitude.

(EDIT)

I have to make a correction.  The specific times that I cited apply only to Year 1.  If Kerbin's year consisted of an exact number of integer days, then those times would reoccur every year.  But since Kerbin's sidereal year contains a fractional day (1 year = 426.09 days), each new calendar year begins with Kerbin in a slightly different part of its orbit than the year before.  It take an extra 32.41 minutes each year to get back to its staring place.  So Kerbin doesn't reach 180 longitude on Year 2 until Day 1, 01:11:17, and the time continues to advance each year.

 

Edited September 11, 2019 by OhioBob

[Fadamor]

Hmm... never have seen a target orbit when I change focus, but then again I don't get too many contracts with specified orbits. I'll give that a try when I next get one of these overly specific contracts. Many thanks!

10 hours ago, OhioBob said:

I have to make a correction.  The specific times that I cited apply only to Year 1.  If Kerbin's year consisted of an exact number of integer days, then those times would reoccur every year.  But since Kerbin's sidereal year contains a fractional day (1 year = 426.09 days), each new calendar year begins with Kerbin in a slightly different part of its orbit than the year before.  It take an extra 32.41 minutes each year to get back to its staring place.  So Kerbin doesn't reach 180 longitude on Year 2 until Day 1, 01:11:17, and the time continues to advance each year.

 

Ours does the same thing. Our vernal equinox (when it was first defined) put our 000 line in the Aries constellation. Now I believe it is in Scorpio due to the precession of the Earth in the intervening millennia.

Edited September 12, 2019 by Fadamor

[pincushionman]

As the others have said, it's not really necessary to know the reference direction. But I know we're a curious lot here - and you're not the first to ask - so I put a tutorial together as an Imgur album to help you make an estimate of where the reference direction is:

https://imgur.com/gallery/7Ttn8Rz