British Astronomical Association
Supporting amateur astronomers since 1890

Secondary menu

Main menu

Home Forums General Discussion
Terms of use

Lagrange Points: Where on the sky are they?

5 posts / 0 new
Last post
Grant Privett's picture
Offline
Last seen: 4 days 8 min ago
Joined: 28/12/2014 - 18:30
Lagrange Points: Where on the sky are they?

Recently, I was reading a paper in the MNRAS and the author casually referred to pointing their telescope toward the Sun-Earth L5 point. Now, I imagine working out where that is in the sky involves offsetting from the antisolar point by a fixed (slightly varying as our orbit is elliptical) number of degrees along the ecliptic, with the L4 being on the other side of the antisolar point. I looked at The SkyX and some other bits of astronomical software I have knocking about (including AstroPy) but have found none that will tell you the RA/Dec of the L4/5 points from a given location at a given time.

Does anyone know - offhand - what that offset is for the Sun-Earth L4/5 please? Alternatively, can you point me at some software that would?

I can only imagine that finding the Earth-Moon Lagrange points must be much harder.

admin_dcf's picture
Offline
Last seen: 4 days 4 min ago
Joined: 10/01/2013 - 12:42
60 degrees?

L4 and L5 are almost exactly 60 degrees away from the Sun [*], along the plane of the ecliptic, I think?

I say "almost" because as you point out, the Earth's elliptical orbit will presumably perturb that angle slightly over the course of the year, though I imagine the perturbation is tiny.

[*] -- My original forum post was incorrect, so I have edited the wording.

Grant Privett's picture
Offline
Last seen: 4 days 8 min ago
Joined: 28/12/2014 - 18:30
Perspective

Is that 60 degrees as viewed from the sun or the earth?

admin_dcf's picture
Offline
Last seen: 4 days 4 min ago
Joined: 10/01/2013 - 12:42
All the angles are 60 degrees!

A correction to my previous post. Seen from the Sun, L4/5 are both 60 degrees away from the Earth. Seen from the Earth, L4/5 are both 60 degrees away from the Sun.

The Sun, Earth, and L4/5 form equilateral triangles, with all three interior angles being 60 degrees, and all the sides being 1 AU.

Grant Privett's picture
Offline
Last seen: 4 days 8 min ago
Joined: 28/12/2014 - 18:30
Brilliant

Brilliant. Really hadnt realised you just had to look 60 degrees in front or following the sun. That makes it pretty easy, get the sun's position on ecliptic in RA/Dec, convert to ecliptic cooords, add 60 degrees and convert back to RA/Dec again. Sure I can find something in AstroPy to do most of that. I imagine that drops it nicely into the edge of the Zodiacal light.  

Thanks again.