# I Reference point of geocentric lunar latitude?

1. Jun 27, 2017

### jwberck

I'm attempting to use an algorithm translated from Jean Meeus: Astronomical Algorithms. The algorithm in question finds a geocentric position of the moon given a certain day. I have been able to find the geocentric longitude of the moon, but I don't know what this longitude is in reference to. What would 0 degrees of longitude point to? Would it be the vernal equinox point like heliocentric systems or something else?

Thanks!

EDIT: I meant longitude, not latitude

2. Jun 27, 2017

### stefan r

http://www.dictionary.com/browse/geocentric-longitude

Is that enough? I think it is geometry from there.

0 degrees longitude would probably point to an observatory in Greenwich England. If it is an old French book it might point to the observatory near Paris France. There was a big argument about this back in the day. The United States butted in and decided to side with the Brits in 1884.

3. Jun 27, 2017

### jwberck

My bad, I think I need to be more specific. I'm referring to an ecliptic coordinate system found here https://en.wikipedia.org/wiki/Ecliptic_coordinate_system . From this page I think 0 degrees would be the vernal exquinox. However, now I need to know the position of the earth in this system as well... anyone have an idea of how to calculate that? I can't find it in Astronomical Algorithms.

4. Jun 28, 2017

### Bandersnatch

Yes, that's the primary direction, just as with the heliocentric system. Only pointing the other way (from Earth to Sun, rather than Sun to Earth)
You won't be able to do that, since, true to its name, in the geocentric ecliptic coordinate system Earth is at the centre of the sphere of coordinates projected onto the sky. It's like asking what is the longitude of the north pole.
But I don't understand why would you need that for, so there's likely still some misunderstanding here.

5. Jun 28, 2017

### jwberck

I'll elaborate on what I'm working on to show you what the problem is. I'm building a software display that shows the actual position of the moon and earth in relation to the sun from a top down 2d perspective of the ecliptic plane. So I've found the position of the moon in relation to the earth using a geocentric ecliptic system and I know that the 0 degree point of this system points towards the vernal equinox. Now I need to figure out how to have the earth in the correct position using a heliocentric ecliptic coordinate system. Currently the top of the display represents January 1st UTC and makes one revolution in one year. I'm thinking the top needs to be the date of the march or September equinox for everything to be positioned realistically in relation to the sun assuming 0 degrees in the moons system points towards the top of the display. This is the sort of thing I'm building, but with actual positioning. http://bl.ocks.org/clayzermk1/9142407

6. Jun 29, 2017

### Bandersnatch

The position of the Earth during the (northern)vernal equinox in the heliocentric system is the opposite of the position of the Sun in the geocentric system at the same moment in time.
Whether you make that the top of the animation or somewhere else, it doesn't matter. You can still count from 1st Jan at 12 o'clock, and just make sure that the direction to the 0 point of the coordinate system you're using for placement of the Moon points towards the Sun on 20th March (or whatever it falls on).

So, if
means the 0 (=primary) direction of the geocentric system, and you want to keep the orientation, then the Earth should be placed at the bottom of the display (which would mark 20th March of this year), so that it correctly points towards the Sun during the northern vernal equinox.
You can then rotate the whole thing to make 12 o'clock wherever you want.

Or keep everything as it is, just change 12 o'clock to the autumn equinox instead of 1st Jan.

Last edited: Jun 29, 2017
7. Jun 30, 2017

### jwberck

I got it working correctly, thank you all for your help!