# Does prime meridian on earth line up with vernal equinox?

#### ncc1701d

##### Member
This is probably really easy question but I am just learning this stuff so dont laugh. but...

What earth longitude degree line directly faces and lines up center to Vernal Equinox vector? I am guessing the prime meridian 0 degrees longitude on earth lines up with Vernal Equinox every year which is 0 hour on the celestial sphere? But this is a guess. My readings on the web so far kind of skip that connection and explanation possibly infering but not confirming it for me.
or I could just be totally off.
thanks

#### george7378

##### DON'T PANIC
Donator
I think the Vernal Equinox is a fixed point in space, and because the Earth rotates relative to the stars, there is no one point that aligns with it. It's like saying 'which longitude line is aligned with Sirius' - there isn't one because the Earth rotates.

#### ncc1701d

##### Member
From what I understand yes the vernal equinox is a fixed point. And when the earth spins and goes around the sun, their is one point aprox around March 22 (Spring Vernal Equinox) every year when the earth rotates to a position when the suns rays hit directly perpendicular to the earth equator. The other time that happen is the Fall Equinox. I am assuming that the same side of earth faces perpendicular to the sun every year at that time...therefore there would, I would think, be a corresponding earth longitude that faces that Vernal Equinox direction.

I think in this picture
https://en.wikipedia.org/wiki/File:Ecliptic_path.jpg
the yellow arrow is pointing at that fixed point in space.

the caption reading looks like it says "When the Sun seems to pass through the vernal equinox (longitude 0°), the longitude of the Earth itself is 180° longitude" so maybe that is my answer. 180 degrees longitude would be my answer which would be mostly atlantic ocean pointing twords the Vernal Equinox direction. Do you agree?

#### Fizyk

##### Member
ncc1701d said:
I am assuming that the same side of earth faces perpendicular to the sun every year at that time...
Nope. That would mean that there is an integer number of days in a year, and this is not true.

#### Quick_Nick

##### Passed the Turing Test
Donator
Based on this time table, it looks like the equinox is about 5 hours and 50 minutes "later" each year. (But on the same calendar day) So that's about a quarter turn of the Earth different. (I.e. nearly 90 degree change in longitude each year)

[ame="http://en.wikipedia.org/wiki/Equinox"]Equinox - Wikipedia, the free encyclopedia[/ame]

Last edited:

#### csanders

Based on this time table, it looks like the equinox is about 5 hours and 50 minutes "later" each year. (But on the same calendar day) So that's about a quarter turn of the Earth different. (I.e. nearly 90 degree change in longitude each year)

http://en.wikipedia.org/wiki/Equinox

Yep, since there is X + .25 days in a year, the same side (give or take a little) should align every 4 years (4X + 1 day).

Fun fact: X = 365

#### ncc1701d

##### Member
how do celestial orbs handle this that you buy in the store?
I would imagine they are not as accurate as the real stuff but I would think given thier physical limitations some aligments between earth and the celestial globe would start to repeat themselves and I am looking for that pattern.
I am trying to make my own celestial globe but dont want to pay 200$for one to look at but need a starting point for positioning earth longitude lines with the celestial spheres prime maridian. Any ideas? csanders: I think you may have answered me. If I put earth long 180 degree line facing vernal equinox it should come back to that postion every 4 years...(at least in the celestial globe manufuctures world) unless someone has one to check and differs. #### Quick_Nick ##### Passed the Turing Test Donator how do celestial orbs handle this that you buy in the store? I would imagine they are not as accurate as the real stuff but I would think given thier physical limitations some aligments between earth and the celestial globe would start to repeat themselves and I am looking for that pattern. I am trying to make my own celestial globe but dont want to pay 200$ for one to look at but need a starting point for positioning earth longitude lines with the celestial spheres prime maridian. Any ideas?

csanders: I think you may have answered me.
If I put earth long 180 degree line facing vernal equinox it should come back to that postion every 4 years...(at least in the celestial globe manufuctures world) unless someone has one to check and differs.

Well, your accuracy would be far less than degrees that way. (I think)
From what I understand from Wikipedia and from csanders' wording, the same 'SIDE' of Earth points in the equinox direction each four years, but it seems far from the same POINT.
I don't have a full understanding of what you're trying to do, but I'm sure epochs will factor in somewhere. In other words, you'll have to make adjustments every year or two for one reason or another. (probably many reasons compounded, which may be difficult to calculate for yourself)

Others will be here shortly to clean up my nonsense.

#### QM2SS

##### New member
This is probably really easy question but I am just learning this stuff so dont laugh. but...

What earth longitude degree line directly faces and lines up center to Vernal Equinox vector? I am guessing the prime meridian 0 degrees longitude on earth lines up with Vernal Equinox every year which is 0 hour on the celestial sphere? But this is a guess. My readings on the web so far kind of skip that connection and explanation possibly infering but not confirming it for me.
or I could just be totally off.
thanks
I understand your frustration. I have a brother who is a surveyor and he looks at Geo positions much differently than I. In fact navigation has been stepping farther away from the Art Celestial Navigation . With the move to Omega, Loran C, Radar and Satellite Navigation and finally GPS that's gives us automatic updates on electronic charts any where any time. Few people concern themselves with pub 229 the Nautical Almanac or Bowditch . It's like trying to interest a kid in HAM Radio so he can talk to friends around the world. But I think people should know something about the Science and mechanics of the world.
The idea that you can fix your position with a Sextant, Sounding or land formation gives me a fuzzy feeling at sea. Where anything that can go wrong, Will go wrong. Give me paper charts a sharp pencil a set of Chonometers ,compass and dividers and a ruler . And my sacred Pubs.
But I digress the PM and CM and FPA do line up where the Equator and Ecliptic intersect. That point is projected into space and onto the Celestial Sphere that we for all intent and purpose st at the center. Practical Navigation is Practical not necessarily the way astronomy is teached in Cosmos Class. For instances it does me no good to know that one degree of arc is 69 statue miles. When a nautical mile works much better with 60 NM. Nice round numbers. Like the Universe.

#### QM2SS

##### New member
Your hunch is correct 0° or Greenwich meridian, Ecliptic and Equator are in line is what marks the first point of Aries. Which means that the FPOA is always a bit different. That's why Aries is no longer at the point due to the procession. The Celestial sphere is a projection on our own terrestrial sphere. It is the angle between the meridian of your Assumed Position and the meridian of the geographical position of the celestial body. All stars have a geographic position. Its how we are able to predict the stars we want to shoot at Nautical Twilight. They are like lights on very high lighthouses.

#### Voyager_VI

##### Sailor Tim
Excellent explanation QM2SS. I came into during Omega and retired as GPS & DGPS moved to the forefront of navigation. Paper charts were replaced with electrons & now my beloved Bowditch comes as a pdf file.

QMCS ret.

#### Arvil

##### Well-known member
ut a lazy-susan on a table and rotate it very slowly, like once a year. Then place on the susan a gyroscope and spin it, like once a day. One end of the table is the vernal equinox. Now rotate the table once every 26000 years. A mark on the gyroscope corresponding to the prime meridian does not necessarily line up with the vernal equinox on the appointed day, it'll be almost a quarter turn past where it was a year ago when it reaches the equnox. For another planet it could be anywhere. So, one does not equate the prime meridian and the equinox. And the equinox will be 1/26000 of the way around each year. So, the gyroscope rotation are days, the lazy-susan rotation is years, and the table rotation is precession.