I am just beginning to try and understand things like solar day, sidereal day, apsides, aphelion, perihelion, vernal equinox, autumn equinox, winter solstice, summer solstice, celestial sphere, and a host of other terms associated with basic astronomy.

 

In particular, right now I am trying to understand "celestial meridian."

 

They spoke about zenith, and nadir. But none of the sites I went to was clear on these terms. Suppose that you are located somewhere on the equator. Send a line through your location and the center of the earth.

 

Is it necessarily true that where the line meets the celestial sphere above you, "zenith" and where the line below you "nadir" meets the "celestial sphere", that those two points must lie on the "celestial meridian"?

 

So that someone standing somewhere else on earth, would have a different celestial meridian?

 

Did I understand that right?

 

Thank you

The celestial meridian is just your line of longitude, projected above you. It intersects with the earth's axis (extend the N and S poles) and also includes a point that's directly above you.

The celestial meridian is just your line of longitude, projected above you. It intersects with the earth's axis (extend the N and S poles) and also includes a point that's directly above you.

 

... your line of longitude projected above you...

 

...the "celestial meridian" is just your line of longitude projected above you...

 

your line of longitude, is a meridian around the earth.

 

Suppose you are in Greenwich England.

 

Your latitude is 51 degrees north, and your longitude is zero, because you are on the earth's prime meridian.

 

Prime meridian is a great circle that wraps around the the earth. It passes through both earth's north pole, and earth's south pole.

 

Now I want to project this great circle onto the celestial sphere?

 

I think so...

 

You said that there are three points which are on the "celestial meridian"

 

The north pole of earth... projected up of course onto the sphere, and the south pole of earth down, and to fix it in place, the point directly above your head... which is called the zenith if I'm not mistaken.

 

Did I get all that correctly?

 

Thanks

Yes.

Yes.

 

Don't you mean that the celestial meridian is your line of latitude projected onto the stars? You said longitude.

 

Regards

Don't you mean that the celestial meridian is your line of latitude projected onto the stars? You said longitude.

 

Regards

 

No, I meant longitude. It's the "finish line" that the sun has to cross to measure solar noon in my example. The sun moves east-west, so the line it crosses has to be north-south.

 

Think of the circle of longitude, and make its radius larger. That circle is now the celestial meridian. All the celestial sphere is, is a mapping of earth coordinates onto a larger sphere.

No' date=' I meant longitude... The sun moves east-west, so the line it crosses has to be north-south.

 

[/quote']

 

So earthbound observers, with different longitudes, will define different "celstial meridians" simultaneously?

 

Thank you

So earthbound observers' date=' with different longitudes, will define different "celstial meridians" simultaneously?

 

Thank you[/quote']

 

Yes. The sun could only be overhead for one of them at any given time, if they have different longitudes.

Yes. The sun could only be overhead for one of them at any given time, if they have different longitudes.

 

Ok I got it, thank you very much Dr Swanson.

 

Quick question, do you know about Euler angles?

Quick question, do you know about Euler angles?

 

I used to. I know what they are, but haven't done any rotation calculations in a long time.

I used to. I know what they are, but haven't done any rotation calculations in a long time.

 

Well I'm going to learn about them now. Perhaps I could help you this time.

 

Kind regards