[SJC: Achyuta Gurukul] Fwd: Parallax and topocentric positions

[Guest guest]

| Q: What is Parallax?

| ...

| When light rays starting from a point under the water reach the water

| surface while reaching us, they are "refracted" by an angle depending

| on the densities of the two media (water and air) on the two sides of

| the surface. When light leaves one medium and enters another, it

| bends. This refraction is the reason for pen appearing bent.

 

A clarification. The explanation given above for parallax is not

technically correct, although it is possible that the term 'parallax' is

also used in this context (change in apparent position because of

refraction)

 

m-w.com defines the word parallax as follows:

 

The apparent displacement of the difference in apparent direction of an

object as seen from two different points not on a straight line with the

object; _especially_: the angular difference in direction of a celestial

body as measured from two points on the earth's orbit.

 

--

 

Here are a couple of pages which explain this diagramatically:

 

http://zebu.uoregon.edu/~soper/Stars/parallax_ly.html

http://www.ast.cam.ac.uk/~mjp/parallax.html

 

Using this meaning, the term 'parallax' would mean the difference in

apparent position of the planets as seen from two different points on the

earth's surface against the "fixed" background of the zodiac. Note that

this error *can* be measured fairly accurately using our knowledge of the

earth's diameter.

 

So in essence the term parallax refers to the difference between the

geocentric position and the topocentric position. That is the difference

between the position as measured from the center of the earth (which is

the "true" position) and the position as seen from the surface of the

earth (which depends on where on the surface we are looking from).

 

Note that in addition to this parallax there is another error induced

because of refraction as explained by Guru Narasimha. This error can't be

accurately determined in advance since it depends on atmospheric

conditions, etc. as explained earlier.

 

I am a novice in astrology and as I said at the outset, it is possible

that the term 'parallax' is used to include the error due to refraction

also.

 

Vinod