Deconstructing Bradley

Simon, I have a question concerning Ch. 22.

max elongation

In this diagram, what does MAX. elongation mean?

I think it is refering to the angle that was recorded in September?

Dear Schoepffer,

Here’s an extract from “The Aberration of Starlight” - by Dr. David P. Stern (2006) :

“For instance Polaris, the pole star, seemed to travel annually around an ellipse whose width was 40”, 40 seconds of arc. As discussed in the section on parallax, that might suggest that the distance to Polaris was 1/40 of a parsec or 0.1 light year. However, the shifts in position did not occur at the times they were expected. The greatest shift of Polaris in any given direction occurred not when the Earth’s was at the opposite end of its orbit, as it should have been, but 3 months later. (…) The apparent position of Polaris should have been shifted the furthest in the direction of ‘December’ when Earth was in its ‘June’ position, which is far as it can go in the opposite direction. Instead, it happened in September, when the Earth had moved 90° from its position in June.

As you can see in my above graphic, this is just what would be expected in the TYCHOS : the greatest shift of the position of Polaris will occur between June and September (points 2 and 3 in my graphic) since, under this period, an earthly observer’s elongation from Polaris will reach its maximum (in the course of a full year of observation).

Thank you Simon. I am not familar with astronomical jargon and this might benefit others as well.

Elongation may also refer to the angular distance of any celestial body from another around which it revolves or from a particular point in the sky. Britannica

It would be interesting to know what method astronomers in the 19th century used to determine said elongation.

With my very limited knowledge of land navigation, I would speculate that position 1 and 2’s azimuth were 180 degrees apart with a very similar altitude*.

Position 3’s azimuth would be interesting to know. But the altitude must have been < the first two.

*The following are two independent horizontal angular coordinates:

  • Altitude (alt.), sometimes referred to as elevation (el.) or apparent height, is the angle between the object and the observer’s local horizon. For visible objects, it is an angle between 0° and 90°.[b]
  • Azimuth (az.) is the angle of the object around the horizon, usually measured from true north and increasing eastward. Exceptions are, for example, ESO’s FITS convention where it is measured from the south and increasing westward, or the FITS convention of the Sloan Digital Sky Survey where it is measured from the south and increasing eastward.

Schoepffer… This is good revision for all but the very brightest. I hope you do more of it…kind of like The Tychos 101 :smile:

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