Thursday, 2 October 2014

Vitruvious's analemma (part 2).

Part 1.
  • Line EI is the horizon.
  • line NC is at the angle of the equator, therefore 90 degrees to line QP.
  • QP is at the angle of latitude for your location, measured from the horizon line EI.
  • AGD and ADH are at an angle equal to the obliquity of the ecliptic, drawn either side of the equator line NC.
  • When lines are drawn to represent declination AGD and ADH, solstice (maximum and minimum) shadow lengths are indicated by T and R (C is the equinoctial shadow length).

Part 2 

Join the ends of the ecliptic lines: point G with L for summer solstice and points R and K for winter. A semi circle is drawn.

Points S and V:
Are where the lines LG (summer) and KH (winter) cross the horizon.

S  extended to edge of solstice semi-circle shows summer solstice sun rise and sun set angle.

V, extended in the same way, shows winter solstice sun rise and set angle.

The angle of sun rise and sun set is the same.
Not the azimuth.

The angle between sun rise at an azimuth of 49, and east (90 degrees) is 41 degrees.
The angle between sun set at an azimuth of 311 degrees and west (270 degrees) is 41 degrees.

The division of time.
The sun moves approximately 15 degrees in one hour, therefore according to modern practice, the division of the analemma into hours is pretty easy.

Mark the 12, 15 degree divisions around the outer edge of the semi-circles,

Time is also divided into seasonal hours- see previous post.
This is why point S and V are drawn on the analemma.

Place protractor on M and measure angle LMD.
Divide this into six, and mark on analemma between D and L instead of the 12 hours between G and L.

Be careful not to use S as the point of angle measurement. S is used to find D.
M is where the latitude line crosses the the line LG.

Part 3
Construction of the summer solstice shadow track, to follow.

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