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Success! Near sunset today I used a variation of the method suggested by
Modris: 1) set sextant to calculated angle between Sun center and Venus,
2) set horizon shades to view Sun, 3) acquire Sun through horizon glass,
4) rotate sextant frame from vertical to the predicted inclination angle
while keeping Sun in sight, 5) oscillate the inclination angle a little
until you see Venus moving perpendicular to the split in the horizon
glass and passing through Sun center, 6) rotate sextant frame to
vertical while keeping Venus in sight, 7) retract horizon shades, 8)
adjust index arm to bring Venus and horizon into coincidence.

Step 7 was the most difficult. Even with a 3.5x telescope, at first I
had trouble holding Venus in view while rotating the shades down.

Step 4 was easy for me since my Lunar software gives separation angle
and position angle (inclination of sextant frame from vertical). Lacking
that, both angles can be calculated by sight reduction formulas. First,
calculate azimuth and altitude of both bodies in the normal way.

Then do a second sight reduction, this time with the zenith as the
"pole" and Sun as the "observer". Azimuth difference is the "hour
angle", Sun altitude is "observer latitude", and Venus altitude is
"declination".

Subtract the calculated "altitude" from 90 to get separation angle. The
calculated "azimuth" is the inclination of the sextant frame with
respect to vertical.

Three previous attempts failed. I think the main problem was Sun
altitude. It was more than 45°, so the viewing position was very
uncomfortable. Today I waited until the Sun was 6° high in the west. The
sextant frame was inclined 18° counter clockwise. It was a comfortable
position, and this time Venus was easy.

--
Paul Hirose
sofajpl.com

   

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