How does the optimal viewing requirement for general iridescence differ from that for Noctilucent Clouds (NLCs)?
Answer
For iridescence, one must look toward the sun (within 10 degrees), whereas for NLCs, one must look toward the northern horizon during deep twilight when the sun is below the horizon.
Iridescence requires viewing the cloud near the sun (less than 10 degrees away). NLCs require viewing during deep twilight when the sun is 6 to 16 degrees below the horizon, looking toward the darkened northern horizon (in the Northern Hemisphere).
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