#sun position

"Seeing a rainbow can feel like a reward. After a violent thunderstorm, it's nice to spot a colorful arch crossing the calming sky. But you might (or might not) be surprised to know that rainbows aren't really arches, nor are they "bows." They're actually full circles.

So why do we only see an arch? Oftentimes, the rainbows we see are partly blocked by the ground and horizon. To observe one in all its circular glory, you'd have to find a nice high vantage point. We'll explain how the phenomenon happens."

"Mediums matter: In air, light cruises along at 186,000 miles per second (300,000 kilometers per second). But since liquid water is denser, light can't move through it as quickly. So once a beam of light that's been zipping through the air hits a body of water, it slows down quite a bit.

In the case of rainbows, sunlight that enters individual water droplets bends — or refracts — multiple times. First, it bends upon passing into a bead of H2O. After that, the light bounces off the inside wall at the far side of the droplet and reenters the air. The light gets refracted again while exiting.

Through refraction, the droplets separate sunlight into its component colors. Although it looks white, rays of sunshine are in fact a mixture of all the colors within the visible light spectrum."

"A key factor here is the location of the antisolar point. This is the spot in the sky — or on the ground — that's exactly 180 degrees away from the sun relative to your perspective. On a bright, sunny day, the head of your shadow marks the antisolar point. Every rainbow is a perfectly circular ring centered around this very spot.

Yet if you're standing at ground level, you won't be able to see the circle's lower half. Indeed, from this vantage point, basically any part of a rainbow that dips below the horizon is rendered invisible. One of the reasons for this is that the close proximity of Earth's surface limits the amount and concentration of raindrops within your line of sight.

As such, the percentage of a rainbow that's visible to most people is directly correlated with the sun's position. When our solar neighbor is just barely peeking over the horizon, the antisolar point will be fairly high up, affording you the chance to see a much bigger rainbow than you would when the sun climbs higher.

Conversely, if the sun is more than 42 degrees above the horizon, it becomes impossible for ground-based observers to see any portion of a rainbow whatsoever. But when you're soaring in an aircraft, things get more interesting. On rainy or misty days, airplane passengers and pilots occasionally see full circular rainbows. Better yet, in 2013, photographer Colin Leonhardt captured this picture of a circular double rainbow while flying around Australia's Cottesloe Beach."

計画案CG

 ３DCG上で緯度経度を組み込むことによって日時指定による太陽の当たり方が検証できます。

およその高さですが近隣住宅を入力すれば、『境界線からの離隔距離や窓の位置等によって、季節ごとで部屋の雰囲気がどう変わるのか』が分かりやすく分かるのが良いですね。

私自身がCGを作成していますので、いろいろな検証ができます。

PCの中の世界ですが、没頭していると現地で検討しているかのような気分になってあっという間に時間が過ぎてしまいます。

 Software：Blender

Proposed plan.

SunClac is a nice web app for calculating sun position and sunrise/sunset/twilight times given location and date.