# 目に見える影を落とすには、太陽の円盤のどのくらいをカバーする必要がありますか？

しかし、再び、金環日食の間、月は太陽の円盤の約$$99$$％を占めており、それはたくさんあります。確かに、太陽の円盤のカバーされている部分が少なければ、影は見えません。しかし、火星の赤道から見た太陽の円盤の≈$$25$$％をカバーするフォボスが、非常に非常に明確な影を投げかけることを知った太陽の円盤の4分の1のみをカバーしています。

そのとき目に見える影を落とすためのカットオフポイントは正確には何ですか？金星は通過中に太陽の円盤の$$0.108$$％のみをカバーするため、目に見える影は地球に投影されません。金星がもっと大きい場合、たとえば木星のサイズだとすると、それは太陽の円盤の$$14.52$$％をカバーします。地球の表面に目に見える影を落とすにはそれで十分でしょうか？確かに、それはアンブラルシャドウではありませんが、それは無関係です。

The answer for your question is quite complicated. Basically, the cutoff point you are saying about depends on the amount of light, which is reflected from the certain surface experiencing the eclipse. Another factor is the sensitivity of your eye, which seems to be individual... http://www.mkrgeo-blog.com/the-role-of-contrast-in-ability-of-human-vision/

This cutoff is nothing like the contrast difference between two separate areas, where one of them is less illuminated. Since the albedo of the Earth is various, we shouldn't expect the fixed border between shadowed area being visible. The biggest albedo feature clouds, snow, and glacial areas, and consequently the deserts. The satellite view is usually perfect for emphasizing the shadowed area. This is driven by the huge amount of reflected light, which comes from the outside, where Sun shines in 100%. As we are moving down to the ground, the visible cutoff changes. The phenomenon, which could explain this situation is the light transition. The light transition defines the border between non-shadow and half-shadow surface, and next the half-shadow and full shadow surface. This is better described here:

http://www.mkrgeo-blog.com/what-is-a-light-transition-what-examples-of-it-can-we-see/

where you have got the experiment on a white background, which reflects the light perfectly. An analog situation happens during every solar eclipse seen from the space, even the partial one. As our sight is higher from the surface, we are able to see a bigger area. Obviously our shadowed area becomes smaller, which yields eventually the regions from outside the penumbra, fully illuminated. The details of this situation have been described here: