Near sunrise and sunset, when θ = 0° and cot(θ) = ∞, shadows can be extremely long. The length of a shadow cast on the ground is proportional to the cotangent of the sun's elevation angle-its angle θ relative to the horizon. The sun casts shadows that change dramatically through the day. Conversely, a shadow cast by the Moon onto the Earth is a solar eclipse. Night is caused by the hemisphere of a planet facing its orbital star blocking its sunlight.Ī shadow cast by the Earth onto the Moon is a lunar eclipse. The only astronomical objects able to project visible shadows onto Earth are the Sun, the Moon, and in the right conditions, Venus or Jupiter. An astronomical object casts human-visible shadows when its apparent magnitude is equal or lower than -4. The names umbra, penumbra and antumbra are often used for the shadows cast by astronomical objects, though they are sometimes used to describe levels of darkness, such as in sunspots. Three moons ( Callisto, Europa and Io) and their shadows parade across Jupiter. a person standing on the ground, or a pole in the ground) the shadows converge at the point of contact.Ī shadow shows, apart from distortion, the same image as the silhouette when looking at the object from the sun-side, hence the mirror image of the silhouette seen from the other side. The absence of diffusing atmospheric effects in the vacuum of outer space produces shadows that are stark and sharply delineated by high-contrast boundaries between light and dark.įor a person or object touching the surface where the shadow is projected (e.g. The lighting of an overcast sky produces few visible shadows. The more diffuse the lighting is, the softer and more indistinct the shadow outlines become until they disappear. If there is more than one light source, there will be several shadows, with the overlapping parts darker, and various combinations of brightnesses or even colors. A viewer located in the penumbra region will see the light source, but it is partially blocked by the object casting the shadow. A viewer located in the umbra region cannot directly see any part of the light source.īy contrast, the penumbra is illuminated by some parts of the light source, giving it an intermediate level of light intensity. The umbra region does not receive any direct light from any part of the light source and is the darkest. The outlines of the shadow zones can be found by tracing the rays of light emitted by the outermost regions of the extended light source. This is known as the shadow blister effect. If two penumbras overlap, the shadows appear to attract and merge. The wider the light source, the more blurred the shadow becomes. For a non-point or "extended" source of light, the shadow is divided into the umbra, penumbra, and antumbra. A point source of light casts only a simple shadow, called an " umbra".
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