Shadows: Light That Does Not Pass

Narrator (April Hobart, CXC): You are relaxing with a book on a nice sunny day when a friend leans over your shoulder and the page goes dark. "Hey," you might say, "you're blocking my light!" It is a familiar experience – any time an object blocks the light from another source, it forms a shadow. But did you know all of the places that shadows occur?

We are most used to shadows of, say, people on a beach. As their bodies block the sunlight, this prevents the light from reaching the sand behind them and a darker region is formed.

But much larger objects such as the Earth and the Moon can also cast shadows. During what astronomers call a lunar eclipse, the alignment of the Earth, Moon, and Sun can result in the Earth briefly blocking most sunlight getting to the Moon. Some of the sunlight does make it to the lunar surface, but it is filtered by the Earth's atmosphere, and it is also slightly bent. This causes the sunlight reflected from the Moon during a lunar eclipse to often appear red. The exact color of the shadow across the Moon during a lunar eclipse can depend on the amount of dust and clouds in our atmosphere.

A different type of eclipse can also produce a spectacular shadow. Once again, this involves a special configuration of the Earth, Moon, and Sun. Instead of the Earth blocking light getting to the Moon, the Moon passes in between the Earth and Sun. This is called a solar eclipse. When this happens, the Sun can appear to be completely blocked when viewed from a small portion of the Earth. This phenomenon is called totality and some people travel all over the world to sit inside this shadow that lasts for just a few minutes.

Shadows occur on other planets as well. One excellent place to look for shadows in our Solar System is the planet Jupiter. This gas giant has several larger moons. When one of the bigger Moons passes in front of the face of the planet as seen from Earth, it blocks sunlight shining onto Jupiter. This creates a shadow on the gaseous clouds that make up Jupiter's atmosphere.

It's also important to note that shadows can happen with different types of light. Astronomers call what humans can see with their eyes "visible light." However, there are also many other types of light ranging from radio waves up to more energetic X-rays and gamma rays. And the nature of a shadow depends both on the object and the light that it is blocking.

For example, our shadow looks different in visible light as compared to X-ray light. Bone is denser than muscle tissue and skin, so it stops more of the X-rays. Fewer X-rays make it to the region of the film that's behind the bone, creating an X-ray shadow image. In a similar way, astronomers use distant X-ray sources as tools to "see inside" so to speak, dark clouds and learn about the elements inside those clouds.

So the next time you're on the beach enjoying the sunshine, take an extra look at the shadows around you. They may be common on Earth, but shadows are also found throughout the Solar System and even across the Universe.