An opaque substance lets no light through. It reflects, scatters or absorbs all of the light, and that is why it can cast a shadow. Materials that let light through are called transparent. A shadow is a dark shape that forms when something blocks light.

Astronomical noon is when the Sun is highest in the sky. One way to find it is to find when shadows are shortest. Solar noon is close to, but usually not exactly, the instant the Sun reaches its apparent highest point. The Sun makes shadows that change during the day, because the Sun seems to move across the sky as Earth rotates on its axis.

A new moon happens about once a month, but solar eclipses do not. The Moon's orbit is tilted by about five degrees, so the new Moon usually passes above or below the Sun and its shadow misses Earth. The Moon's phases depend on where the Moon is in its orbit around Earth and where Earth is in its orbit around the Sun. They are not caused by Earth's shadow.

Even when the Moon does line up, its shadow has more than one part. Within the antumbra the occluding body looks entirely inside the disc of the light source. An observer there sees an annular eclipse, with a bright ring around the Moon. In an annular eclipse the Moon is too far away to cover the whole Sun.

In a lunar eclipse the Moon passes through Earth's shadow. The Moon can be in Earth's umbra, which is a total lunar eclipse, or only in its penumbra. A penumbral eclipse looks like a subtle dimming of the Moon.

People have also made shadows on purpose. Shadow puppet theatre likely began in Central Asia-China or in India in the 1st millennium BCE, and shadow plays reached Europe at the end of the 17th century, probably via Italy.

Words to know

• opaque: no light through
• antumbra: ring-eclipse shadow
• annular: ring-shaped
• penumbral: partial-shadow