On level ground a shadow's length depends on the Sun's elevation angle: the lower the Sun, the longer the shadow. When the Sun is directly overhead, no shadows are cast. Astronomical noon is when the Sun is highest in the sky, and one way to find it is to find when shadows are shortest. Earth completes one rotation about every 23.9 hours, which is why the Sun appears to cross the sky each day and shadows swing and change length. The Sun is directly overhead at noon on the equator twice a year, at the two equinoxes, but outside the tropics the Sun is never directly overhead, so shadows are never absent at noon there.

An opaque substance reflects, scatters or absorbs all of the light, and that is why it can cast a shadow. A point source of light casts only a simple shadow called an umbra, but a wider source, like the Sun, makes a shadow with three parts, and the wider the light source, the more blurred the shadow's edges become. A shadow in sunlight is not pure black, because the object blocks the light of the Sun but not the ambient light of the sky, and such a shadow can have a bluish tinge.

The Sun is about 1.4 million kilometres wide, more than 100 times wider than Earth, and about 150 million kilometres away. The umbra of a round body blocking a round light source forms a cone. The distance from the Moon to the apex of its umbra is roughly equal to the distance between the Moon and Earth, so the tip of the Moon's umbra only just reaches Earth. Inside the umbra the Sun is completely covered and an observer sees a total solar eclipse. In the penumbra only part of the Sun is covered, so an observer sees a partial eclipse.

A new moon happens about once a month, but solar eclipses do not. The new Moon usually passes above or below the Sun and its shadow misses Earth. Lunar eclipses occur at the full moon phase, but not at every full moon, because the Moon's orbit is inclined by about five degrees to Earth's orbit, so the full Moon often passes above or below Earth's shadow. The Moon's phases themselves are not caused by Earth's shadow. Within the antumbra the occluding body looks entirely inside the disc of the light source, and an observer there sees an annular eclipse, with a bright ring around the Moon.

Earth is much bigger than the Moon, so its umbra reaches farther: roughly 1.4 million km, beyond the Moon's orbit. Earth's shadow is as curved as the planet is, and in a lunar eclipse the Moon passes through that shadow. The Moon can be in Earth's umbra, which makes a total lunar eclipse, or only in its penumbra. A penumbral eclipse looks like a subtle dimming of the Moon. In a total lunar eclipse the Moon appears orange or red, because sunlight bent through Earth's atmosphere still reaches it. This is the same effect that turns sunrises and sunsets reddish.

Words to know

• equinoxes: two special days
• apex: the tip
• inclined: tilted
• umbra: darkest shadow part