The Sun rises in the east and sets in the west. A sundial is a tool that uses the Sun to tell the time of day, and it works because the Sun seems to move across the sky, so its shadow lands in different places as the day goes on. The earliest sundials known from the archaeological record are shadow clocks, and the part that casts the shadow is the gnomon. The gnomon came to Greece from Babylon, and Anaximander is credited with introducing this Babylonian instrument to the Ancient Greeks, while in ancient China it was widely used from the second millennium BC onward. Because the sun's height changes shadow length, a vertical gnomon was used to observe the Sun's altitude.

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 appears to revolve around Earth at about 15 degrees per hour. On an equatorial sundial the shadow turns 15 degrees each hour and the hour lines are equally spaced, but on most horizontal sundials the hour lines are not equally spaced. For a sundial to be accurate all year, its style, the time-telling edge of the gnomon, must be parallel to Earth's axis of rotation, so its angle from horizontal equals the sundial's latitude. Even a perfect sundial shows local time only. A clock at solar noon reads higher or lower than 12:00 because of the equation of time, so a correction is needed to match clock time. Solar noon is close to, but usually not exactly, the instant the Sun reaches its apparent highest point.

Eclipses cast shadows on a far greater scale. 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. The Moon's umbra moves eastward across Earth at over 1700 km/h, and typically it is 100 to 160 km wide while the penumbra is more than 6400 km across. A total solar eclipse is seen only where the small umbra touches Earth's surface, while a partial one is seen over a much larger area. Inside the Moon's umbra the Sun is completely covered and an observer sees a total solar eclipse, and in the penumbra only part of the Sun is covered, so an observer sees a partial one. Within the antumbra the occluding body looks entirely inside the disc of the light source, and an observer there sees an annular one, 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. A lunar eclipse can be seen from the entire night half of Earth. It occurs 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. The Moon can be in Earth's umbra or only its penumbra, and a penumbral one looks like a subtle dimming of the Moon. The Moon's phases are not caused by Earth's shadow. 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

• gnomon: shadow caster
• equatorial: on the equator
• latitude: distance from equator
• umbra: darkest shadow part