Every day the Sun rises in the east and sets in the west. Earth completes one rotation approximately every 23.9 hours, which is why the Sun appears to cross the sky each day and shadows swing and change length. Astronomical noon is when the Sun is at its highest in the sky, and one way to determine it is to identify when shadows are shortest, although solar noon is close to, but usually not exactly, the instant the Sun reaches its apparent highest point. Sunlight takes approximately 8 minutes to reach Earth. Light usually travels in a straight line, and we represent these lines as rays, and a shadow forms where an opaque object obstructs light from the Sun or another source. Crepuscular rays are sunbeams that are almost parallel to one another, but they appear to radiate outwards because of a perspective effect, like parallel lines of a long road that appear to meet in the distance.

A shadow is a dark shape that forms when something obstructs light, and it has the identical shape to the object that makes it. 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. Things that let light through so you can see clearly through them are called transparent. Near sunrise and sunset, when the Sun is low in the sky, shadows can be extremely long, and around midday, when the Sun is high, shadows are short. A shadow in sunlight is not pure black, because the object obstructs the light of the Sun but not the ambient light of the sky, and such a shadow can have a bluish tinge.

The Moon is always half lit by the Sun, and the Moon's phases are not caused by Earth's shadow. It has a day side and a night side. The lunar terminator is the division between the lit and dark hemispheres of the Moon. Near it the Sun is low, and consequently shadows cast by craters and other features are elongated, and images of the terminator can be used to map topography. In a composite lunar image the terminator regions show the longest and most prominent shadows, and the contrast and length of those shadows make the flat photograph appear three-dimensional. Artemis II crew studied the Moon's day-night boundary partly because its lighting resembles conditions at the Moon's South Pole. The low-angle light casts long, dramatic shadows that highlight the rugged surface. On the Moon, when the Sun is low, shadows of craters and mountains get long, like shadows on Earth when the Sun is low.

An eclipse happens when an object passes into the shadow of another body. In a solar eclipse the Moon passes in front of the Sun, and inside the Moon's umbra the Sun is entirely covered, and consequently an observer sees a total solar eclipse, while in the penumbra only part of the Sun is covered, and consequently an observer sees a partial eclipse. Only people in the center of the Moon's shadow see a total solar eclipse. In a lunar eclipse the Moon passes through Earth's shadow, and it can be in Earth's umbra or only its penumbra. Earth is much bigger than the Moon, and consequently its umbra extends farther: approximately 1.4 million km, beyond the Moon's orbit. A penumbral eclipse resembles a subtle dimming of the Moon, while in a total lunar eclipse the Moon appears orange or red because sunlight bent through Earth's atmosphere still reaches it.

A new moon happens approximately once a month, but solar eclipses do not. The Moon's orbit is inclined by approximately five degrees, and consequently the new Moon usually passes above or below the Sun and its shadow misses Earth, and lunar eclipses occur at the full moon phase, but not at every full moon, for the same reason. The saros repeats a solar or lunar eclipse every 6,585.3 days, a little over 18 years, and there can be from four to seven eclipses in a calendar year. A lunar eclipse is visible from the entire night half of Earth, while a total solar eclipse is seen only where the small umbra contacts Earth's surface, and the Moon's umbra moves eastward across Earth at over 1700 km/h. Never look straight at the Sun without special eye protection, even during an eclipse, because sunglasses do not make it safe.

The Sun is approximately 1.4 million kilometres wide, more than 100 times wider than Earth, and approximately 150 million kilometres away. The distance from the Moon to the apex of its umbra is approximately equal to the distance between the Moon and Earth, and consequently the apex of the Moon's umbra only just reaches Earth. In an annular eclipse the Moon is too far away to cover the whole Sun, and consequently a bright ring of Sun shows around the Moon, and within the antumbra the occluding body looks entirely inside the disc of the light source, and consequently an observer there sees that bright ring. As of November 2026, totality of a solar eclipse can never exceed 7 min 32 s. The Moon's apparent size varies, and millions of years from now it will be too far away to fully cover the Sun, and consequently no total eclipses will occur.

Words to know

• crepuscular: like sunbeams
• terminator: day-night line
• antumbra: ring-eclipse shadow
• saros: eclipse cycle