Description: Night is a period of darkness when the Sun is below the horizon. Night lasts from sunset to sunrise. On most of the Earth, each 24-hour day has one night. However, north of the Arctic Circle and south of the Antarctic Circle, there are periods of darkness around the winter solstice where the polar night can last for months. These areas have no night around the time of the summer solstice.

The period when the Sun is just below the horizon is called twilight. During twilight, the Sun still illuminates part of the sky. Twilight is divided into three equal periods: Civil twilight, nautical twilight, and astronomical twilight. During civil twilight – when the Sun is zero to six degrees below the horizon – the sky is relatively bright so artificial illumination such as street lighting is often not required. Nautical twilight – when the Sun is six to 12 degrees below the horizon – has both bright stars visible and a sky bright enough to see the horizon even on a moonless night. This allows sailors to navigate using celestial navigation. Astronomical twilight is when the Sun is 12 to 18 degrees below the horizon.

Related Terms:

• Degree
• Horizon
• Solstice
• Sun
• Day
• Arctic Circle
• Antarctic Circle
• Polar Day
• Polar Night

Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".