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Why are lunar days extremely hot and lunar nights extremely cold?

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near and far sides of Earth's Moon — Moon (Left) Near side of Earth's Moon, photographed by the Galileo spacecraft on its way to Jupiter. (Right) Far side of the Moon with some of the near side visible (upper right), photographed by the Apollo 16 spacecraft.

The extreme temperature fluctuations on the Moon are primarily due to its lack of a substantial atmosphere. Unlike Earth, which has an atmosphere that helps regulate surface temperatures, the Moon is unprotected, leaving its surface directly exposed to solar radiation during the day and the cold of space at night.1

Without an atmosphere to trap heat or distribute it across the surface, the lunar daytime temperature can reach nearly 400 kelvins (approximately 127 °C or 260 °F). Conversely, during the lunar night, the surface loses heat rapidly, causing temperatures to plummet to below 100 kelvins (approximately −173 °C or −279 °F). While the top layer of the lunar regolith acts as an efficient insulator due to its high porosity, these dramatic temperature swings are confined to the uppermost surface and do not penetrate deeply into the soil.1