How is a 'day' scientifically defined when NASA shifts focus to planetary science and exploration?

Answer

The time it takes for a celestial body to complete one full rotation on its axis

For planetary missions, a day is scientifically defined by the specific celestial body's rotation period on its axis, which determines the length of that world's solar day.

How is a 'day' scientifically defined when NASA shifts focus to planetary science and exploration?

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