According to astronomer Dang Vu Tuan Son, President of the Vietnam Astronomy and Space Science Association (VACA), the concept that the Earth’s rotation creates day and night is something that almost everyone knows.
The reason for this is that our daytime light comes from the Sun, which can only illuminate half of the Earth’s surface at a time. Thanks to its rotation, different regions of the Earth are continually exposed to sunlight, creating a cycle where one part is in daylight while the other is in darkness. This day-night cycle on Earth has a duration of 24 hours, known as a “solar day” in astronomy.
According to the researcher, the statement about day and night on Earth should be understood as follows: The Earth’s rotation generates a 24-hour day-night cycle. To the question of whether there would be day and night if the Earth didn’t rotate, many people might think the answer is a definite no. However, Mr. Sơn asserted that it’s not quite that simple.
“If the Earth had no other motion apart from its rotation, then the Earth would be illuminated like a globe model in a dark room illuminated by a flashlight. Only when the globe rotates do different areas on its surface receive light. When it doesn’t rotate, naturally, one half of the globe would never receive light,” Mr. Son explained.
VACA
The issue is that the Earth also has a motion around the Sun. Many people mistakenly believe that the Earth always faces the Sun on one side, even if it doesn’t rotate anymore. This belief might be acceptable in a certain geometrical context, taking the Sun as the coordinate origin and the line connecting the Earth and the Sun as the main axis of the coordinate system.
In that case, even the Earth’s motion around the Sun might be considered nonexistent because, in that reference frame, the Earth would remain stationary relative to the Sun. Unfortunately, physics doesn’t accept such a coordinate system because the distribution of matter in the Sun, the Earth, and the Earth’s orbit are not perfect, and furthermore, they are influenced by other celestial bodies.
In astrophysics, the most accurate coordinate system for considering the motion of Earth and planets is based on the distant stars (which, in reality, do have slight changes, but they are extremely slow, and over a few years or even centuries, they can be considered as having no changes). In this reference frame, if the Earth always faced the Sun on one side, it would mean that it both rotated one full turn and completed one orbit simultaneously. This phenomenon is known as “tidal locking.”
The Moon is a classic example of tidal locking; it is tidally locked with the Earth and always shows the same face to our planet. However, no astronomer would claim that it doesn’t rotate.