The observable universe is about 93 billion light-years across in diameter. This is the largest distance we can see in all directions from Earth, limited by how far light has traveled since the Big Bang.

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Day and night occur because Earth rotates on its axis, causing different parts of the planet to face toward and away from the Sun. As your location rotates toward the Sun, you experience day; as it rotates away, you experience night.

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We know Earth is round through multiple lines of evidence including satellite photos, the way ships disappear over the horizon, gravity measurements, and observations of Earth's shadow on the Moon during lunar eclipses. These observations have been confirmed by thousands of scientific measurements and observations over centuries.

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The speed of light is a universal limit because it is the maximum speed at which information and matter can travel through space, established by the laws of physics. Nothing with mass can reach or exceed light's speed because it would require infinite energy.

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Time is a dimension in which events happen in sequence, moving from the past through the present toward the future. It can be measured using the movement of objects like the sun and moon, and is divided into seconds, minutes, hours, days, and years.

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