The term “actual sunrise” refers to the moment when the Sun crosses the horizon. The time difference between the actual and apparent sunrise (when the Sun becomes visible) is about 2 minutes, and this difference is attributed to the bending of sunlight by atmospheric refraction.
What is the significance of the term “actual sunrise” in the context of the time difference between the actual and apparent positions of the Sun?
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The term “actual sunrise” is used in the context of the time difference between the actual and apparent positions of the Sun to refer to the moment when the Sun’s upper limb just begins to touch the horizon as seen from a specific location on Earth. This is the time officially recognized as the sunrise. However, due to atmospheric refraction, the Sun appears slightly above the horizon before it physically reaches this point.
Atmospheric refraction causes the Sun’s rays to bend as they pass through the Earth’s atmosphere. This bending effect is more pronounced when the Sun is near the horizon. As a result, observers on Earth see the Sun slightly earlier than it would be visible if there were no atmosphere. The apparent sunrise, or the time when the Sun’s disk is first visible, occurs a few minutes before the actual sunrise.
Understanding the significance of “actual sunrise” is essential for precise timekeeping, especially in astronomy and navigation, where accurate knowledge of the Sun’s position is crucial.