Question

The sun begins to be seen a little before it rises and continues to be seen for several minutes after it has actually set. Explain.

Answer 1

The sun becomes visible slightly before the actual sunrise and continues to be seen for several minutes after the actual sunset due to the atmospheric refraction of light.

The atmosphere extends up to about 300 km above the Earth’s surface. As we ascend higher in the atmosphere, the density of air decreases, causing a decrease in the refractive index. Hence, the atmosphere can be considered as composed of several concentric spherical layers, where the refractive index increases gradually from the outer layers towards the Earth’s surface.

When light rays from the sun enter these layers, they pass through progressively denser air, causing the rays to bend continuously toward normal due to refraction. This bending causes the sun’s rays, which are actually below the horizon, to appear to an observer on Earth as if they come from a position just above the horizon. This apparent shift in position raises the sun by about 33 minutes of arc above its true position near the horizon.

Because of this, the sun appears to rise a few minutes earlier than its actual geometric sunrise and remains visible for a few minutes after the sun has geometrically set below the horizon. This atmospheric refraction effect effectively lengthens the day by several minutes.

Thus, the phenomenon of atmospheric refraction causes the sun to be seen slightly before it rises and to continue being visible for some time after it has actually set, making the days longer than what geometric calculation alone would indicate.