The Sun is visible for a few minutes after actual sunset due to atmospheric refraction of light, as well as light pollution. Actual sunset is the actual crossing of the horizon by the Sun. When the Sun is slightly below the horizon, the Sun's light comes from less dense air to more dense air. Therefore, light is refracted downwards as it passes through the atmosphere. As a result, the Sun appears to be raised above the horizon and we are able to see the Sun minutes after the actual sunset. Thus, the time difference between actual sunset and the apparent sunset is about 2-5 minutes.

Maghrib is not at actual sunset. Rather, as with every other visible phenomena that is linked with Muslim worship, it is the time of the actual observation. Therefore, dilligence must be observed for if a fast is broken at actual sunset while the Sun is still visible on the horizon, then it will have been prematurely broken, and therefore, null and void.

Apparent Sunset

Click here to read more about apparent sunset and maghrib time.