The earliest sunrise occurs a number of days before the longest day and the latest a number of days after the shortest. There is a knock-on effect on the times of sunrise and sunset. However, unlike the term twilight, which describes a time span, the terms dawn and dusk refer to moments during the transitions between day and night. However, there are 3 more rigorously defined types of twilight and dusk civil, nautical, and astronomical. Dusk: Dusk is the darkest stage of twilight in the evening. By averaging out the length of each day like this, the clock time at which the sun reaches its highest point slowly drifts backwards and forwards as the months progress. Twilight: Twilight is the illumination of the Earths lower atmosphere when the Sun itself is not directly visible because it is below the horizon. Each is therefore fixed at the average length of a natural day (this is where the ‘mean’ in Greenwich Mean Time comes from). The reason for this has to do with the slight variation in the length of 'natural' days throughout the year (the longest natural day is about 51 seconds longer than the shortest).įor a clock to work, all days measured need to have a fixed, equal length. Many people notice that the time of sunrise continues to get later after the winter solstice. They occur at the time of the solstices, either on or around 21 June and 21 December in the UK and most of the northern hemisphere. The twilight phases in the morning are also colloquially called dawn, while the twilight phases in the evening are referred to as dusk. The longest and shortest days of the year are those with the greatest and least amount of daytime. Why isn’t the earliest sunrise on the longest day of the year?
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