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ISLAMABAD Some of our readers have thrown questions at Dawn about the 'confusing' sunset and sunrise timings in major cities.
Readers particularly wanted to learn why Islamabad and Karachi, which were so far apart, had the same sunset timings while Hyderabad that was so close to the port city experienced sunsets some four to five minutes early/late.
Inquiries from the meteorological department, Comsats Institute of Information Technology, showed that in winter months, the difference in the sunset timing was higher than the difference in the summer months of different locations of the world.
Exact local time of sunsets depended on each city's precise longitude in a given time zone. The timing of sunsets also varied with the time of the year as well as the latitude of the cities from which it is viewed.
However, minor and major daily and annual changes in timing of sunsets were also driven by axial tilt of the Earth, the spherical shape of the globe, and the planet's movement in its annual orbit around the sun.
According to the meteorological department, variations in the timings of the sunrise and sunset depended mainly on the longitude and latitude positions of any location.
The light coming from the sun would first shine on the higher longitude area rather than the lower longitude area, said an official from meteorological department, Comsats.
'This means any city of Pakistan that has higher longitude value will be exposed to sun beam first than other cities falling on lower longitude values in the county. For example, Lahore has 74 degree longitude value and Hyderabad has 68 degree longitude. The sun light will first expose Lahore and later Hyderabad,' a met department official explained.
According to the official, latitude values also affected visibility timing of sunrises and sunsets when moving towards higher latitudes.
'Topography is an important factor to change the sunrise and sunset timing of each city daily to some extent,' he said, adding that variations in local topography, observer elevation, atmospheric conditions, and other factors could affect the time by a minute or more, especially at very high latitudes.
Since first light or sunset depends on longitude increments, the official said the reason why Lahore had sunrise/sunset first was because of the highest longitudinal value of 74 degrees. Islamabad followed at 73 degrees and then sitting at 68 degrees longitude, Hyderabad experienced sunrise and sunset before Karachi that had a longitudinal value of 66 degrees.
Owing to the Earth's tilt on its axis 23.5° and its elliptical orbit around the sun, the relative location of the sun above the horizon was not constant from day to day when observed at the same time on each day, the official said.
