What determines the time of sunrise and sunset?

During a recent trip to the Andaman Islands, I noticed that the sunrise and sunset times in the city of Port Blair were almost exactly an hour earlier than those in my hometown of Bengaluru in mainland India. My father believed that sunrise and sunsets are determined by the latitude, while my mother said that it was caused by the longitude. Both cities follow the same time zone (UTC+5.5hrs), which prompted me to study the factors that determine the sunrise and sunset times for a particular place.

Longitude

The Earth has 360 degrees longitude which runs from 180 degrees in the West to 180 degrees in the East. The longitude of a particular place denotes its location with respect to the Prime Meridian. As the Earth rotates from West to East, places further east will experience sunsets and sunrises earlier. Since 360 degrees of longitude corresponds to one whole day, we can calculate that 1 degree corresponds to a 4-minute time difference for the sun’s movement (assuming both places are at the same latitude). If City A is 15 degrees east of City B, then the sun will rise and set an hour earlier in City A when compared to City B.

Latitude and seasons

The Earth’s axis is tilted by 23.5 degrees, which gives rise to varying lengths of days and the seasons. If there were no axial tilt, then the longitude would be the only factor determining when sunrises and sunsets occur. Since the axial tilt affects the length of the day, we need to take this into account. As we move further away from the equator, the days get longer in the summer and shorter in the winter. This difference in the length of the days becomes larger the further we move from the equator (higher latitudes). This implies earlier sunrises and later sunsets in summer, and later sunrises and earlier sunsets in winter.

If City A is at a higher latitude than City B, then City A will experience earlier sunrises and later sunsets in summer, and later sunrises and earlier sunsets in winter. The latitude difference will lead to two additional factors that can affect the sunrises and sunsets.

1. Difference in latitude: The further apart they are spaced in latitude, the larger the difference in times.

2. Season: The time difference caused by the latitude depends on the season. The difference is maximum during the summer and winter solstices. During the spring and autumn equinoxes, the difference will be zero and all places on Earth will have 12-hour days and nights.

Time difference between Port Blair and Bengaluru

Port Blair is 15.1 degrees to the east and 1.4 degrees to the south of Bengaluru. The longitude difference corresponds to 60 minutes of time difference, while the small latitude difference will only cause a couple of minutes of difference depending on the season. This is why the sun rises and sets almost exactly an hour earlier in Port Blair than in Bengaluru.

Time difference between Delhi and Bengaluru

Delhi is located 0.5 degrees to the west and 15.7 degrees to the north of Bengaluru. The longitude difference corresponds to only about 2 minutes of time difference, while the latitude difference can cause up to 30 minutes of time difference for sunrises and sunsets. Since Delhi is further away from the equator than Bengaluru, it will experience longer days in summer and shorter days in winter. This means that during the winter solstice (22 December), the sun will rise 30 minutes later and set 30 minutes earlier in Delhi when compared to Bengaluru. The opposite is true during the summer solstice (21 June). During the equinoxes, both cities will experience 12-hour days and nights.

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