Understanding the Time Difference in iPhone Sunset Times Between Atherton CA and New York City

For March 25, 2019, why did my iPhone display sunset times of 7:23 PM in Atherton, CA, and 7:12 PM in New York City? This discrepancy can be explained by the complex factors that influence the calculation of sunrise and sunset times. In this article, we will break down these factors and provide a comprehensive understanding of why such differences exist.

Factors Determining Sunrise and Sunset Times

Multiple elements come into play when determining the exact time of sunrise and sunset. The mere presence of time zones is not enough to solely determine the times. Time zones are a human construct aimed at normalizing interactions across nations and globally. However, they only serve as an approximation since the earth is divided into 24 equally spaced longitudinal segments starting from the Royal Observatory in Greenwich, England. The U.S., and most places around the world, approximate this ideal division.

The calculation of sunrise and sunset also takes into account the location's latitude, longitude, and elevation. To understand this process, some knowledge of trigonometry would be required. Therefore, the basic explanation here will be in layman's terms.

Explanation of Time Zone Differences

Why do the sunset times differ between Atherton, CA, and New York City? The primary reason is their location in different time zones. Atherton, CA, is part of the Pacific Time Zone, while New York City is in the Eastern Time Zone. Moreover, the geographical locations play a vital role. Atherton is west of the center of the Pacific Time Zone, whereas New York City is east of the center of the Eastern Time Zone.

Another factor is the latitude. Atherton is located at a slightly higher latitude compared to New York City. This difference in latitude is relatively small, but it significantly impacts the calculation of sunrise and sunset times. As the date approaches the summer solstice, sunset times for both locations will approach each other, nearly coinciding.

How is the Time of Sunrise Calculated?

The accuracy of sunrise and sunset times on your iPhone or any other device is often based on the algorithm used by the device's operating system. Apple likely uses a combination of data from the U.S. Naval Observatory and other reputable sources to compute these times.

The calculation involves the following key steps:

Geographical location: The device determines your location using GPS. Time zone: The device identifies the correct time zone for your location. Solar position: The device calculates the position of the sun based on the time of year and your location's latitude and longitude. Golden hour definition: The time of sunrise and sunset is determined based on the sun's position in the sky relative to the horizon. This is often defined as the period when the sun's center is 4 degrees below the horizon.

Conclusion

The differences in sunset times between Atherton, CA, and New York City are a result of geographical and time zone differences. These factors, including latitude and time zone, significantly impact the accuracy of sunrise and sunset times. Understanding these factors can help demystify why your iPhone displays different sunset times for different locations.

References

Sunrise equation - Wikipedia
ESRL Global Monitoring Division
Sunset and sunrise times
NOAA Improved Sunrise/Sunset Calculation