Do you really know when the sun gets up?


The statistics of the time of hundreds of sunsets and sunrises indicate

Our calculation is flawed

Every 24 hours, the earth will experience a dark night. We are like sitting on a carousel that gallops in the dark, whizzing past the morning faint line at a speed of 1675 km / h. In the dark, we either fall into the deep blue sky of the stars, or are lost in the artificial light flow of the modern city dizziness.

Fortunately, we understand the laws of the Earth’s movement and know that we can always greet the dawn after the darkness.

After coordinating with the cosmic clock for thousands of years, you may think that we have been able to know exactly when the sun has risen and fallen. However, Teresa Wilson (now the US Naval Observatory) pointed out in a technical paper at the University of Michigan that our estimates usually have a 1 to 5 minute deviation.

In general, sunrise and sunset are defined as the moment when the top of the sun disk is on the horizon. If we live on a planet without air, we can calculate this moment with simple geometry.

But because of the atmosphere, light does not travel straight from the sun to our eyes. When sunlight hits the Earth’s atmosphere, its path is slightly curved, and this effect is enhanced as the light penetrates into the increasingly dense atmosphere. This bending effect is called refraction and it is most noticeable at the horizon.

All publicly available computer systems that Wilson can find assume a refraction angle of 34 centimeters on the horizon – slightly larger than the full moon. Wilson found it as a standard reference as early as 1865.

 On January 8th, she delivered a speech at the American Winter Astronomical Conference in Seattle: “But the use of a single angle of refraction does not explain the refraction of different meteorological conditions at different locations – in essence, we think 7 The situation in Hawaii during the month is the same as in January in Alaska.” This approach also does not take into account the height of the observer’s location.

Wilson decided to figure out how accurate our estimates of sunrise and sunset were. She identified each record from published and unpublished data, and eventually collected 251 sunrises and 514 sunsets from 30 different geographic coordinates, including the water and land horizons.

 About 600 of them are meteorological data, which can be fitted into three different refraction models with different degrees of complexity. Other data can only be calculated with a 34-angle refraction angle fit.

She found that, in general, differences in location and season can cause different prediction times. Sunrise time on land is generally early in the summer and later in the winter.

The difference in summer is the biggest, which may be due to the obvious refraction effect in these months, and the temperature difference in the atmosphere will be relatively large.

The mirage effect caused by the cold air covering the warm water surface also exacerbates the lag of the sunset time of the year, sometimes lags up to 5 minutes.

 However, if the height of the observer above the horizon is taken into account, the prediction of the horizon on the water surface will be significantly improved.


1. Green Flash: Green flash is an optical phenomenon that sometimes occurs after sunset or before sunrise.

 When the conditions are right, a distinct green dot is visible above the upper edge of the sun disk; the green appearance usually lasts no more than a second or two. In rare cases, the green flash may resemble green light from a sunset (or sunrise) point.

2. NZ effect: The Novaya Zemlya effect is a polar mirage caused by high solar refraction between atmospheric hot layers.

 The Novaya Zemlya effect will give the impression that the sun rises earlier than it actually is (astronomically speaking), and depending on the weather, this effect will present the sun as a straight line or square (sometimes called the “rectangular sun”). , consisting of a flat hourglass shape.

 A mirage requires sun rays to pass through an inversion layer of hundreds of kilometers (at least 400 kilometers) and depends on the temperature gradient of the inversion layer. The sun must bend to the curvature of the earth for at least 400 kilometers to raise the field of view of the sun disk by 5°.

3. Mirage phenomenon: A mirage is a naturally occurring optical phenomenon in which light is curved to produce a displacement image of a distant object or sky.

 Phantoms can be classified as “inferior” (meaning lower), “superior” (meaning higher) and “Fata Morgana” – a super mirage consisting of a series of unusually complex vertically stacked images forming a A rapidly changing mirage.

In addition, more complex refraction models containing meteorological conditions do not play a better role: their inherently limited assumptions about the behavior of the meteorological layer (called the troposphere) in the Earth’s atmosphere cause them to gather near the 34-point score. Regardless of the model, she concluded that the time error of sunrise and sunset cannot be accurate to within 2 minutes.

Two minutes may not sound important, but Wilson reminds that if the GPS fails, the sailor will use celestial navigation. She said: “Most sailors will tell you that they can estimate their position through celestial bodies, and the error will not exceed 1 nautical mile.

” But if the sailor’s calculation includes sunset time, “1 minute may cause 15 nautical miles of error. “Knowing the sunrise and sunset moments is good for both professional and amateur observers.

 A better understanding of how light reaches the horizon through atmospheric refraction will help us make more accurate predictions.


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