Ignore the effect of zenith parameter on the result

M66 2025-05-31

In PHP, the date_sunset() function is used to calculate the sunset time for a specific date, time, and geographical location. This is a very practical function, especially when building applications that rely on geo-time events, such as mosque prayer time reminders, automatic control lighting systems, etc.

However, many developers often ignore a very critical parameter when using this function: zenith . Although this parameter has a default value, its impact on the final result may be much greater than expected.

What is zenith?

In astronomy, zenith means "the vertical angle of the center of the sun and the observer", which can also be understood as the angle of the sun below the horizon at sunset. The default zenith value of date_sunset() of PHP is 90 + 50/60 degrees, which is 90.8333 degrees (also known as "official sunset").

But in fact, the zenith value is customizable. Different application scenarios (civil, navigation, astronomy, etc.) have different definitions of "sunset", and the zenith values used are also different:

type	Zenith value (degree)
Civilian	96
navigation	102
astronomical	108
Official defaults	90.8333

What kind of error will ignoring zenith cause?

Let's use a specific example. On April 26, 2025, Beijing time, assuming we are in Shanghai (latitude 31.2304°, longitude 121.4737°), we call date_sunset() twice, using the default zenith and the custom zenith values to observe the difference.

 <?php
$date = strtotime("2025-04-26");
$latitude = 31.2304;
$longitude = 121.4737;

// Use default zenith
$defaultSunset = date_sunset($date, SUNFUNCS_RET_STRING, $latitude, $longitude);

// Use custom zenith，For example, civilian（96°）
$civilZenith = 96;
$civilSunset = date_sunset($date, SUNFUNCS_RET_STRING, $latitude, $longitude, 0, $civilZenith);

echo "default zenith Sunset time: $defaultSunset\n";
echo "Civilian zenith Sunset time: $civilSunset\n";
?>

The output may be similar to:

 default zenith Sunset time: 18:30
Civilian zenith Sunset time: 18:53

Conclusion: Just because zenith is ignored, the sunset time has a difference of nearly 23 minutes!

Why is this important?

Timing trigger event : If you use the default sunset time to control street lights, ignoring zenith may cause early turn-off or delay turn-on, affecting experience or safety.
Religious uses : Different cultures have strict definitions of "sunset", and the wrong time may directly affect user trust.
Astronomical applications : For precision calculations, even a minute deviation can cause data distortion.

Best Practices

Always make sure that your business semantics align with the default zenith , and if not sure, look up information or specify the zenith value explicitly.
Use constants to record your zenith type, for example:

 define('ZENITH_CIVIL', 96);
define('ZENITH_NAUTICAL', 102);
define('ZENITH_ASTRONOMICAL', 108);

Reserve zenith configuration options in the system design to avoid writing default values.

Conclusion

Although the zenith parameter is optional in the date_sunset() function, it is by no means irrelevant. Ignoring it may make your application appear "unreliable" at critical moments. A number after a decimal point is often a watershed between reliable systems and unreliable applications.

date_sunset