title: PHP date_sunrise and date_sun_info giving different results
body: <p>I have a webcam website and I wanted to display the sunrise time for the actual day. So I used the date_sunrise function in PHP for this purpose. In addition to the coordinates, it takes the suns zenith value as input.</p>
  
  <p>I learned that the correct zenith for sunrise is 90.8333. 90 would be the theoretic angle to the center of the sun (i.e. at the horizon). To account for the diameter of the sun and refraction, 0.8333 is added (16 arcminutes + 34 arcminutes divided by 60).</p>
  
  <p>But I noticed that this input gave different sunrise times that other sites. The most reliable source must be the Astronomical Applications Department of the U.S. Naval Observatory. See their <a href="http://aa.usno.navy.mil/data/docs/RS_OneDay.php" rel="nofollow noreferrer">online calculator</a> and read <a href="http://aa.usno.navy.mil/faq/docs/RST_defs.php#top" rel="nofollow noreferrer">the description of their method</a>. It's stated that they also use a Zenith value of 90.8333 for the purpose of calculating sunrise.</p>
  
  <p>Then I discovered that the php function date_sun_info, which is an array containing various data, gave the correct sunrise time (corresponding to the results from AAD).</p>
  
  <p>To achieve the same sunrise time from the date_sunrise function, I have to enter a zenith value around 90.5.</p>
  
  <p>Why doesn't date_sunrise with zenith=90.8333 give the same result as date_sun_info?</p>