在数据库中实现天文计算，特别是计算日出和日落的时间，对于许多应用来说是非常有用的。例如，农业、气象、能源管理等领域都需要准确的日出日落时间来优化资源分配和计划。本文将介绍如何使用SQL Server实现这一功能，包括相关的存储过程和函数。
本文介绍的实现基于美国海军天文台的日出/日落算法。这个算法可以用于SQL Server，并且可能与市场上大多数基于SQL的数据库引擎兼容（尽管未经测试）。
使用这些函数非常简单。主要包括两个存储过程：DetermineSunrise和DetermineSunset。每个存储过程都需要日期和地理位置参数。
以下是使用这些存储过程的示例代码：
DECLARE @OffSet int
SET @OffSet = dbo.udf_getUTCOffSet()
-- Buenos Aires, Argentina, 34, 35 S, 58, 22 W, 11th May 2012 --
EXEC DetermineSunrise '05/11/2012', -34.58, -58.3, @OffSet
EXEC DetermineSunset '05/11/2012', -34.58, -58.3, @OffSet
执行上述代码后，将输出：
2012-05-11 07:37:19.200
2012-05-11 18:01:19.200
以下是实现这一功能所需的相关函数：
CREATE FUNCTION [dbo].[TimeFromDecimal]
(
@dTime decimal(28,4),
@dateTime datetime
)
RETURNS datetime
AS
BEGIN
-- Add the T-SQL statements to compute the return value here
DECLARE @iHour int, @iMin int, @iSec int, @imSec int
set @iHour = @dTime
set @iMin = ((@dTime - @iHour)*60.)
set @iSec = (((@dTime-@iHour)*60-@iMin)*60)
set @imSec = ((((@dTime-@iHour)*60-@iMin)*60) - @iSec)*1000
if @iHour > 0
begin
set @dateTime = dateadd(hh, @ihour, @dateTime)
end
else
begin
set @dateTime = dateadd(d, 1, @dateTime)
set @dateTime = dateadd(hh, @ihour, @dateTime)
end
set @dateTime = dateadd(mi, @imin, @dateTime)
set @dateTime = dateadd(s, @isec, @dateTime)
set @dateTime = dateadd(ms, @imsec, @dateTime)
RETURN @dateTime
END
CREATE FUNCTION [dbo].[udf_calcDayofYear]
(
@dDate datetime
)
RETURNS int
AS
BEGIN
DECLARE @N1 int, @N2 int, @N3 int, @N int
set @N1 = floor(275 * (select month(@dDate)) / 9)
set @N2 = floor(((select month(@dDate)) + 9) / 12)
set @N3 = (1 + floor(((select year(@dDate)) - 4 * floor((select year(@dDate)) / 4) + 2) / 3))
set @N = @N1 - (@N2 * @N3) + (select day(@dDate)) - 30
RETURN @N
END
CREATE FUNCTION [dbo].[udf_getUTCOffSet]()
RETURNS int
AS
BEGIN
RETURN DATEDIFF(hh, GETUTCDATE(), GETDATE())
END
以下是存储过程的实现：
CREATE PROCEDURE [dbo].[DetermineSunrise]
@localDate datetime,
@latitude decimal(18,3),
@longitude decimal(18,3),
@GMT int
AS
BEGIN
DECLARE @ZENITH decimal(18,3)
SET @ZENITH = 90.83
-- Calculate the day of the year
DECLARE @DayOfYear int
SET @DayOfYear = dbo.udf_calcDayofYear(@localDate)
-- Convert the longitude to hour value and calculate an approximate time
DECLARE @lngHour decimal(18,3)
DECLARE @t decimal(18,3)
SET @lngHour = @longitude / 15
SET @t = @DayOfYear + ((6 - @lngHour) / 24)
-- Calculate the Sun's mean anomaly
DECLARE @Mean decimal(18,3)
SET @Mean = (0.9856 * @t) - 3.289
-- Calculate the Sun's true longitude
DECLARE @SunLon decimal(18,3)
SET @SunLon = @Mean + (1.916 * sin(RADIANS(@Mean))) + (0.020 * sin(2 * RADIANS(@Mean))) + 282.634 - 360
-- Calculate the Sun's right ascension
DECLARE @SunRightAsention decimal(18,3)
SET @SunRightAsention = DEGREES(atan(0.91764 * tan(RADIANS(@SunLon))))
-- Right ascension value needs to be in the same quadrant as L
DECLARE @Lquadrant decimal(18,3)
DECLARE @Rquadrant decimal(18,3)
SET @Lquadrant = (floor(@SunLon/90)) * 90
SET @Rquadrant = (floor(@SunRightAsention/90)) * 90
SET @SunRightAsention = @SunRightAsention + (@Lquadrant - @Rquadrant)
-- Right ascension value needs to be converted into hours
SET @SunRightAsention = @SunRightAsention / 15
-- Calculate the Sun's declination
DECLARE @sinDec decimal(18,3)
DECLARE @cosDec decimal(18,3)
SET @sinDec = 0.39782 * sin(RADIANS(@SunLon))
SET @cosDec = cos(asin(@sinDec))
-- Calculate the Sun's local hour angle
DECLARE @cosH decimal(18,3)
SET @cosH = (cos(radians(@ZENITH)) - (@sinDec * sin(radians(@latitude)))) / (@cosDec * cos(radians(@latitude)))
-- If (cosH > 1) the sun never rises on this location (on the specified date)
-- If (cosH < -1) the sun never sets on this location (on the specified date)
DECLARE @H decimal(18,3)
SET @H = 360 - degrees(ACOS(@cosH))
SET @H = @H / 15
-- Calculate local mean time of rising/setting
DECLARE @MeanTime decimal(18,3)
SET @MeanTime = @H + @SunRightAsention - (0.06571 * @t) - 6.622
DECLARE @UT decimal(18,3)
SET @UT = @MeanTime - @lngHour
SELECT dbo.TimeFromDecimal(@UT + @GMT, @localDate)
END