Class AstronomicalCalculator

  • All Implemented Interfaces:
    Cloneable
    Direct Known Subclasses:
    NOAACalculator, SunTimesCalculator

    public abstract class AstronomicalCalculator
    extends Object
    implements Cloneable
    An abstract class that all sun time calculating classes extend. This allows the algorithm used to be changed at runtime, easily allowing comparison the results of using different algorithms.
    Author:
    © Eliyahu Hershfeld 2004 - 2018
    To Do:
    Consider methods that would allow atmospheric modeling. This can currently be adjusted by setting the refraction.
    • Field Summary

      Fields 
      Modifier and Type Field Description
      private double earthRadius
      The commonly used average earth radius in KM.
      private static double GEOMETRIC_ZENITH
      The zenith of astronomical sunrise and sunset.
      private double refraction
      The commonly used average solar refraction.
      private double solarRadius
      The commonly used average solar radius in minutes of a degree.
    • Field Detail

      • refraction

        private double refraction
        The commonly used average solar refraction. Calendrical Calculations lists a more accurate global average of 34.478885263888294
        See Also:
        getRefraction()
      • earthRadius

        private double earthRadius
        The commonly used average earth radius in KM. At this time, this only affects elevation adjustment and not the sunrise and sunset calculations. The value currently defaults to 6356.9 KM.
        See Also:
        getEarthRadius(), setEarthRadius(double)
    • Method Detail

      • getEarthRadius

        public double getEarthRadius()
        A method that returns the earth radius in KM. The value currently defaults to 6356.9 KM if not set.
        Returns:
        the earthRadius the earth radius in KM.
      • setEarthRadius

        public void setEarthRadius​(double earthRadius)
        A method that allows setting the earth's radius.
        Parameters:
        earthRadius - the earthRadius to set in KM
      • getDefault

        public static AstronomicalCalculator getDefault()
        getDefault method returns the default sun times calculation engine.
        Returns:
        AstronomicalCalculator the default class for calculating sunrise and sunset. In the current implementation the default calculator returned is the SunTimesCalculator.
      • getCalculatorName

        public abstract String getCalculatorName()
        Returns the name of the algorithm.
        Returns:
        the descriptive name of the algorithm.
      • getUTCSunrise

        public abstract double getUTCSunrise​(Calendar calendar,
                                             GeoLocation geoLocation,
                                             double zenith,
                                             boolean adjustForElevation)
        A method that calculates UTC sunrise as well as any time based on an angle above or below sunrise. This abstract method is implemented by the classes that extend this class.
        Parameters:
        calendar - Used to calculate day of year.
        geoLocation - The location information used for astronomical calculating sun times.
        zenith - the azimuth below the vertical zenith of 90 degrees. for sunrise typically the zenith used for the calculation uses geometric zenith of 90° and adjusts this slightly to account for solar refraction and the sun's radius. Another example would be AstronomicalCalendar.getBeginNauticalTwilight() that passes AstronomicalCalendar.NAUTICAL_ZENITH to this method.
        adjustForElevation - Should the time be adjusted for elevation
        Returns:
        The UTC time of sunrise in 24 hour format. 5:45:00 AM will return 5.75.0. If an error was encountered in the calculation (expected behavior for some locations such as near the poles, Double.NaN will be returned.
        See Also:
        getElevationAdjustment(double)
      • getUTCSunset

        public abstract double getUTCSunset​(Calendar calendar,
                                            GeoLocation geoLocation,
                                            double zenith,
                                            boolean adjustForElevation)
        A method that calculates UTC sunset as well as any time based on an angle above or below sunset. This abstract method is implemented by the classes that extend this class.
        Parameters:
        calendar - Used to calculate day of year.
        geoLocation - The location information used for astronomical calculating sun times.
        zenith - the azimuth below the vertical zenith of 90°. For sunset typically the zenith used for the calculation uses geometric zenith of 90° and adjusts this slightly to account for solar refraction and the sun's radius. Another example would be AstronomicalCalendar.getEndNauticalTwilight() that passes AstronomicalCalendar.NAUTICAL_ZENITH to this method.
        adjustForElevation - Should the time be adjusted for elevation
        Returns:
        The UTC time of sunset in 24 hour format. 5:45:00 AM will return 5.75.0. If an error was encountered in the calculation (expected behavior for some locations such as near the poles, Double.NaN will be returned.
        See Also:
        getElevationAdjustment(double)
      • getElevationAdjustment

        double getElevationAdjustment​(double elevation)
        Method to return the adjustment to the zenith required to account for the elevation. Since a person at a higher elevation can see farther below the horizon, the calculation for sunrise / sunset is calculated below the horizon used at sea level. This is only used for sunrise and sunset and not times before or after it such as nautical twilight since those calculations are based on the level of available light at the given dip below the horizon, something that is not affected by elevation, the adjustment should only made if the zenith == 90° adjusted for refraction and solar radius. The algorithm used is
         elevationAdjustment = Math.toDegrees(Math.acos(earthRadiusInMeters / (earthRadiusInMeters + elevationMeters)));
         
        The source of this algorthitm is Calendrical Calculations by Edward M. Reingold and Nachum Dershowitz. An alternate algorithm that produces an almost identical (but not accurate) result found in Ma'aglay Tzedek by Moishe Kosower and other sources is:
         elevationAdjustment = 0.0347 * Math.sqrt(elevationMeters);
         
        Parameters:
        elevation - elevation in Meters.
        Returns:
        the adjusted zenith
      • adjustZenith

        double adjustZenith​(double zenith,
                            double elevation)
        Adjusts the zenith of astronomical sunrise and sunset to account for solar refraction, solar radius and elevation. The value for Sun's zenith and true rise/set Zenith (used in this class and subclasses) is the angle that the center of the Sun makes to a line perpendicular to the Earth's surface. If the Sun were a point and the Earth were without an atmosphere, true sunset and sunrise would correspond to a 90° zenith. Because the Sun is not a point, and because the atmosphere refracts light, this 90° zenith does not, in fact, correspond to true sunset or sunrise, instead the centre of the Sun's disk must lie just below the horizon for the upper edge to be obscured. This means that a zenith of just above 90° must be used. The Sun subtends an angle of 16 minutes of arc (this can be changed via the setSolarRadius(double) method , and atmospheric refraction accounts for 34 minutes or so (this can be changed via the setRefraction(double) method), giving a total of 50 arcminutes. The total value for ZENITH is 90+(5/6) or 90.8333333° for true sunrise/sunset. Since a person at an elevation can see blow the horizon of a person at sea level, this will also adjust the zenith to account for elevation if available.
        Parameters:
        zenith - the azimuth below the vertical zenith of 90°. For sunset typically the zenith used for the calculation uses geometric zenith of 90° and adjusts this slightly to account for solar refraction and the sun's radius. Another example would be AstronomicalCalendar.getEndNauticalTwilight() that passes AstronomicalCalendar.NAUTICAL_ZENITH to this method.
        elevation - elevation in Meters.
        Returns:
        The zenith adjusted to include the sun's radius, refraction and elevation adjustment. This will only be adjusted for sunrise and sunset (if the zenith == 90°)
        See Also:
        getElevationAdjustment(double)
      • getRefraction

        double getRefraction()
        Method to get the refraction value to be used when calculating sunrise and sunset. The default value is 34 arc minutes. The Errata and Notes for Calendrical Calculations: The Millenium Eddition by Edward M. Reingold and Nachum Dershowitz lists the actual average refraction value as 34.478885263888294 or approximately 34' 29". The refraction value as well as the solarRadius and elevation adjustment are added to the zenith used to calculate sunrise and sunset.
        Returns:
        The refraction in arc minutes.
      • setRefraction

        public void setRefraction​(double refraction)
        A method to allow overriding the default refraction of the calculator.
        Parameters:
        refraction - The refraction in arc minutes.
        See Also:
        getRefraction()
        To Do:
        At some point in the future, an AtmosphericModel or Refraction object that models the atmosphere of different locations might be used for increased accuracy.
      • getSolarRadius

        double getSolarRadius()
        Method to get the sun's radius. The default value is 16 arc minutes. The sun's radius as it appears from earth is almost universally given as 16 arc minutes but in fact it differs by the time of the year. At the perihelion it has an apparent radius of 16.293, while at the aphelion it has an apparent radius of 15.755. There is little affect for most location, but at high and low latitudes the difference becomes more apparent. My Calculations for the difference at the location of the Royal Observatory, Greenwich show only a 4.494 second difference between the perihelion and aphelion radii, but moving into the arctic circle the difference becomes more noticeable. Tests for Tromso, Norway (latitude 69.672312, longitude 19.049787) show that on May 17, the rise of the midnight sun, a 2 minute 23 second difference is observed between the perihelion and aphelion radii using the USNO algorithm, but only 1 minute and 6 seconds difference using the NOAA algorithm. Areas farther north show an even greater difference. Note that these test are not real valid test cases because they show the extreme difference on days that are not the perihelion or aphelion, but are shown for illustrative purposes only.
        Returns:
        The sun's radius in arc minutes.
      • setSolarRadius

        public void setSolarRadius​(double solarRadius)
        Method to set the sun's radius.
        Parameters:
        solarRadius - The sun's radius in arc minutes.
        See Also:
        getSolarRadius()