Class CalendarAstronomer

java.lang.Object
com.ibm.icu.impl.CalendarAstronomer
public class CalendarAstronomer extends Object
CalendarAstronomer is a class that can perform the calculations to determine the positions of the sun and moon, the time of sunrise and sunset, and other astronomy-related data. The calculations it performs are in some cases quite complicated, and this utility class saves you the trouble of worrying about them.

The measurement of time is a very important part of astronomy. Because astronomical bodies are constantly in motion, observations are only valid at a given moment in time. Accordingly, each CalendarAstronomer object has a time property that determines the date and time for which its calculations are performed. You can set and retrieve this property with

invalid reference 
setDate
, getDate and related methods.

Almost all of the calculations performed by this class, or by any astronomer, are approximations to various degrees of accuracy. The calculations in this class are mostly modelled after those described in the book Practical Astronomy With Your Calculator, by Peter J. Duffett-Smith, Cambridge University Press, 1990. This is an excellent book, and if you want a greater understanding of how these calculations are performed it a very good, readable starting point.

WARNING: This class is very early in its development, and it is highly likely that its API will change to some degree in the future. At the moment, it basically does just enough to support IslamicCalendar and ChineseCalendar.

  • Nested Class Summary

    Nested Classes
    Modifier and Type
    Class
    Description
    private static interface 
    CalendarAstronomer.AngleFunc
     
    static final class 
    CalendarAstronomer.Ecliptic
    Represents the position of an object in the sky relative to the ecliptic, the plane of the earth's orbit around the Sun.
    static final class 
    CalendarAstronomer.Equatorial
    Represents the position of an object in the sky relative to the plane of the earth's equator.
    private static class 
    CalendarAstronomer.MoonAge
     
    private static class 
    CalendarAstronomer.SolarLongitude
     

  • Field Summary

    Fields
    Modifier and Type
    Field
    Description
    static final long
    DAY_MS
    The number of milliseconds in one day.
    private static final double
    DEG_RAD
     
    (package private) static final long
    EPOCH_2000_MS
    Milliseconds value for 0.0 January 2000 AD.
    static final int
    HOUR_MS
    The number of milliseconds in one hour.
    private static final double
    INVALID
     
    (package private) static final double
    JD_EPOCH
     
    static final long
    JULIAN_EPOCH_MS
    The start of the julian day numbering scheme used by astronomers, which is 1/1/4713 BC (Julian), 12:00 GMT.
    private double
    julianDay
     
    private double
    meanAnomalySun
     
    static final int
    MINUTE_MS
    The number of milliseconds in one minute.
    (package private) static final double
    moonA
     
    (package private) static final double
    moonE
     
    private double
    moonEclipLong
     
    (package private) static final double
    moonI
     
    (package private) static final double
    moonL0
     
    (package private) static final double
    moonN0
     
    (package private) static final double
    moonP0
     
    (package private) static final double
    moonPi
     
    private CalendarAstronomer.Equatorial
    moonPosition
     
    (package private) static final double
    moonT0
     
    static final CalendarAstronomer.MoonAge
    NEW_MOON
    Constant representing a new moon.
    private static final double
    PI
     
    private static final double
    PI2
     
    private static final double
    RAD_DEG
     
    private static final double
    RAD_HOUR
     
    static final int
    SECOND_MS
    The number of milliseconds in one second.
    static final double
    SIDEREAL_DAY
    The number of standard hours in one sidereal day.
    static final double
    SIDEREAL_MONTH
    The average number of days it takes for the moon to return to the same ecliptic longitude relative to the stellar background.
    static final double
    SIDEREAL_YEAR
    The average number of days it takes for the sun to return to the same position against the fixed stellar background.
    static final double
    SOLAR_DAY
    The number of sidereal hours in one mean solar day.
    (package private) static final double
    SUN_E
     
    (package private) static final double
    SUN_ETA_G
     
    (package private) static final double
    SUN_OMEGA_G
     
    private double
    sunLongitude
     
    static final double
    SYNODIC_MONTH
    The average number of solar days from one new moon to the next.
    private long
    time
    Current time in milliseconds since 1/1/1970 AD
    static final double
    TROPICAL_YEAR
    The average number number of days between successive vernal equinoxes.
    static final CalendarAstronomer.SolarLongitude
    WINTER_SOLSTICE
    Constant representing the winter solstice.

  • Constructor Summary

    Constructors
    Constructor
    Description
    CalendarAstronomer()
    Construct a new CalendarAstronomer object that is initialized to the current date and time.
    CalendarAstronomer(long aTime)
    Construct a new CalendarAstronomer object that is initialized to the specified time.

  • Method Summary

    Modifier and Type
    Method
    Description
    private void
    clearCache()
     
    private double
    eclipticObliquity()
    Return the obliquity of the ecliptic (the angle between the ecliptic and the earth's equator) at the current time.
    final CalendarAstronomer.Equatorial
    eclipticToEquatorial(double eclipLong, double eclipLat)
    Convert from ecliptic to equatorial coordinates.
    Date
    getDate()
    Get the current time of this CalendarAstronomer object, represented as a Date object.
    double
    getJulianDay()
    Get the current time of this CalendarAstronomer object, expressed as a "julian day number", which is the number of elapsed days since 1/1/4713 BC (Julian), 12:00 GMT.
    double
    getMoonAge()
    The "age" of the moon at the time specified in this object.
    CalendarAstronomer.Equatorial
    getMoonPosition()
    The position of the moon at the time set on this object, in equatorial coordinates.
    long
    getMoonTime(double desired, boolean next)
    Find the next or previous time at which the Moon's ecliptic longitude will have the desired value.
    long
    getMoonTime(CalendarAstronomer.MoonAge desired, boolean next)
    Find the next or previous time at which the moon will be in the desired phase.
    double
    getSunLongitude()
    The longitude of the sun at the time specified by this object.
    (package private) double[]
    getSunLongitude(double julian)
    TODO Make this public when the entire class is package-private.
    long
    getSunTime(double desired, boolean next)
    Find the next time at which the sun's ecliptic longitude will have the desired value.
    long
    getSunTime(CalendarAstronomer.SolarLongitude desired, boolean next)
    Find the next time at which the sun's ecliptic longitude will have the desired value.
    long
    getTime()
    Get the current time of this CalendarAstronomer object, represented as the number of milliseconds since 1/1/1970 AD 0:00 GMT (Gregorian).
    private static final double
    norm2PI(double angle)
    Normalize an angle so that it's in the range 0 - 2pi.
    private static final double
    normalize(double value, double range)
    Given 'value', add or subtract 'range' until 0 invalid input: '<'= 'value' invalid input: '<' range.
    private static final double
    normPI(double angle)
    Normalize an angle into the range -PI - PI
    private static String
    radToDms(double angle)
     
    private static String
    radToHms(double angle)
     
    void
    setJulianDay(double jdn)
    Set the current date and time of this CalendarAstronomer object.
    void
    setTime(long aTime)
    Set the current date and time of this CalendarAstronomer object.
    private long
    timeOfAngle(CalendarAstronomer.AngleFunc func, double desired, double periodDays, long epsilon, boolean next)
     
    private double
    trueAnomaly(double meanAnomaly, double eccentricity)
    Find the "true anomaly" (longitude) of an object from its mean anomaly and the eccentricity of its orbit.

    Methods inherited from class Object

    clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

  • Field Details

    • SIDEREAL_DAY

      public static final double SIDEREAL_DAY
      The number of standard hours in one sidereal day. Approximately 24.93.
      See Also:

    • SOLAR_DAY

      public static final double SOLAR_DAY
      The number of sidereal hours in one mean solar day. Approximately 24.07.
      See Also:

    • SYNODIC_MONTH

      public static final double SYNODIC_MONTH
      The average number of solar days from one new moon to the next. This is the time it takes for the moon to return the same ecliptic longitude as the sun. It is longer than the sidereal month because the sun's longitude increases during the year due to the revolution of the earth around the sun. Approximately 29.53.
      See Also:

    • SIDEREAL_MONTH

      public static final double SIDEREAL_MONTH
      The average number of days it takes for the moon to return to the same ecliptic longitude relative to the stellar background. This is referred to as the sidereal month. It is shorter than the synodic month due to the revolution of the earth around the sun. Approximately 27.32.
      See Also:

    • TROPICAL_YEAR

      public static final double TROPICAL_YEAR
      The average number number of days between successive vernal equinoxes. Due to the precession of the earth's axis, this is not precisely the same as the sidereal year. Approximately 365.24
      See Also:

    • SIDEREAL_YEAR

      public static final double SIDEREAL_YEAR
      The average number of days it takes for the sun to return to the same position against the fixed stellar background. This is the duration of one orbit of the earth about the sun as it would appear to an outside observer. Due to the precession of the earth's axis, this is not precisely the same as the tropical year. Approximately 365.25.
      See Also:

    • SECOND_MS

      public static final int SECOND_MS
      The number of milliseconds in one second.
      See Also:

    • MINUTE_MS

      public static final int MINUTE_MS
      The number of milliseconds in one minute.
      See Also:

    • HOUR_MS

      public static final int HOUR_MS
      The number of milliseconds in one hour.
      See Also:

    • DAY_MS

      public static final long DAY_MS
      The number of milliseconds in one day.
      See Also:

    • JULIAN_EPOCH_MS

      public static final long JULIAN_EPOCH_MS
      The start of the julian day numbering scheme used by astronomers, which is 1/1/4713 BC (Julian), 12:00 GMT. This is given as the number of milliseconds since 1/1/1970 AD (Gregorian), a negative number. Note that julian day numbers and the Julian calendar are not the same thing. Also note that julian days start at noon, not midnight.
      See Also:

    • EPOCH_2000_MS

      static final long EPOCH_2000_MS
      Milliseconds value for 0.0 January 2000 AD.
      See Also:

    • PI

      private static final double PI
      See Also:

    • PI2

      private static final double PI2
      See Also:

    • RAD_HOUR

      private static final double RAD_HOUR
      See Also:

    • DEG_RAD

      private static final double DEG_RAD
      See Also:

    • RAD_DEG

      private static final double RAD_DEG
      See Also:

    • JD_EPOCH

      static final double JD_EPOCH
      See Also:

    • SUN_ETA_G

      static final double SUN_ETA_G
      See Also:

    • SUN_OMEGA_G

      static final double SUN_OMEGA_G
      See Also:

    • SUN_E

      static final double SUN_E
      See Also:

    • WINTER_SOLSTICE

      public static final CalendarAstronomer.SolarLongitude WINTER_SOLSTICE
      Constant representing the winter solstice. For use with getSunTime. Note: In this case, "winter" refers to the northern hemisphere's seasons.

    • moonL0

      static final double moonL0
      See Also:

    • moonP0

      static final double moonP0
      See Also:

    • moonN0

      static final double moonN0
      See Also:

    • moonI

      static final double moonI
      See Also:

    • moonE

      static final double moonE
      See Also:

    • moonA

      static final double moonA
      See Also:

    • moonT0

      static final double moonT0
      See Also:

    • moonPi

      static final double moonPi
      See Also:

    • NEW_MOON

      public static final CalendarAstronomer.MoonAge NEW_MOON
      Constant representing a new moon. For use with getMoonTime

    • time

      private long time
      Current time in milliseconds since 1/1/1970 AD
      See Also:

    • INVALID

      private static final double INVALID
      See Also:

    • julianDay

      private transient double julianDay

    • sunLongitude

      private transient double sunLongitude

    • meanAnomalySun

      private transient double meanAnomalySun

    • moonEclipLong

      private transient double moonEclipLong

    • moonPosition

      private transient CalendarAstronomer.Equatorial moonPosition

  • Constructor Details

    • CalendarAstronomer

      public CalendarAstronomer()
      Construct a new CalendarAstronomer object that is initialized to the current date and time.

    • CalendarAstronomer

      public CalendarAstronomer(long aTime)
      Construct a new CalendarAstronomer object that is initialized to the specified time. The time is expressed as a number of milliseconds since January 1, 1970 AD (Gregorian).
      See Also:

  • Method Details

    • setTime

      public void setTime(long aTime)
      Set the current date and time of this CalendarAstronomer object. All astronomical calculations are performed based on this time setting.
      Parameters:
      aTime - the date and time, expressed as the number of milliseconds since 1/1/1970 0:00 GMT (Gregorian).
      See Also:

    • setJulianDay

      public void setJulianDay(double jdn)
      Set the current date and time of this CalendarAstronomer object. All astronomical calculations are performed based on this time setting.
      Parameters:
      jdn - the desired time, expressed as a "julian day number", which is the number of elapsed days since 1/1/4713 BC (Julian), 12:00 GMT. Note that julian day numbers start at noon. To get the jdn for the corresponding midnight, subtract 0.5.
      See Also:

    • getTime

      public long getTime()
      Get the current time of this CalendarAstronomer object, represented as the number of milliseconds since 1/1/1970 AD 0:00 GMT (Gregorian).
      See Also:

    • getDate

      public Date getDate()
      Get the current time of this CalendarAstronomer object, represented as a Date object.
      See Also:

    • getJulianDay

      public double getJulianDay()
      Get the current time of this CalendarAstronomer object, expressed as a "julian day number", which is the number of elapsed days since 1/1/4713 BC (Julian), 12:00 GMT.
      See Also:

    • eclipticToEquatorial

      public final CalendarAstronomer.Equatorial eclipticToEquatorial(double eclipLong, double eclipLat)
      Convert from ecliptic to equatorial coordinates.
      Parameters:
      eclipLong - The ecliptic longitude
      eclipLat - The ecliptic latitude
      Returns:
      The corresponding point in equatorial coordinates.

    • getSunLongitude

      public double getSunLongitude()
      The longitude of the sun at the time specified by this object. The longitude is measured in radians along the ecliptic from the "first point of Aries," the point at which the ecliptic crosses the earth's equatorial plane at the vernal equinox.

      Currently, this method uses an approximation of the two-body Kepler's equation for the earth and the sun. It does not take into account the perturbations caused by the other planets, the moon, etc.

    • getSunLongitude

      double[] getSunLongitude(double julian)
      TODO Make this public when the entire class is package-private.

    • getSunTime

      public long getSunTime(double desired, boolean next)
      Find the next time at which the sun's ecliptic longitude will have the desired value.

    • getSunTime

      public long getSunTime(CalendarAstronomer.SolarLongitude desired, boolean next)
      Find the next time at which the sun's ecliptic longitude will have the desired value.

    • getMoonPosition

      public CalendarAstronomer.Equatorial getMoonPosition()
      The position of the moon at the time set on this object, in equatorial coordinates.

    • getMoonAge

      public double getMoonAge()
      The "age" of the moon at the time specified in this object. This is really the angle between the current ecliptic longitudes of the sun and the moon, measured in radians.
      See Also:
      • invalid reference 
        #getMoonPhase

    • getMoonTime

      public long getMoonTime(double desired, boolean next)
      Find the next or previous time at which the Moon's ecliptic longitude will have the desired value.

      Parameters:
      desired - The desired longitude.
      next - true if the next occurrance of the phase is desired, false for the previous occurrance.

    • getMoonTime

      public long getMoonTime(CalendarAstronomer.MoonAge desired, boolean next)
      Find the next or previous time at which the moon will be in the desired phase.

      Parameters:
      desired - The desired phase of the moon.
      next - true if the next occurrance of the phase is desired, false for the previous occurrance.

    • timeOfAngle

      private long timeOfAngle(CalendarAstronomer.AngleFunc func, double desired, double periodDays, long epsilon, boolean next)

    • normalize

      private static final double normalize(double value, double range)
      Given 'value', add or subtract 'range' until 0 invalid input: '<'= 'value' invalid input: '<' range. The modulus operator.

    • norm2PI

      private static final double norm2PI(double angle)
      Normalize an angle so that it's in the range 0 - 2pi. For positive angles this is just (angle % 2pi), but the Java mod operator doesn't work that way for negative numbers....

    • normPI

      private static final double normPI(double angle)
      Normalize an angle into the range -PI - PI

    • trueAnomaly

      private double trueAnomaly(double meanAnomaly, double eccentricity)
      Find the "true anomaly" (longitude) of an object from its mean anomaly and the eccentricity of its orbit. This uses an iterative solution to Kepler's equation.
      Parameters:
      meanAnomaly - The object's longitude calculated as if it were in a regular, circular orbit, measured in radians from the point of perigee.
      eccentricity - The eccentricity of the orbit
      Returns:
      The true anomaly (longitude) measured in radians

    • eclipticObliquity

      private double eclipticObliquity()
      Return the obliquity of the ecliptic (the angle between the ecliptic and the earth's equator) at the current time. This varies due to the precession of the earth's axis.
      Returns:
      the obliquity of the ecliptic relative to the equator, measured in radians.

    • clearCache

      private void clearCache()

    • radToHms

      private static String radToHms(double angle)

    • radToDms

      private static String radToDms(double angle)