Class CalendarAstronomer
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
.
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Nested Class Summary
Nested ClassesModifier and TypeClassDescriptionprivate static interface
static final class
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
Represents the position of an object in the sky relative to the plane of the earth's equator.private static class
private static class
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Field Summary
FieldsModifier and TypeFieldDescriptionstatic final long
The number of milliseconds in one day.private static final double
(package private) static final long
Milliseconds value for 0.0 January 2000 AD.static final int
The number of milliseconds in one hour.private static final double
(package private) static final double
static final long
The start of the julian day numbering scheme used by astronomers, which is 1/1/4713 BC (Julian), 12:00 GMT.private double
private double
static final int
The number of milliseconds in one minute.(package private) static final double
(package private) static final double
private double
(package private) static final double
(package private) static final double
(package private) static final double
(package private) static final double
(package private) static final double
private CalendarAstronomer.Equatorial
(package private) static final double
static final CalendarAstronomer.MoonAge
Constant representing a new moon.private static final double
private static final double
private static final double
private static final double
static final int
The number of milliseconds in one second.static final double
The number of standard hours in one sidereal day.static final double
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
The average number of days it takes for the sun to return to the same position against the fixed stellar background.static final double
The number of sidereal hours in one mean solar day.(package private) static final double
(package private) static final double
(package private) static final double
private double
static final double
The average number of solar days from one new moon to the next.private long
Current time in milliseconds since 1/1/1970 ADstatic final double
The average number number of days between successive vernal equinoxes.static final CalendarAstronomer.SolarLongitude
Constant representing the winter solstice. -
Constructor Summary
ConstructorsConstructorDescriptionConstruct a newCalendarAstronomer
object that is initialized to the current date and time.CalendarAstronomer
(long aTime) Construct a newCalendarAstronomer
object that is initialized to the specified time. -
Method Summary
Modifier and TypeMethodDescriptionprivate void
private double
Return the obliquity of the ecliptic (the angle between the ecliptic and the earth's equator) at the current time.eclipticToEquatorial
(double eclipLong, double eclipLat) Convert from ecliptic to equatorial coordinates.getDate()
Get the current time of thisCalendarAstronomer
object, represented as aDate
object.double
Get the current time of thisCalendarAstronomer
object, expressed as a "julian day number", which is the number of elapsed days since 1/1/4713 BC (Julian), 12:00 GMT.double
The "age" of the moon at the time specified in this object.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
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 thisCalendarAstronomer
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 - PIprivate static String
radToDms
(double angle) private static String
radToHms
(double angle) void
setJulianDay
(double jdn) Set the current date and time of thisCalendarAstronomer
object.void
setTime
(long aTime) Set the current date and time of thisCalendarAstronomer
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.
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Field Details
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SIDEREAL_DAY
public static final double SIDEREAL_DAYThe number of standard hours in one sidereal day. Approximately 24.93.- See Also:
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SOLAR_DAY
public static final double SOLAR_DAYThe number of sidereal hours in one mean solar day. Approximately 24.07.- See Also:
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SYNODIC_MONTH
public static final double SYNODIC_MONTHThe 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:
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SIDEREAL_MONTH
public static final double SIDEREAL_MONTHThe 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:
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TROPICAL_YEAR
public static final double TROPICAL_YEARThe 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:
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SIDEREAL_YEAR
public static final double SIDEREAL_YEARThe 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:
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SECOND_MS
public static final int SECOND_MSThe number of milliseconds in one second.- See Also:
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MINUTE_MS
public static final int MINUTE_MSThe number of milliseconds in one minute.- See Also:
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HOUR_MS
public static final int HOUR_MSThe number of milliseconds in one hour.- See Also:
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DAY_MS
public static final long DAY_MSThe number of milliseconds in one day.- See Also:
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JULIAN_EPOCH_MS
public static final long JULIAN_EPOCH_MSThe 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:
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EPOCH_2000_MS
static final long EPOCH_2000_MSMilliseconds value for 0.0 January 2000 AD.- See Also:
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PI
private static final double PI- See Also:
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PI2
private static final double PI2- See Also:
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RAD_HOUR
private static final double RAD_HOUR- See Also:
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DEG_RAD
private static final double DEG_RAD- See Also:
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RAD_DEG
private static final double RAD_DEG- See Also:
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JD_EPOCH
static final double JD_EPOCH- See Also:
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SUN_ETA_G
static final double SUN_ETA_G- See Also:
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SUN_OMEGA_G
static final double SUN_OMEGA_G- See Also:
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SUN_E
static final double SUN_E- See Also:
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WINTER_SOLSTICE
Constant representing the winter solstice. For use withgetSunTime
. Note: In this case, "winter" refers to the northern hemisphere's seasons. -
moonL0
static final double moonL0- See Also:
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moonP0
static final double moonP0- See Also:
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moonN0
static final double moonN0- See Also:
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moonI
static final double moonI- See Also:
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moonE
static final double moonE- See Also:
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moonA
static final double moonA- See Also:
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moonT0
static final double moonT0- See Also:
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moonPi
static final double moonPi- See Also:
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NEW_MOON
Constant representing a new moon. For use withgetMoonTime
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time
private long timeCurrent time in milliseconds since 1/1/1970 AD- See Also:
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INVALID
private static final double INVALID- See Also:
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julianDay
private transient double julianDay -
sunLongitude
private transient double sunLongitude -
meanAnomalySun
private transient double meanAnomalySun -
moonEclipLong
private transient double moonEclipLong -
moonPosition
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Constructor Details
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CalendarAstronomer
public CalendarAstronomer()Construct a newCalendarAstronomer
object that is initialized to the current date and time. -
CalendarAstronomer
public CalendarAstronomer(long aTime) Construct a newCalendarAstronomer
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:
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Method Details
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setTime
public void setTime(long aTime) Set the current date and time of thisCalendarAstronomer
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:
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setJulianDay
public void setJulianDay(double jdn) Set the current date and time of thisCalendarAstronomer
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:
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getTime
public long getTime()Get the current time of thisCalendarAstronomer
object, represented as the number of milliseconds since 1/1/1970 AD 0:00 GMT (Gregorian).- See Also:
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getDate
Get the current time of thisCalendarAstronomer
object, represented as aDate
object.- See Also:
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getJulianDay
public double getJulianDay()Get the current time of thisCalendarAstronomer
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:
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eclipticToEquatorial
Convert from ecliptic to equatorial coordinates.- Parameters:
eclipLong
- The ecliptic longitudeeclipLat
- The ecliptic latitude- Returns:
- The corresponding point in equatorial coordinates.
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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.
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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
Find the next time at which the sun's ecliptic longitude will have the desired value. -
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:
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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.
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getMoonTime
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.
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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
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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.
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clearCache
private void clearCache() -
radToHms
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radToDms
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