Julian day

The library allows to retrieve the Julian day from a supported temporal accessor, such as ZonedDateTime or LocalDateTime either by using the methods of the JulianDayHelper class directly:

  ZonedDateTime sputnikLaunch = ZonedDateTime.parse("1957-10-04T19:26:24Z");

  // Alternatively, we can use JulianDayHelper.getBigDecimalFrom for
  // a higher precision.
  double jd = JulianDayHelper.getDoubleFrom(sputnikLaunch); // 2436116.31

or using the queries defined in the Queriesclass:

  ZonedDateTime sputnikLaunch = ZonedDateTime.parse("1957-10-04T19:26:24Z");

  // Alternatively, we can use Queries.HP_JULIAN_DAY for a higher precision.
  double jd = sputnikLaunch.query(Queries.JULIAN_DAY); // 2436116.31

Decimal Time

In Meeus' book date-times are often specified with a decimal time. For example the Sputnik launch date and time is given as 1957 October 4.81. Amongst methods that can convert to/from temporal accessors, visible in the API reference, there is also a parsing method for handling such dates and convert them in the JDK LocalDateTime object:

  String sputnikLaunch = "1957-10-04.81";

  // Check the API reference for details on the accepted format.
  LocalDateTime dt = DecimalTime.parseDateTime(sputnikLaunch); // 1957-10-04T19:26:24

Please note that a decimal time of 0 indicates midnight, not noon.

The methods to get the decimal time from a temporal accessor can also be accessed through queries:

  LocalTime sputnikLaunch = LocalTime.parse("19:26:24");

  // Queries.HP_DECIMAL_TIME can be used for retrieving a BigDecimal.
  double t = sputnikLaunch.query(Queries.DECIMAL_TIME); // 0.81