Need to know when the New and Full Moons are to plan the dates of your
observing sessions? This Moon Phase Calculator will compute the New & Full
Moons for a given year. Type in a year or use the buttons change the year:
TIME ACCURACY:
The calculated times are in Terrestrial Dynamical Time (TDT or TT),
a replacement for Ephemeris Times (ET). High precision algorithms are not
used, and actual times may be off by several minutes, or more as you move many
hundreds of years from the current date. This tool is intended as an
observing planning aid, to help you know when the moon will be dark, and extreme
precision is not our goal. Note also that the day-of-week will not be
correct when the date is in the Julian calendar
(earlier than 1582-Oct-15).
CIVIL TIME: TDT is
a uniform time used for astronomical calculations. Civil time, such as UTC
(popularly known as GMT, but technically incorrect) or
your local time, is corrected for the non-linear changes in the rotation of the
earth. This is done accurately only through observations. This web page corrects
the the calculated TDT to civil time using a table of observations from 1620
thru 2002. Time corrections outside this range are estimated from predictive
equations. Be aware that for times hundreds of years in the past or future, the
difference between TDT and UTC can be many hours. See Meeus' book for additional details.
LOCAL TIME: Local time zone conversion is done by the JavaScript built-in runtime functions
from the corrected UTC (including daylight or summer time rules), based on information it gets from
your computer's environment settings. If the local time is in the wrong time zone
for you, you need to check and adjust these settings
on your computer, there is nothing in this web page or the code behind it to change this. On
Windows machines, right click on the clock in the task bar and select "Adjust
Date/Time", and then click the "Time Zone" tab.
CREDITS:
The code for the Moon phase computation appeared in Astronomical Computing,
Sky & Telescope, March, 1985, while the
code for Julian Day to Calendar Dates appeared in the same column in May 1984.
In both cases, the Basic code by Roger W. Sinnott was based on algorithms by
Jean Meeus in Astronomical Formulae for Calculators
(Willmann-Bell, 1982). The Basic source
code was obtained from the Sky Publishing Web
Site and converted to JavaScript for this web page. The TDT
to UTC corrections are from a subsequent book by Meeus: Astronomical
Algorithms, Second Edition
(Willmann-Bell, 1998).
CAUTION:
While the output has been spot checked for correctness against current U.S.
Naval Observatory tables (always within 2 minutes), and exactly produces the
test data given in the March 1985 Sky & Telescope article, the code has not
been thoroughly tested and could contain errors. The underlying algorithms are
not exact and actual moon phases could vary by many minutes. Consult the above
sources for detailed information.
