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Lunar Phase Calculator
The date here (initialised to your computer's idea of today's date)
is used to find the closest full moon, from which three complete
lunar cycles are then calculated and displayed. Unless today is a
full moon, then the table will start at some other date than the one
set at the top.
You can change the date shown and get phases of the moon for any
date you choose. However no checks are currently made as to the
validity of whatever you enter, so make sure you don't enter silly
dates, or you will get surprising answers. The Julian date mechanism
is quite robust, and will correctly extend both far into the future
and way back into antiquity, but the moon algorithm may not be valid
for more than a few hundred years ahead or behind the present.
Note that there is no such thing as the year zero (the Romans hadn't
grasped the concept of zero) so you will get an error if you try to
enter such a year. The sequence goes ..., 2BC, 1BC, 1AD, 2AD, ...,
with no 0AD in the middle, but the program will correct for this by
adding one to negative years. Because of changes to the calendar when
things got out of step a few hundred years ago, some days don't exist
at all, so you may see some surprising gaps between moon phases if
you explore these periods.
The code does all its calculations in GMT, but displays the
answers corrected for your local timezone. It gets this from your
computer, via your Web browser, so it may not be correct. If you
don't agree with the values shown at the top of the page, you can
correct them and use the Recalculate button to put the new offset
into effect. Better still, set your computer's date and time
correctly, and tell it your local timezone. MS-Windows and MacOS
have Control Panels to do this, but mainframe or workstation users
will have to ask their System Administrator.
If you are West of the Greenwich Meridian, then your offset will
be a negative value (it is earlier in the day in your timezone),
whereas if you are East of it then your offset will be positive
(it is later in the day in your timezone). Daylight savings time
should be taken into account if it is in effect, by adding one or two
hours to the offset.
The correct value is the one which, when added to GMT, gives your
local time. Eg, EST would be Minus 5, BST would be Plus 1, for Nepal
it would be Plus 5.75. All fractions are rounded to the nearest 15
minutes.
Although the times shown may appear accurate to the last second, this
is almost certainly not true and you should consult a lunar ephemeris
if you need accurate timings. The values given here are only approximate
and are meant to be a rough guide, not for launching moonshots.
The innacuracies are partly inherent in the method used, partly due
to rounding errors, and partly the result of Javascript's occasional
insistence that two plus two equals twenty-two rather than four. I
have tracked down and (I think) squashed all occurrences of the latter
in the code which works out the date, but so far haven't finished
checking the code which calculates the time and applies the timezone
corrections.
Links to related sites
- Phases of the Moon
Astronomy site with moon pictures and a beautiful animated picture of the moon
going through all its phases.
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