A calendar marks four phases because four of the eight names are instants and four are periods. The Moon is
at 0°, 90°, 180° or 270° elongation at one moment each; it is a crescent for about seven days. Only the first
kind can carry a time.
The tables above run twelve months ahead from the build date, and the field recomputes any month you ask
for. All times are UTC.
What “exactly full” means here
The tool solves for the instant the Moon’s elongation reaches 180.000°, using the truncated lunar theory in
src/lib/sky.mjs and Newton iteration on a mean rate. That is a well-defined event and it can be checked
against an almanac to the minute, which is the point.
It is not, however, what a reader usually means by “full moon night”. The Moon is 99.2% lit at 170° and
99.8% at 175°, so the disc looks full for two or three evenings around the instant — the cosine flattens near
its peak. Naming the nearest calendar day is a convention, and a reasonable one; naming the instant is what
this page does.
Where the dates drift
Because 29.53 days does not divide a calendar month, the instants walk backward through the month at about
one day per lunation. A full moon on the 2nd of one month is followed by one on the 31st and then the 29th.
That walk is also what produces blue moons: when the walk reaches the very start of a 31-day month, the
month can contain a second full moon at the end.
The eclipse table on this site depends on the same geometry — an eclipse can only happen at new or full moon,
and only when the Moon is near a node — which is why
eclipses come in seasons rather than at random.
Reading these tables in local time
Subtract your zone’s UTC offset: New York is UTC−5 in winter and −4 in summer, London UTC+0 or +1, Sydney
UTC+10 or +11. A phase at 23:40 UTC in Sydney is 10:40 the next morning, so a calendar printed in local time
will show the following date for roughly a third of these events.
If you want the phase for one specific date rather than a month, the lookup tool
gives the illumination, age and distance as well.
Computed phase instants in UTC. No claim here depends on the Moon having any effect on events on Earth.
Frequently asked questions
- Why four phases and not eight?
- The other four names — waxing crescent, waxing gibbous, waning gibbous, waning crescent — describe a range of elongation, not an instant. A crescent lasts about seven days, so a calendar cannot give it a time. The four principal phases are the moments the elongation hits 0°, 90°, 180° and 270° exactly, which is the only kind of phase a date and time can be attached to.
- Why do the times not match my local calendar?
- They are UTC. A full moon at 02:00 UTC on the 4th is 21:00 on the 3rd in New York and 13:00 on the 4th in Sydney, so a printed calendar that shows local dates will sometimes name a different day. Neither is wrong; they are the same instant in two frames.
- How close is this to a published almanac?
- Within 1.1 minutes at worst across the 2026 lunations, measured against published instants given to the second. The residuals are printed by npm run verify-sky in the site's repository. Three minutes is the tolerance, because published tables disagree with each other by one to two.
- Why is there sometimes a fifth phase in a month?
- Because 29.53 days fits into 30 or 31 days, but not neatly. A month can hold two full moons — the second is a blue moon — or two of some other phase. Between four and five principal phases per calendar month is normal; there is no month with six.