Understanding the lunar cycle
One synodic month—from new moon to new moon—averages about 29.5306 days. Illumination grows while the moon is waxing, peaks near full moon, then shrinks while the moon is waning.
Free lunar cycle tool
Free Moon Phase Calculator
A moon phase calculator estimates the lunar phase for any calendar date. Check illumination, lunar age, waxing or waning status, and upcoming new and full moons, with practical context for reef observations.
Phase by dateMoon illuminationLunar ageReef observation notes
Choose a calendar date
How the estimate works
The calculator places your date within an average 29.5306-day synodic month measured from a known new moon. Local viewing conditions and precise event times can differ by several hours.
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August 9, 2026
Waning Crescent
Waning · estimated lunar day 25.7
Illumination
15.7%
Next full moon
August 28, 2026
Next new moon
August 13, 2026
DarkLunar illuminationFull
Reef observation note
Natural moonlight is relatively dim. If your fixture simulates a lunar cycle, use gradual changes and keep night lighting far below daytime intensity.
Use the date as an observation cue
Lunar timing is one environmental signal among many. Keep lighting, feeding, temperature, and water quality stable while you log repeated behavior; a calculator estimate is not a prediction that spawning will occur.
How to use the moon phase calculator
Step 1
Choose a date
Enter any calendar date from 1900 through 2100, or jump to today.
Step 2
Read the phase
Review the named phase, waxing or waning direction, lunar age, and illuminated percentage.
Step 3
Check upcoming events
See estimated dates for the next full moon and next new moon.
Step 4
Log reef observations
Use the cycle as context when recording coral and livestock behavior without changing normal care from one estimate alone.
Printable reef keeping worksheets
Keep a clean backup log for test day.
The Printable Reef Logbook gives you water testing, dosing, maintenance, and livestock worksheets you can print or save as a PDF.
Moon phase calculator FAQ
What is a moon phase calculator?
A moon phase calculator estimates where a date falls in the repeating lunar cycle. It reports the named phase, illuminated fraction, lunar age, and nearby new and full moon dates.
How accurate is this lunar phase calculator?
It uses the average 29.5306-day synodic month from a known new moon, which is useful for calendar planning. Exact astronomical event times and local visibility can differ by several hours.
What is lunar age?
Lunar age is the estimated number of days since the previous new moon. It runs from about zero days at new moon to about 14.8 days at full moon, then continues to the next new moon.
Do moon phases affect coral spawning?
Some coral species use lunar light together with seasonal, temperature, and sunset cues to synchronize reproduction. The pattern varies by species and location, so record observations instead of treating one phase as a guarantee.
Should a reef tank moonlight match lunar illumination?
A simulated lunar cycle should remain much dimmer than daytime reef lighting and change gradually. Follow your fixture guidance and keep routine husbandry stable while you observe livestock behavior.
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