Moon Phase Calculator (Illumination, Age, Next Full/New)
Time
Calculates moon phase using a mean synodic month of 29.530588853 days anchored to a reference new moon at JD 2451550.1 (6 Jan 2000 18:14 UTC). Phase age is (JD − reference) mod 29.5306, illumination is the standard (1 − cos(2π × age / 29.5306)) / 2 — exact for the simplified two-body model and accurate to within a few hours for civilian use. The illumination curve maps to the eight conventional phase names (new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, last quarter, waning crescent) at the standard 3.69-day boundaries. The SVG renders the lit limb on the correct side: right-hand crescent during the waxing half, left-hand during the waning half — matching the appearance from the Northern Hemisphere. Upcoming new and full moons are listed for the chosen look-ahead window — useful for photography, gardening, astronomy planning and tide estimation.
| # | Date | Phase |
|---|---|---|
| 1 | 2026-06-15 02:27 | New moon |
| 2 | 2026-06-29 20:49 | Full moon |
Mean synodic month (29.530588853 days) anchored to JD 2451550.1. Accurate to ±6 hours; for astronomical-grade timing use VSOP87 / ELP-2000. The SVG glyph shows the Northern-Hemisphere view; mirror it for southern latitudes.
How to use
- Pick a date — defaults to today.
- Read the phase name, illumination percentage and lunar age in days.
- Adjust the look-ahead window (14–180 days) to see all new and full moons in that range.
- Compare to the SVG glyph to see what the moon should look like in the sky on that night.
Frequently asked questions
- How accurate is this?
- Accurate to roughly ±6 hours for the new/full moon timestamps — perfectly fine for photography, hiking, fishing or scheduling. Astronomical-grade accuracy needs a full ELP-2000 or VSOP87 lunar theory accounting for perturbations from the Sun, Earth, planets and Earth's oblateness; the simplified mean-synodic model used here ignores those, so the actual perigee-driven 'supermoon' full moons can be off by a few hours and rare edge-of-day cases can land on the wrong calendar day.
- Why doesn't this show the southern-hemisphere view?
- From the Southern Hemisphere the same phase appears mirrored — what's a right-hand waxing crescent in Seoul looks like a left-hand waxing crescent in Sydney. The phase NAME, illumination percentage and age are identical worldwide, just the lit limb appears on the opposite side of the disk. Mentally flip the glyph if you're south of the equator.
- Why is the full moon sometimes called a 'blue moon'?
- Two definitions: the seasonal blue moon is the third of four full moons in an astronomical season — rare because synodic months (29.5 days) almost-but-don't-quite divide a season (~91 days) into three. The calendrical blue moon is the popular but technically erroneous definition: the second full moon in a Gregorian calendar month. Both happen roughly every 2.5–3 years; neither has anything to do with the moon's actual color.
- What's lunar 'age' in days?
- Time since the last new moon, in days. 0 = new moon, ~7.4 = first quarter, ~14.77 = full moon, ~22.1 = last quarter, ~29.53 = next new moon. Many older calendars (Islamic, Hebrew, Chinese) are luni-solar and track this number directly.
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