Moon Phase Calculator

Moon phase calculation determines the current phase of the moon based on the number of days elapsed since the last new moon.
The lunar cycle repeats every 29.53059 days, which is called the synodic month.

Current moon age formula: Moon Age (days) = Days since known New Moon (mod 29.53059)

Phase determination from age: The four exact phase moments fall at day 0 (new), 7.38 (first quarter), 14.77 (full) and 22.15 (last quarter). Each named window below is centred on its moment rather than starting at it, which matters more than it sounds. Centre the window and anything 99% lit or brighter reads Full Moon, which is what you actually see in the sky. Start the window at the instant instead and a moon at 99.5% is filed under Waxing Gibbous, which nobody looking up would agree with.

Because the calculator reckons at noon UTC and a full moon can fall at any hour, the calendar day before a full moon sometimes reads Full Moon and sometimes Waxing Gibbous. That is not inconsistency, it is the 1.85-day window doing its job: if the exact full moment is more than about 22 hours away, the moon is genuinely still filling out.

• 28.61–0.92 days: 🌑 New Moon
• 0.92–6.46 days: 🌒 Waxing Crescent
• 6.46–8.31 days: 🌓 First Quarter
• 8.31–13.84 days: 🌔 Waxing Gibbous
• 13.84–15.69 days: 🌕 Full Moon
• 15.69–21.22 days: 🌖 Waning Gibbous
• 21.22–23.07 days: 🌗 Last Quarter
• 23.07–28.61 days: 🌘 Waning Crescent

Moon illumination percentage: Illumination % = (1 − cos(Moon Age × 360° ÷ 29.53)) ÷ 2 × 100

Traditional gardening guide by moon phase:

• New Moon to First Quarter (Waxing Crescent): Plant leafy crops and annuals (lettuce, spinach, cabbage), rising sap, good germination
• First Quarter to Full Moon (Waxing Gibbous): Plant fruiting crops (tomatoes, squash, peppers, beans), increased moisture in soil
• Full Moon to Last Quarter (Waning Gibbous): Plant root vegetables, bulbs, and perennials (carrots, beets, garlic, trees)
• Last Quarter to New Moon (Waning Crescent): Rest, weed, harvest, till, apply compost, minimal planting

What each variable means:

• Synodic month (29.53 days): measured from new moon to new moon; different from the sidereal month (27.32 days), which measures the moon’s orbit relative to distant stars
• Gravitational effect on water: the traditional argument for lunar gardening is that the tidal forces moving the oceans also move soil moisture. Worth knowing that controlled trials have not found a measurable effect on germination or yield, so treat the planting guide above as folklore worth trying rather than settled agronomy
• Lunar perigee: when the moon is closest to Earth (largest apparent size); associated with stronger tidal forces and what is popularly called a “supermoon”

Worked example: What phase was the moon on February 5, 2024?

The calculator counts forward from a known new moon, 6 January 2000 at 18:14 UTC, and takes the remainder after whole cycles. To noon on 5 February 2024 that is 8,795.7 days, and 8,795.7 mod 29.53059 leaves the moon 25.16 days into its cycle.

Phase = 23.07–28.61 days → Waning Crescent

Illumination = (1 − cos(25.16 × 360° ÷ 29.53)) ÷ 2 × 100 = (1 − cos(306.7°)) ÷ 2 × 100 = (1 − 0.597) ÷ 2 × 100 = 20.1% illuminated

Count the calendar squares by hand from the January 11 new moon and you get a round 25 days, which works out at 21.5%. The gap is the new moon itself falling at 11:57 UTC rather than at midnight, so a hand count is always a fraction of a day out. Everything here is reckoned at noon UTC, which is why the answer for a given date is the same whether you load the page in Los Angeles or Tokyo.

Gardening action for this phase: focus on weeding, harvesting root vegetables, and preparing beds for the upcoming new moon planting cycle.