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 — 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 the night before a full moon reads Full Moon at 99% lit, which is what you actually see in the sky.

• 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: A known New Moon occurred on January 11, 2024. What phase was the moon on February 5, 2024?

Days elapsed = January 11 to February 5 = 25 days Moon age = 25 mod 29.53 = 25 days (under one full cycle) Phase = 23.07–28.61 days → Waning Crescent

Illumination = (1 − cos(25 × 360° ÷ 29.53)) ÷ 2 × 100 = (1 − cos(304.8°)) ÷ 2 × 100 = (1 − 0.570) ÷ 2 × 100 = 21.5% illuminated

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