Moon Phase Calculator - Phase, Age, and Illumination

A moon phase calculator is a date-driven tool that takes any calendar day and returns the named lunar phase, the lunar day within the current synodic cycle, the illuminated percent of the visible disc, and the dates of the next new and full moons. This moon phase calculator uses the standard 29.53058770576-day synodic month and a known reference new moon, so the same date always gives the same answer whether you check it today or ten years from now.

• • Plan a stargazing night: Check whether the Moon will be a thin crescent or a full disc on a planned trip date, so you can pick a darker window for meteor showers or deep-sky viewing.
• • Time a full-moon photo shoot: Look up exactly which date in a target month carries a full moon, then plan your shoot around moonrise or moonset for the best landscape light.
• • Plan outdoor activities by lunar phase: Fishing, gardening, hunting, and tide-sensitive coastal trips are often scheduled around specific phases, and a quick date lookup replaces a paper almanac.
• • Look up a historical or future phase: Type in a past birthday or a future wedding date to see what the Moon will look like that night without paging through a multi-year table.

The Moon goes through eight named phases on a roughly 29.5-day cycle, and the same cycle repeats for thousands of years with very small drift. Anchoring the calculation to a known new moon and using a fixed mean synodic month gives a useful answer for any date within a couple centuries of the present.

When you want to verify the math by hand, Days Between Dates Calculator gives you the exact whole-day count between the reference new moon and your chosen date.

The calculator converts your selected date to a count of days since a known reference new moon, takes the remainder when that count is divided by the length of the synodic month, and maps the remainder to one of the eight named lunar phases. The illumination percent comes from a simple cosine model of the Moon's elongation from the Sun.

• target_date: The calendar day the user wants the phase for, parsed into a UTC date at midday for a clean whole-day count.
• days_since_reference: Whole days and fraction between the target date and the reference new moon at 2000-01-06 00:00 UTC.
• synodic_month: The mean length of one full new-moon-to-new-moon cycle, fixed at 29.53058770576 days for this everyday calculator.

The reference new moon (2000-01-06) is the cycle's day zero. Picking any date and counting whole days forward gives a number we reduce modulo the synodic month. Anything in the first or last day of the cycle is a New Moon, around 7 days is a First Quarter, around 15 days is a Full Moon, and around 22 days is a Last Quarter.

The illumination percent uses the same lunar day. A cosine curve from 0 to 100 percent and back to 0 across the cycle gives a smooth estimate that matches the visual appearance of the disc for everyday use.

According to Wikipedia (Lunar month), the mean synodic month is 29 days 12 hours 44 minutes 3 seconds, or about 29.53059 days, and is the cycle that determines the visible lunar phases.

For a deeper look at the orbital period that drives this whole cycle, Synodic Period Calculator explains how the synodic month is derived from two sidereal orbital periods.

Four ideas make the result make sense: the synodic month that drives the cycle, the reference new moon that anchors the count, the named-phase boundary table, and the simple cosine model used for the illumination percent.

Waxing means the lit portion is growing, waning means it is shrinking. Crescent means less than half is lit, gibbous means more than half is lit. Together these four words describe the eight named phases in plain English.

If you are curious about the traditional Chinese lunar age system rather than the astronomical cycle, Lunar Age Calculator compares the two side by side.

Pick a date, read the named phase, then use the next-phase dates to plan ahead.

1. 1 Enter the date: Type a date in the YYYY-MM-DD field, or leave it blank to look up the Moon phase for today.
2. 2 Read the named phase: The primary result shows one of the eight named phases (New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter, Waning Crescent).
3. 3 Check the lunar day in the cycle: A second row shows how many days into the 29.53-day cycle the selected date is, starting from 0 at the most recent new moon.
4. 4 Read the illumination percent: The third row shows the percent of the visible disc that is lit by the Sun, useful for judging how bright the night will be.
5. 5 Find the next new and full moons: The last two rows give the dates of the next new moon and the next full moon from your selected date, so you can plan a week or a month ahead.

For example, type in 2026-06-14 to see today's Moon phase. The calculator will return the named phase, the lunar day, the illumination percent, and the next new and full moon dates so you can plan a stargazing night, a photo shoot, or a tide-sensitive activity without paging through a paper almanac.

If you would rather work with raw date arithmetic than a phase band, Date Calculator lets you add or subtract days and see the resulting calendar date.

A few seconds of typing replaces a paper almanac and gives you four pieces of information that are useful for everyday planning.

• • Plan around the light: Knowing the illumination percent tells you how bright the night sky will be, which matters for stargazing, astrophotography, and nocturnal wildlife watching.
• • Time tide-sensitive activities: Coastal fishing, kayaking, and beach walks are easier to plan when you know whether a spring tide is approaching (full or new moon) or a neap tide is in effect (first or last quarter).
• • Pick a wedding or event date: A full moon makes a striking visual for outdoor evening events, while a new moon is better for outdoor lighting setups that should not fight the sky.
• • Look up any date in seconds: Type in a past birthday, a planned trip, or a wedding date and get the phase immediately, without paging through a multi-year table or buying a paper almanac.
• • Cross-check astronomy apps: Use the eight named phases and the illumination percent to confirm what your sky-map app or astronomy software is showing, especially for unusual dates.

The math is the same simple model that printed almanacs have used for a century, and it is accurate to within a day or so for any date within a couple centuries of now.

When you are planning a multi-day stargazing trip, Date to Date Calculator gives you the exact duration between your arrival and the next new moon.

The calculator uses a simple fixed-length synodic month, so a few astronomical details can shift the result by a small amount.

• • The calculator uses a constant mean synodic month, so it can return the next new or full moon one day earlier or later than the official USNO ephemeris for some dates.
• • It does not return rise, set, or culmination times, nor the Moon's position in the sky. For horizon times and altitude, use a planetarium app alongside this tool.

For most everyday planning, a half-day tolerance on the next-phase date is perfectly adequate, and the named phase and illumination percent are reliable to within the rounding shown.

According to Wikipedia (Lunar phase), the approximate age of the Moon can be calculated for any date by counting the days since a known new moon (such as 6 January 2000) and reducing that modulo 29.53059 days, with the understanding that this simple model is accurate to within hours for everyday use.

According to the USNO Astronomical Applications Department, primary Moon phases occur on specific geocentric dates and times published in an authoritative almanac. For results that must match that almanac, verify the next-phase dates from this calculator against USNO tables rather than the simple fixed-cycle estimate.

If you want to know which named phase the Moon will be in on a specific upcoming birthday, Birthday Countdown Calculator shows the countdown in days to that date so you can plan a Moon-themed party.