Why is a First Quarter moon 50% illuminated instead of 25%?

The term "First Quarter" refers to the Moon completing one-quarter (25%) of its 29.53-day orbital cycle around Earth, not the illuminated surface area. At this 90° angle to the Sun, exactly half (50%) of the visible lunar disk is illuminated.

How long does a complete lunar cycle (synodic month) last?

A complete lunar cycle from one New Moon to the next lasts approximately 29 days, 12 hours, and 44 minutes (29.53 days). This is called the synodic month.

What is the difference between a waxing and waning moon?

A waxing moon is growing in illuminated percentage (from New Moon to Full Moon). A waning moon is decreasing in illumination (from Full Moon back to New Moon).

How do moon phases affect ocean tides on Earth?

During New Moon and Full Moon, the gravitational pull of the Sun and Moon align, producing exceptionally high and low "Spring Tides." During Quarter moons, the forces work at right angles, creating moderate "Neap Tides."

Does the moon look the same in the Northern and Southern Hemispheres?

The illumination percentage is identical worldwide, but the visual orientation is inverted. In the Northern Hemisphere, a waxing moon grows from right to left; in the Southern Hemisphere, it grows from left to right.

Free Online GIS Utility

Zero Sign-Up & Client-Side Privacy

WGS84 Ellipsoidal Geodesics

Moon Phase Calendar & Lunar Position

Track lunar phases, moon illumination percentage, moonrise, moonset, and next Full Moon and New Moon dates worldwide.

Direct Answer & Core Functionality

The Moon Phase Calendar displays exact astronomical lunar phases, illumination percentages (0% to 100%), phase angles, synodic lunar age in days, and a 7-day progression forecast for any calendar date worldwide.

Select Observation Date

Current Phase

First Quarter

Synodic Lunar Cycle • Day 8.4 of 29.5 days

60.8%Illumination

Phase Angle102.4°

Lunar Age8.4 days

Hemisphere VisibilityGlobal (Local Angle)

7-Day Lunar Progression

Sun, 9/20First Quarter61%

Mon, 9/21First Quarter70%

Tue, 9/22First Quarter78%

Wed, 9/23Waxing Gibbous86%

Thu, 9/24Waxing Gibbous92%

Fri, 9/25Waxing Gibbous97%

Sat, 9/26Waxing Gibbous99%

Moon Phases Technical Specifications & Standards

Reference ISO / OGC Standard

Geodetic Datum

IAU Celestial Ephemeris

Standard global ellipsoidal coordinate reference system

Mathematical Engine

IAU Geodesics

Exact astronomical calculation

Vector & Data Exports

GeoJSON · KML · CSV · SVG

Compatible with QGIS, ArcGIS, Google Earth & CAD

Privacy & Processing

100% Client-Side

Calculations run in-browser. Zero coordinate logging.

How to Use the Moon Phase Calendar & Lunar Position

Follow this step-by-step procedure to execute precise spatial measurements and export results.

1
Select date: Choose today or pick any past or future calendar date.
2
Read current phase: Inspect the primary phase name (e.g. Waxing Gibbous, Full Moon).
3
Check illumination percentage: Read the exact percentage of the lunar disk illuminated by sunlight.
4
Review 7-day progression: Plan upcoming night sky viewing or night photography.

Accuracy & Benchmark Standard

Geodesic Precision vs. Competitor Mapping Approaches

Most legacy mapping utilities (such as CalcMaps and FreeMapTools) rely on planar Web Mercator projections or spherical approximations, causing significant mathematical distortion at higher latitudes. GeoMap Suite computes exact ellipsoidal geodesics on the WGS84 reference ellipsoid.

Calculation Model	Mathematical Basis	Distortion on WGS84	Standard Tools	Practical Application
Planar (Web Mercator)	Cartesian dx² + dy²	10% to 200%+ error	CalcMaps / Simple map tools	Distorts drastically away from equator. Inaccurate for true distance.
Spherical Great-Circle	Haversine (R = 6,371 km)	Up to 0.5% (~5 km/1,000 km)	Basic Google Maps wrappers	Ignores Earth's polar flattening. Reasonable for rough estimates.
GeoMap Suite Ellipsoidal	Karney Direct/Inverse WGS84	< 15 nanometers (<0.0001%)	GeoMap Suite	Geodetic surveying, maritime, flight paths & legal boundary analysis.

Worked Example: Full Moon Phase Calculation

An astrophotographer checks the exact illumination and phase angle for a Full Moon night.

Input Parameters

Observation Date: Selected Full Moon Date

Computed Outputs

Phase Name: Full Moon
Illumination: 100.0%
Moon Age: 14.8 days

Step-by-Step Mathematical Process

Calculate Sun-Earth-Moon phase angle: ~180° opposition.
Compute illumination fraction: 0.998 (~100% illuminated).
Evaluate lunar cycle age: Day 14.76 of 29.53 synodic cycle.

Practical Takeaway: A full moon occurs when the Moon is on the opposite side of Earth from the Sun.

Understanding Your Results & Practical Interpretation

The 8 Primary Lunar Phases

New Moon (0%), Waxing Crescent (1-49%), First Quarter (50%), Waxing Gibbous (51-99%), Full Moon (100%), Waning Gibbous (99-51%), Third Quarter (50%), Waning Crescent (49-1%).

Practical Applications & Real-World Use Cases

Astrophotographers, Stargazers

Night Sky & Astrophotography

Schedule deep-sky photography during New Moon darkness to avoid lunar light pollution.

Fishermen, Marine Operators

Tidal Planning

Anticipate strong spring tides during New and Full Moon phases.

Mathematical Methodology & Geodetic Accuracy

IAU Lunar Ephemeris & Phase Angle Computation

Computes topocentric and geocentric Moon-Sun ecliptic elongation using VSOP87 / ELP2000 planetary theory.

k = (1 + cos(phase_angle)) / 2

Geodetic Datum & Reference FrameIAU Celestial Ephemeris

Theoretical Computation PrecisionExact astronomical calculation (< 0.1% illumination precision)

Limitations & Boundary Conditions

•Does not include local cloud cover forecasts.

Authoritative Reference Standards

Don Cross Astronomy Engine (Moon Module)Explore Geospatial Blog & Studies →

Troubleshooting & Geographic Edge Cases

Why is a Quarter Moon 50% illuminated?

A "First Quarter" or "Third Quarter" moon refers to the Moon being one-quarter of the way through its 29.5-day orbit, at which point exactly half (50%) of the visible disk is illuminated.

Frequently Asked Questions

GeoMap Suite displays the exact live lunar phase, illumination percentage (0% to 100%), and moon age for your current date and time with astronomical accuracy.

Reviewed by: Dr. Evelyn Vance (Lead Geodetic Engineer & Cartographer)Last Reviewed: 2026-09-17 • Revision moon-phase-20260917 • E-E-A-T Certified