Moon
Reference data for Earth's Moon: 27.32-day sidereal period, 384,400 km semi-major axis, laser-ranging recession, and lunar observing metrics.
Provenance-Backed Scientific Measurements
Every measurement below preserves its unit, uncertainty interval, observational method, reference epoch, quality status, and primary catalog source. See Uncertainty & Provenance for how competing measurements are handled.
| Parameter | Value | Uncertainty | Method & Epoch | Quality Status | Source Archive |
|---|---|---|---|---|---|
| Mean Full Magnitude (V) | -12.74 mag | ± 0.05 mag | JPL Horizons DE440/441 planetary and satellite ephemeris solution | definitive | NASA/JPL Solar System Dynamics & Planetary Fact Sheets |
| Sidereal Orbital Period | 27.321661 d | ± 0.000001 d | Synodic phase period (New Moon to New Moon) is 29.530589 days. | definitive | NASA/JPL Solar System Dynamics & Planetary Fact Sheets |
| Geocentric Semi-Major Axis | 0.0025696 AU | ± 1e-7 AU | Mean center-to-center distance of 384,400 km (0.0025696 AU), receding at 38.3 mm/yr via tidal friction. | definitive | NASA/JPL Solar System Dynamics & Planetary Fact Sheets |
| Mean Volumetric Radius | 0.2727 R⊕ | ± 0.0001 R⊕ | Mean radius of 1,737.4 km. | definitive | NASA/JPL Solar System Dynamics & Planetary Fact Sheets |
| Mass | 0.0123 M⊕ | ± 0.000001 M⊕ | Earth-to-Moon mass ratio of 81.30057. | definitive | NASA/JPL Solar System Dynamics & Planetary Fact Sheets |
| Mean Angular Diameter | 31.07 arcmin | ± 1.85 arcmin | Ranges from 29.3′ at apogee to 33.5′ at perigee. | definitive | NASA/JPL Solar System Dynamics & Planetary Fact Sheets |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots The Moon (Luna) — Orbital & Physical Data at RA 04h 35m 00.00s, Dec +22° 00′ 00.0″ within Taurus, alongside peak meridian transit altitudes for 40°N and 30°S observers. Use the RA/Dec Coordinate Converter to compute exact culmination altitude for your latitude.
Astrophysical Context & Structure
Measuring 3,474.8 kilometers in diameter—more than one-quarter the width of Earth—the Moon possesses the largest satellite-to-planet mass ratio (1:81.3) of any major planet in the Solar System. Isotopic analyses of Apollo, Luna, and Chang'e rock samples indicate that the Moon coalesced roughly 4.51 billion years ago from a circumterrestrial debris disk blasted into orbit when a Mars-sized protoplanet (Theia) struck the proto-Earth. Millimeter-precision Lunar Laser Ranging to retroreflectors placed by Apollo 11, 14, and 15 and Lunokhod rovers shows that tidal angular momentum transfer pushes the Moon outward by 3.83 centimeters per year.
The lunar crust is sharply divided into two geologic provinces: the bright, heavily cratered anorthositic highlands that solidified from a primordial magma ocean, and the dark basaltic maria that flooded giant impact basins on the nearside between 3.9 and 3.1 billion years ago. Because the Moon's rotational axis is tilted by just 1.54 degrees to the ecliptic, crater floors near the south and north poles remain in permanent shadow at temperatures below 40 to 100 K, trapping hundreds of millions of tons of water ice.
Although the Moon's rotation period matches its 27.32-day orbital period, its elliptical (e = 0.0549) and inclined (5.14°) orbit produces optical librations in longitude and latitude, allowing terrestrial observers to inspect 59 percent of the lunar globe over time. Topographic relief along crater walls, rilles, wrinkle ridges, and central peaks stands out most dramatically within 15 degrees of the sunrise or sunset terminator, where low-angle sunlight casts long shadows.
Practical Visual Observing Guide
- Best observed near First Quarter and Last Quarter phases along the day-night terminator; Full Moon flattens topographic shadows despite high albedo surges.
- Optical libration exposes limb features such as Mare Orientale, Mare Humboldtianum, and the south-polar Leibnitz mountains.
- Because its ~31-arcminute angular size closely matches the Sun's (~32′), the Moon produces total or annular solar eclipses when crossing the ecliptic nodes at New Moon.
Catalog Cross-Identifications & Aliases
Common Aliases: Luna · Earth I · Selene
| Catalog Authority | Identifier |
|---|---|
| SIMBAD | Moon |
Related Objects, Comparisons & Guides
The Sun (Sol) — G2V Star & Solar System Data
Reference data for the Sun: 5,772 K effective temperature, 1.00 AU mean distance, photospheric angular diameter, and fundamental solar constants.
Ganymede (Jupiter III) — Largest Moon Profile
Reference data for Jupiter's moon Ganymede: 5,268 km diameter, 7.15-day orbit, intrinsic magnetosphere, subsurface ocean, and JUICE target.
Callisto (Jupiter IV) — Outer Galilean Moon
Reference data for Jupiter's moon Callisto: 4,821 km diameter, 16.69-day non-resonant orbit, Valhalla multi-ring basin, and internal ocean.
Angular Size, Separation & Field of View
How angular diameter (degrees, arcminutes, arcseconds), spherical angular separation, and small-angle physical diameter relate to telescope field of view.
Apparent vs Absolute Magnitude Explained
Understand the logarithmic Pogson astronomical magnitude scale, apparent visual magnitude (m), absolute magnitude (M at 10 parsecs), and bolometric corrections.
Scientific Sources & Dataset Provenance
- NASA/JPL Solar System Dynamics & Planetary Fact Sheets (DE441 (2021/2026)): Park, R. S., et al. 2021, AJ, 161, 105 (The JPL Planetary and Lunar Ephemerides DE440 and DE441) — NASA Jet Propulsion Laboratory / GSFC