Mercury

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.

J2000 Equatorial Sky Chart & Culmination Altitude

The chart below plots Mercury — Orbital & Physical Reference Data at RA 18h 40m 00.00s, Dec -24° 10′ 00.0″ within Sagittarius, 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

Sweeping along the most eccentric orbit of any major planet (e = 0.2056), Mercury ranges from 46.0 million kilometers from the Sun at perihelion to 69.8 million kilometers at aphelion across its 87.97-day year. Radar Doppler echoes transmitted from Arecibo in 1965 revealed that instead of being synchronously locked in a 1:1 state, Mercury rotates exactly three times on its axis (58.646 days) for every two revolutions around the Sun—resulting in a solar day lasting 175.94 Earth days. Its anomalous perihelion advance of 42.98 arcseconds per century beyond Newtonian planetary perturbations provided Albert Einstein with the first triumph of general relativity in 1915.

Although Mercury is slightly smaller in diameter than the moons Ganymede and Titan, its uncompressed density is the highest in the Solar System. Gravity and magnetic measurements from NASA's MESSENGER orbiter confirmed that a partially molten metallic iron-nickel-sulfur core occupies roughly 82 percent of the planet's radius and 70 percent of its mass, overlain by a thin 400-kilometer silicate mantle and crust. As the interior cooled over billions of years, the planet's radius shrank by up to 7 kilometers, buckling its surface with global lobate thrust-fault scarps.

Because Mercury orbits inside Earth's path, its angular elongation from the Sun never exceeds 17.9 degrees at perihelic elongations or 27.8 degrees at aphelic elongations. Observers must catch it low in bright morning or evening twilight, where it displays a complete cycle of lunar-like phases.

Practical Visual Observing Guide

• Maximum elongation from the Sun ranges between 18° and 28°; best viewed from mid-northern latitudes during spring evening apparitions and autumn morning apparitions when the ecliptic stands steep against the horizon.
• Apparent disk diameter varies from 4.5″ (full phase at superior conjunction) to 13.0″ (thin crescent at inferior conjunction); a crescent or half-phase disk is easily resolved at 100x–150x.
• Transits across the solar disk occur 13 or 14 times per century in May or November (next on November 13, 2032).

Catalog Cross-Identifications & Aliases

Common Aliases: Hermes · Innermost Planet · Sol I

Related Objects, Comparisons & Guides

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