☿ Mercury

How to follow Mercury live

The panel above recomputes Mercury every second in your browser: how far it is from the Sun and from Earth (in AU and in kilometers), its apparent magnitude, the illuminated fraction, the phase angle and the elongation from the Sun. The magnitude tells you at a glance whether Mercury is visible to the naked eye tonight or needs a telescope.

The top-down map of the Solar System then shows exactly where Mercury sits among the other planets right now. Run time with the day slider, zoom and pan, click a second planet to compare distances, or press "Next event" to jump to the next opposition or closest approach to Earth, the best moment to observe it.

Quick facts

About Mercury

Mercury is the smallest planet in the Solar System and the one that orbits closest to the Sun. At about 4,879 km across, it is only a little bigger than the Moon, and that extreme nearness to the star makes it a world of records: the most brutal temperature swing of any planet, the fastest orbit and the proportionally largest metallic core in the entire Solar System.

Despite sitting closest to the Sun, Mercury is not the hottest planet. With almost no atmosphere to hold heat, it loses energy fast after sunset. One side cooks at 430 °C while the other freezes at -180 °C, sometimes separated by just a few kilometres at the terminator line.

Physical characteristics

Mercury's diameter is 4,879 km, less than half Earth's. The surface is grey and densely pitted with impact craters, looking a lot like the Moon, with one standout feature: the Caloris Basin, an impact scar over 1,550 km wide formed about 3.9 billion years ago when a giant asteroid struck the planet and sent shockwaves through the interior to the opposite side, creating a jumbled region called the chaotic terrain.

Mercury holds the proportionally largest metallic core in the Solar System, filling roughly 85 percent of the planet's radius. The most accepted explanation is that a catastrophic collision stripped away much of the original mantle. That oversized core still generates a weak magnetic field, which surprised scientists who expected to find it dormant.

The crust wrinkles into long cliffs called rupes, or lobate scarps, stretching hundreds of kilometres. They form because the core cools and contracts over billions of years, shrinking the planet like a drying fruit. Estimates suggest Mercury has lost between 5 and 7 km of radius since it formed.

Orbit and motion in the sky

Mercury completes one lap around the Sun in just 88 Earth days, the fastest orbit of any planet, reaching 47 km/s at perihelion. Its average distance from the Sun is about 58 million km (0.39 AU), but the orbit is quite elliptical: at its closest point (perihelion) it is 46 million km out, and at its farthest (aphelion) it stretches to 70 million km.

Its rotation is slow and locked in a 3:2 resonance with its orbit: the planet spins three times for every two trips around the Sun, a stable orbital pattern. As a result, one full solar day on Mercury, from one sunrise to the next, lasts about 176 Earth days, two full Mercurian years. For part of the day the Sun actually appears to slow down, reverse briefly in the sky, and then move forward again, because the varying orbital speed outpaces the slow rotation.

How to see Mercury tonight

Mercury is the hardest planet to catch with the naked eye, not because it is faint, but because it always clings to the Sun in the sky. It only shows up at twilight: just after sunset, low on the western horizon, or shortly before sunrise on the eastern horizon. The viewing window is short, a matter of days per apparition.

• The ideal moment is when the planet reaches its greatest elongation from the Sun (maximum angular separation), which ranges from 18° to 28° depending on its position in the elliptical orbit.
• Look for a clean horizon with no buildings or mountains, and watch in the minutes when the sky still holds an orange glow. Binoculars help you find it, but never aim them toward the Sun.
• Through a telescope Mercury shows phases like the Moon: crescent, quarter, gibbous and full, changing as it shifts position relative to the Sun.
• Apps like Stellarium or SkySafari can tell you exactly where Mercury is tonight and whether it is well-placed for viewing from your location.

Space exploration and missions

Mercury is one of the least visited planets. Distance is not the main hurdle; speed is. Getting there and slipping into orbit means shedding enormous velocity against the Sun's strong gravity, which burns through fuel fast.

NASA's Mariner 10 flew past Mercury three times in 1974 and 1975, photographing about 45 percent of the surface. MESSENGER, also from NASA, became the first spacecraft to orbit the planet in 2011, mapping 100 percent of the surface before being deliberately crashed in 2015. Among its findings: permanent water ice at the bottom of polar craters and sulfur deposits on the surface.

BepiColombo, a joint ESA and JAXA mission launched in 2018, consists of two separate orbiters. They arrive at Mercury in December 2025 and begin science operations in 2026, equipped with far more advanced instruments than MESSENGER to study the magnetic field, core and crust in detail.

Ice at the poles and other paradoxes

It sounds contradictory, but Mercury has permanent water ice, and plenty of it. Detected by ground-based radar in the 1990s and confirmed by MESSENGER, the ice sits at the bottom of deep polar craters that never receive direct sunlight. With no atmosphere to spread heat, those regions stay permanently frozen below -170 °C, while just a few kilometres away the sunlit surface exceeds 400 °C.

Another paradox: Mercury's surface is coated in a thin layer of dark regolith, but MESSENGER found patches of very low-albedo material concentrated near the polar ice, possibly complex organic compounds delivered by comets and asteroids over billions of years.

Facts and records

• Mercury is the smallest planet and has the second-largest metallic core proportional to its size of any planet, behind only Earth.
• Its daily temperature swing is the largest of any planet: more than 600 °C between the hottest and coldest points.
• Mercury's orbit shows a precession (a slow rotation of the orbital axis) that classical Newtonian physics could not fully explain. Only Einstein's general relativity, in 1915, accounted for the missing 43 arc-seconds per century.
• The BepiColombo mission is named after Italian mathematician Giuseppe "Bepi" Colombo, who worked out the Mariner 10 trajectory and identified Mercury's 3:2 spin-orbit resonance.
• Mercury has no moons and no rings: its weak gravity combined with the Sun's proximity makes it impossible to hold natural satellites in stable orbit.

In mythology and culture

For the Romans, Mercury was the messenger of the gods, linked to speed and commerce. The name is a direct reference to the planet's rapid motion across the sky, covering the entire zodiac in just 88 days. The Greeks called it Hermes, and both the chemical element mercury (Hg, from the Latin hydrargyrum) and the planet share that mythological root tied to quickness and fluidity.

In traditional astrology Mercury governs communication, intellect and short journeys. The popular phrase "Mercury retrograde", used in astrology to describe a period when the planet appears to drift backward in the sky (a geometric optical effect with no demonstrated physical influence), has become a broad cultural shorthand for explaining misunderstandings and communication glitches.

Other planets

• ♀ Venus
• ♂ Mars
• ♃ Jupiter
• ♄ Saturn
• ♅ Uranus
• ♆ Neptune