How Far Is Mercury From the Sun?

Mercury is an average distance of 36 million miles (58 million kilometers) from the Sun. When answering the question How Far Is Mercury From the Sun? we have to think about how this small world stems from its close proximity to the Sun. From its formation to its thin exosphere, the smallest planet and our Solar System’s brightest star are intertwined.

Scientists measure a planet’s distance from the Sun by astronomical units (AU). And one AU is the distance from Earth to the Sun. Since Mercury is the closest planet, it is only 0.4 astronomical units away. That means it takes sunlight a brief 3.2 minutes to warm the small world.

Let’s delve into other ways how far Mercury is from the Sun affects this small world with a big core.

Mercury Images
MESSENGER Mercury Images: NASA

Learning About Mercury

Mercury’s closeness to the Sun made the job of early Earth-bound scientists very difficult. And it wasn’t until the 1960s that the scientists could finally determine the length of the small planet’s day.

NASA’s Mariner 10 flew by Mercury three times in 1974-75 while it orbited the Sun. And scientists used flight information from that mission to understand the amount of energy needed to shape a spacecraft’s trajectory from Earth to Mercury.

Then in 2004, NASA launched a probe to answer some of Mercury’s mysteries. The MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging) probe flew past the planet three times in 2008-09 before settling into orbit in 2011. Then it mapped Mercury’s surface before its 2015 crash into it.

Keeping the spacecraft’s instruments at a low operating temperature challenged MESSENGER’s designers. In addition to the extreme heat from Mercury’s closeness to the Sun, the planet’s reradiated heat also causes coolant issues.

How Far Is Mercury From the Sun?
NAC Mosaic Impact Crater – Hokusai: NASA/Carnegie Institution of Washington/Johns Hopkins University Applied Physics Laboratory

Mercury’s Astronomical Data

Because of how close (or far) Mercury is from the Sun, it is a tiny but extreme planet.

Average Orbital Distance From the Sun~36 million miles (~58 million km)
Perihelion (Closest) Distance To the Sun28,583,820 miles (46,001,200 km)
Aphelion (Furthest) Distance From the Sun43,382,210 miles (69,816,900 km)
Length of Day59 Earth days (to complete one axis rotation)
Solar Day176 Earth days (to complete one day-night cycle)
Shortest Year of all Planets88 days
Smallest major planet radius1,516 miles (2,440 km)
Moons larger than MercuryJupiter’s Ganymede, Saturn’s Titan
Dense for its MassHighest of all planets
Iron Core Radius1,300 miles (2,100 km)
Surface Crust and Mantle Thickness200 miles (300 km)

Orbital and Rotational Effects On Mercury From the Sun

Mercury zips around the Sun every 88 days. The speedy planet travels almost 29 miles (47 km) per second and is faster than all the other planets. It also has an egg or oval-shaped, high eccentric orbit, so Mercury darts closer to the Sun before zooming back away.

Orbital Eccentricity

The elongated orbit makes the Sun appear twice as bright when the planet is closest to it. Early astronomers thought Mercury was tidally locked. That would mean it perpetually kept one side to the Sun while the other remained in darkness.

But radar observations in the 1960s proved otherwise. Scientists initially thought that Mercury’s rotation and orbital period were synchronous, meaning the rotation period matched the planet’s 88-day revolution. 

However, they now know that Mercury’s rotation period of 58.6 days is precisely two-thirds of its orbital period. So, during formation, Mercury likely spun faster. But strong solar tides and the friction between its molten core and solid mantle slowed the small world down. So, instead of slowing to a synchronous rate, the planet got trapped at three rotations for every two Sun orbits. 

Perihelion and Aphelion

You may still wonder how much traveling the planet does as it moves closer and then further away from the sun, and how far Mercury is from the Sun. On average, Mercury stays about a third of the distance between the Sun and Earth. But since it has the greatest orbital eccentricity (0.205) of all Solar System planets (except dwarf-planet Pluto), Mercury travels in an elongated path.

  • Perihelion (Closest) Distance To the Sun: 28,583,820 miles (46,001,200 km)
  • Aphelion (Furthest) Distance From the Sun: 43,382,210 miles (69,816,900 km)

Spin-Orbit Resonance

Mercury rotates slowly over 59 days and an 88-day orbital period. So that gives it a 3.2 spin-orbit resonance, and as mentioned above, the planet spins on its axis three times for every two trips around the Sun.

That means there’s a massive difference in how long Mercury rotates on its axis (completing a sidereal day) and when the Sun reappears in the same place in the sky (completing a solar day.) 

So it takes 176 Earth days for Mercury to see a complete day-night cycle with the Sun rising, setting, and returning to its exact place. 

Perihelion Precession

Precession also affects Mercury’s perihelion (close Sun orbit), meaning that over a century, the planet’s Sun orbit shifts by 0.0119 degrees (42.98 arcseconds.) So after about 12 million rotations around the Sun, Mercury goes around an extra time before returning to its start.

Sidenote: Aren’t mathematics astounding? For example, Einstein’s theory of relativity accounted for the perihelion orbit, and the precession allowed him to test his theory. Honestly, though, it’s a level beyond my comprehension!

At any rate, Mercury’s proximity to the Sun has a giant perihelion precession compared to nearby planets.

  • Earth 0.001° (3.84 arcseconds)
  • Venus 0.0024° (8.62 arcseconds)
  • Mars 0.00037° (1.35 arcseconds)

So the close dance between Mercury and the Sun affects many factors in this small world. Next, let’s see what it does to the temperature.

MESSENGER Color Composition Image: NASA/Carnegie Institution of Washington/Johns Hopkins University Applied Physics Laboratory

Temperature Effects: How Far Mercury is From the Sun

Mercury has a mere two-degree tilt to its axis of rotation. So it spins nearly upright with respect to its orbital plane around the Sun. As a result, the planet doesn’t experience seasons like we do.

The slow rotation and how far Mercury is from the Sun create extreme temperature variations. For example, daytime temperatures can reach 800 degrees Fahrenheit (430 degrees Celsius.)

But the planet’s thin exosphere means there are no atmospheric insulation properties to hold that heat in. So, it escapes into space, and nighttime temperatures plummet to negative 290℉ (-180℃.) Now, that’s quite a swing!

Scientists think water ice could be inside Venus’s permanently shadowed north and south pole regions. In addition, they believe it could be cold enough deep within craters to keep the ice water from melting or evaporating. 

So even though the Sun’s proximity to Mercury creates insanely high temperatures in the world’s sunlit areas, the shadows stay permanently cold.

Summary: How Far is Mercury From the Sun

As the first planet from the Sun, Mercury challenged astronomers and scientists throughout time to learn more about her formation, rotation, and temperatures. NASA’s MESSENGER probe finally provided some clarity and gave the Mercury/Sun dance a clearer perspective.

The Sun pushes Mercury away and then pulls her close in an elongated, egg-shaped, eccentric orbit. And then it superheats the planet’s days while freezing its nights.

On average, Mercury remains about 36 million miles (58 million kilometers) from the Sun. That’s still close enough for mighty solar winds to whip surface particles into the atmosphere, only to strip them away into space later. It’s an intricate dance between this planet and our solar system’s most significant star.