Mercury is the closest planet to our Sun. That may be the most obvious fact about the planet. However, there are many more interesting features that aren’t so obvious. Here are ten fascinating facts about Mercury that make it unique.
1. The Smallest Planet
Mercury's diameter measures in at about 3,032 miles (4,880 km), making it the smallest planet in our solar system. 
In comparison, it is less than half the diameter of the Earth.
This title was previously held by Pluto, which has a diameter of about 1,485 miles (2,390 km) . Of course, Pluto was demoted from planet to dwarf-planet in 2006, when the definition of planet was modified and Pluto did not fit the new definition. 
2. Greatest Range in Temperature
With a range of 1,080 degrees, Mercury’s surface has the greatest range in temperature out of all the planets in our solar system. 
At night time, the dark side of the planet can get as low as -279 degrees Fahrenheight (-173 Celsius). The lowest temperature ever recorded on Earth was -129 degrees Fahrenheight (-89 degrees Celsius) in Antarctica. 
During the day, the light side of the planet can reach up to 801 degrees Fahrenheight (427 degrees Celsius). The highest temperature ever recorded on Earth was 136 degrees Fahrenheight (57 degrees Celsius) in Africa. 
If you traveled to the coldest place on Earth and then crawled into a typical consumer-grade oven that was turned up as hot as it could get (mine reaches 600 degrees Fahrenheight), you still would not even get close to experiencing the temperature range on Mercury.
3. Most Eccentric Orbit
Mercury has the highest orbital eccentricity of all the planets in our solar system, with a score of .2056. 
Orbital eccentricity is plainly explained on the NASA website:
“Orbital Eccentricity - This is a measure of how far a planet's orbit about the Sun (or the Moon's orbit about the Earth) is from being circular. The larger the eccentricity, the more elongated is the orbit, an eccentricity of 0 means the orbit is a perfect circle. There are no units for eccentricity.” 
Mercury ranges from 29 million miles (46,670,976 kilometers) at its closest distance from the sun to 44 million miles (70,811,136 kilometers) at its furthest. 
4. Thinnest Atmosphere
Of all the planets in our solar system, Mercury has the thinnest atmostphere, weighing in at approximately 0.00000000000001 bars. Where 1 bar (1.0) is the average atmospheric pressure at sea level on Earth. 
Credit: Image courtesy of NASA
Due to this lack of an atmosphere, the surface of Mercury is full of craters. It looks very similar to our moon. 
5. Slightest Axis Tilt
At zero degrees, Mercury’s axis is tilted less than any other planet in our Solar system. 
The tilt of a planet’s axis is measured with respect to its orbital plane around the sun. In Mercury’s case, its equator is facing the sun directly and its north and south poles point at 90 degree angles from the sun. In comparison, the Earth is tilted at 23.45 degrees.
6. Lowest Escape Velocity
Mercury has an escape velocity of 2.64 miles per second (4.25 kilometers per second), which is the lowest excape velocity of all the planets in our solar system. 
Gravity is what holds our solar system together. It pulls the planets in toward the sun and pulls the moons toward their respective planets. The planet’s velocity (speed of something in a given direction) counteracts with the gravitational pull of the sun to prevent the sun from immediately absorbing the planet. If the velocity of a planet were steadily increasing over time, it would eventually overcome the gravitational pull of the sun and leave its orbit (called “escape velocity”). In the case of Mercury, if its velocity were to increase to 2.64 miles per second (4.25 kilometers per second) it would escape its orbit around the sun and eventually leave our solar system.
However, there is no worry of this event occuring. Mercury is not going anywhere. The escape velocity is a hypothetical measurement.
7. A Day on Mercury Lasts Two Thirds of its Year
A single day on Mercury lasts just barely over two thirds of its year. 
One full day is the time it takes for a planet to completely spin 360 degrees on its axis. In comparison to Earth, it takes 59 Earth days for Mercury to complete one full rotation. That’s nearly two months of sunlight, or two months of darkness!
One full year is the time it takes for a planet to completely revolve around the sun. Since Mercury is much closer to the sun and has a much shorter orbit, it takes much less time. It takes Mercury 88 Earth days to completely revolve around the sun.
It makes you wonder how many hours are in a work week on Mercury!
8. Greatest Orbital Inclination
Of all the planets in our solar system, Mercury has the greatest orbit inclination of 7 degrees. 
If you were to draw a line of a planet’s orbit around the Sun, it would form a plane called an ecliptic. All of the planets in our solar system revolve around the sun on ecliptics that to not vary much from each other. However, Mercury has the greatest inclination. Coming in at second is Venus with a 3.39 degree inclination. Earth’s orbital inclination is only 0.00005 degrees!
9. Second Hottest Planet
Mercury can reach up to 801 degrees Fahrenheight (427 degrees Celsius) at its surface, making it the second hottest planet in our solar system. 
Since Mercury is the closest planet to the sun, many would assume it’s the hottest planet. However, that is just not the case! If you recall fact number 4, Mercury has an extremely thin atmosphere. On the other hand, Venus (the second planet from the sun) has a very thick atmosphere. Therefore, Venus can reach temperatures over 860 degrees Fahrenheight (460 degrees Celsius).
10. Black Sky All Day and Night
Since Mercury has such a thin atmosphere, it’s sky is completely black even during the day. 
Credit: Image courtesy of NASA
Imagine waking up in the morning and walking outside to full sunlight and a completely black sky (other than the fact that the Sun would be gigantic!) Since Mercury has such a thin atmosphere, there isn’t much there to hold in all the gases. Therefore, when you look up into the sky you would see the darkness of space rather than the beautiful blue we see here on Earth.