Facts About Mercury: All You Need to Know

~7 min

Mercury is very different from other planets in our Solar System. Today we’ll tell you more about its peculiarities and explain how they affect this little planet. Let’s get started!


Mercury Planet Facts

  • Planet type: terrestrial
  • Radius: 2440 km (1516 miles)
  • Mass: 3.3011×10^23 kg
  • Aphelion: 69.8 million km (43.4 million miles)
  • Perihelion: 46.0 million km (28.6 million miles)
  • Average distance from the Earth: 77 million km (48 million miles)
  • Surface temperature: -173°C to 427°C (−280 °F to 800 °F)
  • Solar day length: 176 Earth days
  • Sidereal day length: 59 Earth days
  • Year length: 88 Earth days
  • Age: 4.503 billion years
  • Named after: Roman god of commerce

How big is Mercury?

Mercury is the smallest planet in the Solar System in both mass and diameter. It’s 18 times less massive than the Earth, and its diameter barely reaches two-fifths of the Earth’s size. To give you a better understanding of Mercury’s size, let’s say that it’s only a third larger than the Moon.

Surprisingly enough, this small planet got even smaller over time. Cooling of its core has caused the entire planet to shrink, reducing Mercury’s volume by about 5-10 kilometers in radius.

How hot is Mercury?

The temperature of Mercury’s surface varies more than on any other planet in our Solar System. It can range from −173 °C (−280 °F) at night to 427 °C (800 °F) during the day. This happens because the smallest planet has almost no atmosphere to trap heat, and it quickly loses the energy received from the Sun during the daytime.

Although Mercury is the closest planet to the Sun, it isn’t the hottest one (Venus is).

How long is a day on Mercury?

A solar day — the amount of time it takes the Sun to rise, set and rise to the same place again — on Mercury lasts about 176 Earth days. And here is another weird fact about Mercury: one year on this planet is twice shorter than a single day and takes 88 Earth days. How come?

The smallest planet spins slowly about its axis — one rotation takes 59 Earth days. However, its sunrises and sunsets are quite different from the Earth’s ones. Due to the planet’s eccentric orbit, the Sun appears to rise twice: once, shortly before setting, and then again from some parts of the surface. The same thing occurs in reverse at sunset. Thus it takes much longer for the Sun to appear in the same place again, and one solar day lasts almost twice as long as a year.

Even though it spins slowly, Mercury travels around the Sun faster than any other planet — at 47 km/s (29 mi/s). In comparison, the second-fastest planet in our Solar System, Venus, has a speed of 35 km/s (21 mi/s).

What is Mercury made of?

Since Mercury is a terrestrial planet, it’s mainly composed of iron, nickel, and silicate rock. It has a large iron core, which is about 61% of the planet’s volume (in comparison, the Earth’s core is only 16%), and a 400 kilometers (250 miles) thick outer shell. By the way, Mercury’s core is about the same size as our Moon. Another similarity with the Moon is that the planet’s dry and rocky surface is full of craters.

Craters are the results of impacts with asteroids or other space objects — the more craters a planet has, the older it is. Since Mercury’s surface is heavily cratered, it’s most likely ancient.

What is Mercury retrograde?

Mercury is said to be in retrograde when it appears to go “backward” for observers from the Earth. In reality, the planet doesn’t change its direction — this is only an optical illusion in the sky. You can read more about the phenomenon of retrograde motion in one of our recent articles.

The speedy little planet goes retrograde three or four times in a calendar year for about three weeks. In 2023, Mercury will be in retrograde from April 21 to May 14, from August 23 to September 15, and from December 13 to January 1, 2024.

When is Mercury visible in the night sky?

In 2023, Mercury will be visible in the morning sky from January 23 to February 13, from May 22 to June 12, and from September 16 to September 30. In the evening, look for the planet from April 2 to April 18, from July 27 to August 17, and then from November 20 to December 11.

Since the planet is very close to the Sun, there are not many opportunities during a year to observe it. The easiest way to find Mercury in the sky is to use the planetary guide Star Walk 2. Just type the name of a planet in the search field, and the app will show you its position in the sky.

Upcoming Events

Mercury is not easy to see. The planet often gets close to the Sun in the sky, so the sunlight outshines it. Learn when to see the elusive planet with the Sky Tonight app. Tap the magnifier icon in the lower part of the main screen, enter the planet’s name in the search field, and choose the corresponding result. Then, go to the Events tab and see the list of all Mercury-related astronomical events. A quick tip: the planet is best visible at greatest elongation when its apparent distance from the Sun is the largest.

September 22: Mercury at greatest elongation west

On September 22, at 11:27 GMT (7:27 a.m. EDT), Mercury will reach the farthest apparent distance from the Sun (17°52′). The greatest elongation is the best time to observe Mercury, as the elusive planet isn’t lost in the Sun’s glare. On the same day, the planet will also climb the highest in the morning sky and will be shining brightly at a magnitude of -0.4. Observe the planet before sunrise in the constellation Leo.

September 23: Mercury at perihelion

On September 23, at 18:06 GMT (2:06 p.m. EDT), Mercury will reach perihelion, meaning it will be at its closest to the Sun in space. The distance between the two bodies will be 0.31 AU. Although the event has an incredible effect on the planet’s surface temperatures, it makes little difference to Mercury’s appearance in the sky. The planet will be visible in the morning sky in the constellation Leo.

October 14: Moon near Mercury

  • Close approach time: 08:58 GMT (4:58 a.m. EDT)
  • Close approach distance: 0°36'

On October 14, the New Moon will meet Mercury (mag -1.4) in the constellation Virgo. Both celestial objects will rise almost simultaneously with the Sun, so it will be hard to see them close together. However, on this day, observers from the United States, Mexico, Belize, Honduras, Nicaragua, Panama, Colombia, and Brazil will see a rare celestial event called “ring of fire”, or annular solar eclipse, which occurs when the Moon passes in front of the Sun.

November 14: Moon near Mercury

  • Conjunction time: 14:04 GMT (9:04 a.m. EST)
  • Conjunction distance: 1°37'

On November 14, the Moon will meet Mercury (mag -0.4) in the constellation Scorpius. The elusive planet will be an evening object and will set an hour after the Sun. The lunar disc will be almost invisible, as the New Moon will occur the day before.

December 14: Moon near Mercury

  • Conjunction time: 05:47 GMT (0:47 a.m. EST)
  • Conjunction distance: 4°22'

On December 14, the 5%-illuminated Moon will meet Mercury (mag 0.8) in the constellation Sagittarius. The lunar disc will be almost invisible, and the planet will set one hour after the Sun, so you won't have much time to find it in the evening sky.


What color is Mercury?

Mercury is primarily dark gray. A planet’s color depends on the color of its surface and its reflective abilities. Unfortunately, Mercury’s rocky exterior covered with dust doesn’t give a wide variety of colors.

How many moons does Mercury have?

None. Without going into details, Mercury just cannot form, capture, or acquire a moon due to the planet’s small size, weak gravity, and proximity to the Sun.

How far is Mercury from the Sun?

Its average distance from the Sun is about 58 million km (36 million miles). Thus, among other planets in the Solar System, Mercury is the closest one to the star.

When was Mercury discovered?

We don’t know for sure — the first recorded observation was made in 265 BC. Because Mercury is so close to the Sun, it’s often hidden by its glare. The first people who observed Mercury through a telescope were Galileo Galilei and Thomas Harriot in the 17th century.

Why is Venus hotter than Mercury?

Despite Mercury being the closest planet to the Sun, Venus is hotter because of the thick Venusian atmosphere trapping heat. Mercury, in its turn, has no significant atmosphere and can’t hold the Sun’s energy.

What does the Sun look like from Mercury?

If you could stand on Mercury, at aphelion (the farthest Mercury can get from the Sun), you would see the Sun twice bigger than it appears from the Earth. At perihelion (the closest distance), the Sun would appear three times larger than it does from our planet.

Did you know?

  • There are no seasons on Mercury because its axis has almost no tilt (only 2 degrees).
  • Ancient astronomers believed Mercury was two different objects because it can appear in the evening western sky or rise in the morning eastern sky. By the way, they thought the same about Venus.
  • Like the Moon and Venus, Mercury has phases, but you can see those only through a telescope.
  • Mercury is almost certainly not habitable. Its rough temperatures and closeness to the Sun make the planet too extreme for living organisms.
  • Mercury has a tail. Many people know comets have tails. But have you heard about a planet that has one? Mercury has a cometlike tail produced by the solar wind that pushes sodium atoms off the planet’s surface. It’s not easy to see: you’ll have to take a long-exposure photo using a telescope and a special filter.
  • In Sanskrit, Mercury is called Budha (not to be confused with Buddha, the founder of Buddhism). The Vedic texts call it Budh planet and refer to it as a deity, the son of Soma (or Chandra, the Moon god) and Tara (wife of Bṛhaspati, the god of Jupiter).

Mercury is, without a doubt, one of the most extreme planets of all, and now you know why. Share this article with your friends and keep learning more about astronomy with Star Walk 2. You can also watch the fun and educational cartoon about Mercury that explains the main facts about the planet in simple words.

Wishing you clear skies and happy observations!