Facts About Mercury: All You Need to Know

Image Credit: Vito Technology

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: 3.3011×10²³ 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 2021, its next retrograde motion will start on September 27 and last till October 17.

When is Mercury visible in the night sky?

In 2021, Mercury will be visible in the morning sky from June 11 to July 31 and from October 9 to November 28. In the evening, look for the planet from August 1 to October 8 and then from November 29 to December 31.

The best morning apparition will be between October 18 and November 1 — Mercury will shine at its brightest. 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

Learn about the future events that will occur with Mercury in the night sky.

Greatest elongation of Mercury

On July 5, 2021, at 4:11 GMT (12:11 a.m. EDT), Mercury will reach its greatest elongation west or greatest separation from the Sun. The speedy little planet will be visible in the morning in the eastern sky, shining at a magnitude of 0.3. This will be a perfect time to observe the planet in its June-July apparition.

Moon-Mercury conjunction

On July 8, 2021, at 04:39 GMT (12:39 a.m. EDT), Mercury will pass close to the Moon in our skies — the two space objects will share the same right ascension. 28 days old Moon and the small planet will stay in the constellation Taurus, shining at a magnitude of -8.8 and -0.0, respectively.

Mercury at dichotomy

On July 9, 2021, at 17:48 GMT (1:48 p.m. EDT), Mercury will reach dichotomy (or the half phase). Observers from the Earth will see its Earth-facing side half-illuminated. Like the Moon, Mercury has phases that vary depending on its position relative to our planet. Dichotomy occurs at or near Mercury’s greatest elongation when it appears furthest from the Sun.


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 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!



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