Facts About Mercury: All You Need to Know
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
- How big is Mercury?
- How hot is Mercury?
- How long is a day on Mercury?
- What is Mercury made of?
- What is Mercury retrograde?
- When is Mercury visible in the night sky?
- Upcoming Events
- Did you know?
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 2022, Mercury will be in retrograde from May 10 to June 3, from September 9 to October 2, and then from December 29 to January 18, 2023.
When is Mercury visible in the night sky?
In 2022, Mercury will be visible in the morning sky from June 2 to July 3 and from October 3 to October 17. In the evening, look for the planet from April 18 to May 10, from August 1 to September 15, and then from December 7 to December 31.
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.
Learn about the future events that will occur with Mercury in the night sky.
October 2: Mercury’s retrograde motion ends
On October 2, 2022, Mercury ends its retrograde motion that lasted from September 9. It means that the planet will appear to move prograde, or “normally” – from east to west – until the next retrograde period that will start on December 29, 2022. By the way, Mercury is not the only planet that appears to move backward sometimes: check our infographic to learn what other celestial bodies can be in retrograde.
October 6: Mercury at perihelion
On October 6, 2022, at 4:57 p.m. EDT (20:57 GMT), Mercury’s orbit will carry it to the closest point to the Sun. The distance between the star and the planet will be 0.31 AU. For comparison, at aphelion (farthest point from the Sun), the distance between these two objects is about 0.4 AU. The difference makes up more than 50%, meaning Mercury gets twice as much energy from the Sun at perihelion than it does at aphelion. Because of this, its surface heats up by about 1.5 times. However, it doesn’t affect Mercury’s appearance in the sky: you will not notice much difference even if you look through a telescope.
October 8: Mercury at greatest western elongation
On October 8, 2022, at 4:59 p.m. EDT (20:59 GMT), Mercury (magnitude -0.6) will appear farthest from the Sun in the sky: the apparent distance between the two bodies will be 18°. As the innermost planet, most of the time, Mercury is near the Sun and, therefore, hard to observe. One of the best chances to see it is during elongation when the planet gets the farthest from the star. This time, Mercury will rise about two hours before dawn and will not have time to rise high; stargazers better find a vantage point with an unobstructed horizon.
October 24: Moon-Mercury conjunction
On October 24, 2022, at 15:00 GMT (11:00 a.m. EDT), the Moon will approach Mercury at a distance of 18'; both objects will be in the constellation Virgo. This will happen the day before the New Moon; therefore, our natural satellite will be almost invisible. Mercury, shining at a magnitude of -1.1, will rise an hour before the Sun, so there won't be much time to spot it.
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!