Mercury: Facts & Retrograde

Instructor: Katie Chamberlain

Katie has a PhD in Microbiology and has experience preparing online education content in Biology and Earth Science.

Bigger is not always better. For example, just check out the tiniest planet in our solar system, Mercury. In this lesson you will learn that despite Mercury's miniature dimensions, it tops the others in several ways.


Mercury is one of eight planets (including Earth) that orbit around our sun. It is one of the four rocky terrestrial planets that formed in a protoplanetary ring near the sun approximately 4.5 billion years ago. It was first identified by humans thousands of years ago, and it is named after the Roman messenger of the gods because of its fast trips around the sun.

Basic Stats

Mercury has a circumference of 15,329 km. Its surface area is 0.147 times that of the Earth; this makes it slightly larger than our moon. Mercury itself has no known natural moon satellites.

Mercury is the closest planet to the sun at approximately 57 million km away. To a Mercury resident, the sun would look three times larger than we see it on Earth. It is also the fastest planet in our solar system, and it cruises through space at 112,000 mph. For comparison, the Earth moves around the Sun at only 67,000 mph. Since Mercury is so speedy and so close to the sun, a year on Mercury is the equivalent of 88 Earth days.

While its year is quite short, its days are quite long. A single day on Mercury lasts for 58 long Earth days because it rotates very slowly on its axis. Oddly, since Mercury has an extremely elliptical orbit, the sunrise and sunset are not continuous events. At sunrise, the sun appears to rise briefly, set briefly, and then rise completely and travel across the sky. Likewise, at sunset, there is a brief sunset, sunrise, and then a final sunset.

Mercury as viewed by the Mariner 11 spacecraft

It would be tough to survive a single day on Mercury because of the harsh climate. The slow rotation means that the Sun has plenty of time to scorch the planet during the day and the planet has plenty of time to cool down at night. In conjunction with the lack of a protective atmosphere, this results in dramatic shifts in temperature from about -173 to 427 degrees Celsius!

The surface of Mercury is covered with many craters from asteroid impacts. This is also a consequence of having very little atmosphere. Below the surface, there is likely a very dense iron core. Scientists predict that the core is possibly molten since Mercury has a small magnetic field.

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