Night Sky Gaze
Planet Mercury

Mercury: More Than Just Retrograde

When we think about the planet Mercury–if it happens at all–it’s usually because someone is complaining about “Mercury in retrograde.” Astrology likes to blame everything that goes wrong on poor little Mercury, but we know that there’s no truth to it.

Here are some actual scientific facts about the planet Mercury!

No Moons for You

Mercury is the only planet that doesn’t have any moons or rings. In fact, it’s only slightly bigger than our moon! It’s just over 3000 miles in diameter, which makes Mercury smaller than the moons Ganymede and Titan.

A Crater the Size of Texas

Despite its tiny size, Mercury survived a massive asteroid impact at some point in its past. Caloris Basin is almost 1000 miles across! Astronomers estimate that the asteroid that caused the crater would have been about 60 miles wide.

Major Hot Flashes

The difference between temperatures during daytime and night on Mercury are extreme. Not in the sense that you’d better pack layers for a visit, either. During the day, the closest planet to the sun can experience surface temperatures of up to 840 degrees Fahrenheit! But at night, that temperature drops dramatically, as low as negative 275 F.

With a temperature fluctuation of up to 1100 degrees, Mercury experiences the most extreme highs and lows in the solar system.

The Years Are Short But the Days Are Long

One of the strangest things about Mercury is the difference between its rotation and orbit. Most planets spin on their axis as they orbit the sun. For a long time, astronomers thought that Mercury didn’t rotate at all. Now, we know that it does–but very, very slowly.

The planet experiences “gravitational lock” thanks to its close proximity to the sun. This has slowed its rotation to the point that, although a year on Mercury is 88 Earth days, a single day from sunrise to sunset takes 176 Earth days!

Maybe those astrologers are onto something after all. A planet where a day lasts 2 years is just plain weird.

The Iron Planet

Okay, one more Mercury fact for the road. Mercury’s core is a huge, molten mass of iron. It takes up about 75% of the planet’s diameter–that’s a way bigger percentage than any other planet in the solar system.

The size and density of the core mean that Mercury is basically a thin crust of dirt and ice over a metal ball. You might think that because it contains so much iron, Mercury would have a strong magnetic field, but you’d be wrong! In fact, its magnetic field is only about 1% of what we experience on Earth.

Deana Adams

When Deana Adams was a kid, she begged her parents for a backyard telescope every birthday and Christmas until they finally caved. As an avid amateur stargazer, she memorized the constellations and hunted for the planets, collecting facts about astronomy the way other kids collected baseball cards or Barbies.

Deana is a contributing writer at NightSkyGaze. She still has her very first telescope.