---
source: https://iinventedlight.com/library/notes/mercury-basics.md
title: "Mercury basics: the smallest, fastest planet"
license: CC-BY-4.0
license_url: https://creativecommons.org/licenses/by/4.0/
source_license: "US government work, public domain; restated in our own words"
attribution: "Summary by iinventedlight (AI-assisted). Sources: NASA, Mercury: Facts, US government work, public domain; NASA, Mercury, US government work, public domain."
author: "iinventedlight (synthesis of cited sources); text drafted with AI assistance"
date: 2026-11-04
status: published
checked_on: 2026-11-04
as_of: 2026-11-04
confidence: medium
untested: "No observation or experiment was done; only the fetched government pages were read."
category: coverage-gaps
topic_area: "science-explain: astronomy and space"
tags: science, astronomy, mercury, planets
sources:
  - id: S1
    title: "Mercury: Facts"
    publisher: NASA
    url: https://science.nasa.gov/mercury/facts/
    accessed: 2026-11-04
    license: "US government work, public domain"
  - id: S2
    title: "Mercury"
    publisher: NASA
    url: https://science.nasa.gov/mercury/
    accessed: 2026-11-04
    license: "US government work, public domain"
---
# Mercury basics: the smallest, fastest planet

## Short answer
Mercury is the smallest planet and the closest to the Sun, only slightly larger than Earth's Moon. It is the fastest planet, going around the Sun every 88 Earth days. [S1][S2]

## Key points
- Days reach about 800°F (430°C), but with no atmosphere to hold heat, nights get very cold. [S1]
- It is not the hottest planet; Venus is, because of its dense atmosphere. [S1]
- It is named for the swiftest Roman god. [S2]

## Claims
- id: C1
  claim: Mercury is only slightly larger than Earth's Moon.
  source: S2
  evidence: "It's only slightly larger than Earth's Moon."
- id: C2
  claim: Mercury orbits the Sun every 88 Earth days.
  source: S2
  evidence: "It's the fastest planet, zipping around the Sun every 88 Earth days."
- id: C3
  claim: Daytime temperatures on Mercury can reach 800°F.
  source: S1
  evidence: "day temperatures can reach highs of 800°F (430°C)"
- id: C4
  claim: Venus, not Mercury, is the hottest planet.
  source: S1
  evidence: "Mercury is not the hottest planet in our solar system"

## Sources
- [S1] Mercury: Facts. NASA. https://science.nasa.gov/mercury/facts/ . License: US government work, public domain. Accessed 2026-11-04.
- [S2] Mercury. NASA. https://science.nasa.gov/mercury/ . License: US government work, public domain. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).