Background Info

On September 23, 1846, French scientists discovered Neptune, the outermost planet in our solar system. Neptune is one of only two planets in the solar system (along with Uranus) only visible through a telescope.

Neptune is the only planet discovered by mathematical prediction. Long before its discovery, astronomers hypothesized the existence of Neptune because Uranus seemed to be affected by an unknown source of gravity. Several mathematicians successfully calculated the approximate position of the planet, which helped astronomers know where to look.

Since its discovery, scientists have learned much about Neptune. In 1989, NASA’s Voyager 2 spacecraft provided images of the planet, which appears bright blue due to the methane in its upper atmosphere. Neptunian rings and moons were also identified.

Vocabulary

Term Part of Speech Definition Encyclopedic Entry

approximate

Adjective

generally or near an exact figure.

astronomer

Noun

person who studies space and the universe beyond Earth's atmosphere.

atmosphere

Noun

layers of gases surrounding a planet or other celestial body.

Encyclopedic Entry: atmosphere

calculate

Verb

to reach a conclusion by mathematical or logical methods.

gravity

Noun

physical force by which objects attract, or pull toward, each other.

hypothesize

Verb

to form a statement or suggestion that explains certain questions about certain facts.

mathematician

Noun

person who studies the theory and application of quantities, groupings, shapes, and their relationships.

methane

Noun

chemical compound that is the basic ingredient of natural gas.

moon

Noun

natural satellite of a planet.

Encyclopedic Entry: moon

planet

Noun

large, spherical celestial body that regularly rotates around a star.

Encyclopedic Entry: planet

solar system

Noun

the sun and the planets, asteroids, comets, and other bodies that orbit around it.

telescope

Noun

scientific instrument that uses mirrors to view distant objects.

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Editor

Caryl-Sue, National Geographic Society

Producer

Mary Crooks, National Geographic Society

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