Scientists Say: Transit | Science News for Students

Scientists Say: Transit

This is when a planetary body passes in front of a star, or a moon in front of a planet
Aug 21, 2017 — 6:50 am EST
Io transit

This is the moon Io in transit across the face of Jupiter.

NASA/JPL/University of Arizona

Transit (noun, “TRAN-sit”)

A transit has a specific meaning in the field of astronomy. It is when a planet, asteroid or moon passes in front of a star. A moon passing in front of a planet is also a transit. To our eyes, a transit usually looks as though a small, dark dot is crossing the face of a star or planet.

Transits are useful to astronomers searching for new planets outside of our solar system. If the scientists see the light from a star dip a little, this could be because a planet is in transit in front of that star.

In a sentence

When an object in transit is big enough to block out the light from the object behind it, scientists call it an eclipse.

This video shows the transit of Venus across the face of the sun, an event that happened in 2012. The next such transit will occur more than a century later, in 2117.
NASA Goddard

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Power Words

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asteroid     A rocky object in orbit around the sun. Most asteroids orbit in a region that falls between the orbits of Mars and Jupiter. Astronomers refer to this region as the asteroid belt.

astronomy     The area of science that deals with celestial objects, space and the physical universe. People who work in this field are called astronomers.

eclipse     This occurs when two celestial bodies line up in space so that one totally or partially obscures the other. In a solar eclipse, the sun, moon and Earth line up in that order. The moon casts its shadow on the Earth. From Earth, it looks like the moon is blocking out the sun. In a lunar eclipse, the three bodies line up in a different order — sun, Earth, moon — and the Earth casts its shadow on the moon, turning the moon a deep red.

field     An area of study, as in: Her field of research was biology. Also a term to describe a real-world environment in which some research is conducted, such as at sea, in a forest, on a mountaintop or on a city street. It is the opposite of an artificial setting, such as a research laboratory. (in physics) A region in space where certain physical effects operate, such as magnetism (created by a magnetic field), gravity (by a gravitational field), mass (by a Higgs field) or electricity (by an electrical field).

mean     One of several measures of the “average size” of a data set. Most commonly used is the arithmetic mean, obtained by adding the data and dividing by the number of data points.

moon     The natural satellite of any planet.

planet     A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood. To accomplish the third feat, the object must be big enough to have pulled neighboring objects into the planet itself or to have slung them around the planet and off into outer space. Astronomers of the International Astronomical Union (IAU) created this three-part scientific definition of a planet in August 2006 to determine Pluto’s status. Based on that definition, IAU ruled that Pluto did not qualify. The solar system now includes eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

solar system     The eight major planets and their moons in orbit around our sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.

star     The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.

transit     (in astronomy) The passing of a planet, asteroid or comet across the face of a star, or of a moon across the face of a planet.