At a Glance: Transits

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Transits can reveal characteristics of the transiting body that are otherwise difficult to observe. 

Produced by the Space Telescope Science Institute, Office of Public Outreach, in collaboration with NASA’s Universe of Learning partners: IPAC, Jet Propulsion Laboratory, Smithsonian Astrophysical Observatory, and Sonoma State University.
Images and Animations:
  • 2017 total solar eclipse composite: Zolt Levay
  • 2017 total solar eclipse, view of Earth from the Earth Polychromatic Imaging Camera (EPIC) on NOAA’s Deep Space Climate Observatory (DSCOVR): NASA EPIC Team
  • Total solar eclipse animation: STScI
  • 2012 transit of Venus with animated light curve: based on animation by James Gilbert
  • 2012 transit of Venus composite from the Solar Dynamic Observatory (SDO): NASA, Goddard, SDO
  • Atmospheric halo of Venus during the 2012 transit from the Solar Optical Telescope on the Hinode satellite: JAXA, NASA, Hinode
  • Sunlight filtered by the Venusian atmosphere: STScI
  • Exoplanets orbiting a distant star: NASA, ESA, and G. Bacon (STScI)
  • Orbiting exoplanets with animated light curve: STScI, based on animation from NASA, JPL Exoplanet Exploration Program
  • Starlight filtered through an exoplanet atmosphere: STScI
  • Wide Field Infrared Survey Telescope (WFIRST); Transiting Exoplanet Survey Satellite (TESS), Hubble Space Telescope, James Webb Space Telescope, Kepler Space Telescope, Spitzer Space Telescope: NASA, STScI

Writing: Margaret W. Carruthers
Design: Dani Player
Science review lead: Dr. Brandon Lawton
Education lead: Timothy Rhue II
Additional editorial input from: Dr. Lynn Cominsky, Sonoma State University; Dr. Sabrina Stierwalr, IPAC/Caltech; Dr. Tiffany Meshkat, IPAC/Caltech.
Music: APM

at-a-GLANCE. Transits.
Photos showing the evolution of a solar eclipse
Once every 18 months or so, Earth is treated to a spectacular celestial event: a total eclipse of the Sun.
As the Moon passes in front of the Sun, its shadow falls over Earth's surface and the sky grows dark.
Photo of Earth from space, showing the shadow of the Moon.
An eclipse is a special type of event known as a transit.
animation of the Moon orbiting the Earth
A transit occurs when one celestial body passes in front of another.
Telescopic images of a small planet passing in front of the Sun
In 2012, Venus passed directly between Earth and the Sun. Although a transit of Venus does not cast a visible shadow on Earth's surface, it does block some of the Sun's light.
Transits can reveal characteristics of the transiting body that are otherwise difficult to observe.
It was during a transit in 1761 that the hazy atmospheric halo surrounding Venus was first discovered.
Telescopic photo showing Venus's hazy halo
More recently, astronomers have been able to determine the composition and structure of Venus's atmosphere by analyzing sunlight that passes through it.
illustration of light passing through Venus's atmosphere
There are other transits that can be detected with sophisticated space telescopes.
A star will appear to dim slightly if a planet moves in front of it.
A repeating pattern of change in brightness reveals the existence of a planet -- or many planets -- orbiting a distant star.
Scientists can also use transits to study exoplanet atmospheres by analyzing the starlight that passes through them.
NASA's current and next generation of space telescopes will continue to analyze the shadows of transiting exoplanets and their atmospheres.
From the awe-inspiring darkness of a total solar eclipse, to the rare sight of Venus silhouetted against the Sun, to the barely detectable dimming of a distant star transited by an exoplanet...
Transitory shadows reveal secrets of the planets, moons, and star systems.