WEBVTT FILE

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NASA's Kepler telescope trails
behind Earth in its orbit.

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Since 2014, Kepler has been
looking outward along the

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ecliptic, the plane
of our solar system.

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In this orientation, the
telescope can observe the

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superior planets like Neptune,
which passed through its field

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of view in late 2014
through early 2015.

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As we zoom into Kepler's array
of detectors, we see the images

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taken by the telescope during
this period, sped up from over

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two months of observations
into a matter of seconds.

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The dot orbiting Neptune is its
largest moon, Triton, while the

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smaller moon Nereid
trails faintly to the east.

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About halfway through the
observation, Neptune appears to

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stop its westward
march and reverse course.

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In reality, Neptune always moves
east,

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but at a slower rate than
Kepler.

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This causes an apparent
retrograde motion, or backwards

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step, each time that the
telescope laps the planet.

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The Kepler Telescope was
originally designed to search

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for planets in other solar
systems

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using the transit method.

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When a planet passes in front of
its star, it causes a small dip

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in starlight that can be
measured by Kepler's sensitive

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detector, revealing the
presence and size of the planet.

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During the Neptune observations,
Kepler instead looked

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at changes in reflected
sunlight.

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It measured fluctuations in
Neptune's brightness of less

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than a single percent, caused by
factors including the planet's

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daily rotation, the movement of
clouds, and even seismic waves

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within the Sun itself.

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By measuring tiny variations
in Neptune's brightness, Kepler

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teased out clues to the planet's
hidden dynamics, pushing the

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limits of its detector, and
laying the groundwork for more

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detailed studies of
exoplanets in the years ahead.

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[Music]

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[Satellite beeping]