What does "dip" mean here?

[NewHopeR]

Context:

Kepler discovers planets and planet candidates by measuring dips in the brightness of more than 150,000 stars to search for planets that cross in front, or "transit," the stars. Kepler requires at least three transits to verify a signal as a planet.

More:
NASA - NASA's Kepler Mission Confirms Its First Planet in Habitable Zone of Sun-like Star

 

[rainous]

Context:

Kepler discovers planets and planet candidates by measuring dips in the brightness of more than 150,000 stars to search for planets that cross in front, or "transit," the stars. Kepler requires at least three transits to verify a signal as a planet.

Not a teacher

Dips in the brightness means "decreases" in the brightness.
If a planet passes in front of a twinkling twinkling little star,
naturally your eyes will register less brightness unless you have clairvoyant powers.
That's how these folks figure out where planets are.

 

[SoothingDave]

A "dip" would be a temporary decrease.