Galileo attempted to measure the speed of light, but the technology of that era just wasn't advanced enough.
Tyson begins the episode by explaining the nature of the speed of light and how much of what is seen of the observable universe is from light emanated from billions of years in the past. Tyson further explains how modern astronomy has used such analyzes via deep time to identify the Big Bang event and the age of the universe.
Tyson proceeds to describe how the work of Isaac Newton, William Herschel, and James Clerk Maxwell contributed to understanding the nature of electromagnetic waves and gravitational force.
Isaac Newton attempts to understand gravity and it's affect on distant objects.
And how this work led towards Albert Einstein's Theory of Relativity, that the speed of light is a fundamental constant of the universe and gravity can be seen as distortion of the fabric of space-time.
Tyson describes the concept of dark stars as postulated by John Michell which are not visible but detectable by tracking other stars trapped within their gravity wells, an idea Herschel used to discover binary stars.
The 19th century scientist Michael Faraday discovered invisible fields of force that help him understand how gravity works.
Tyson then describes the nature of black holes, their enormous gravitational forces that can even capture light, and their discovery via X-ray sources such as Cygnus X-1.
Tyson uses the Ship of Imagination to provide a postulate of the warping of spacetime and time dilation as one enters the event horizon of the black hole, and the possibility that these may lead to other points within our universe or others, or even time travel.
The Hubble Telescope was able to capture light from some of the vey first stars, which existed before Earth.
Tyson ends on noting that Herschel's son, John would be inspired by his father to continue to document the known stars as well as contributions towards photography that play on the same nature of deep time used by astronomers.
Animated sequences in this episode feature caricatures of William and John Herschel; Sir Patrick Stewart provided the voice for William in these segments
The stars of the Pleiades Cluster are 400 light years away.
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