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[Night sky pans across, moon passes through upper left corner]

How far away are the stars in the night sky?

Most are at least hundreds of trillion of miles away from Earth. Galaxies are even more distant.

[Pan down to Earth’s night side from low-orbit, Magellanic clouds visible in sky. Sky continues to rotate, view moves away from Earth, North American cities lit at night.]

Such astronomical distances can be measured using light travel time, the time light takes to travel a specified distance.

[Continue moving away from Earth throughout video. Earth now shrinking into the distance, orbit of Moon comes into view.]

Sunlight reflected off the Moon takes 1.3 seconds to reach Earth, so the Moon is 1.3 light seconds away.

[Blue circle marking the Moon is labeled “1.3 light seconds.” Orbits of other planets come into view.]

Sunlight travels 93 million miles from the Sun to Earth in 8.3 minutes

The Sun is 8.3 light minutes away.

[Sun enters view, in center of planets’ orbits. Blue circle marking the Sun is labeled “8.3 light minutes.”]

Neptune is 4 light hours away.

[All planets’ orbits in view. Blue circle marking Neptune is labeled “4 light hours.”]

The more distant an object is, the longer its light takes to reach us.

How long has light from the stars or galaxies traveled to reach us?

Years? Centuries? Millennia?

[Solar system fades into distance, continues moving away, among the stars.]

Alpha Centauri, the closest star system to the Sun, is 4.2 light years away.

What were you doing 4.2 years ago?

[Timeline appears at bottom of screen, a marker at its middle. 4.2 years is at marker then moves to the right as view moves farther away from Earth, looking farther back in time.  Logarithmic scale on timeline moves left to right, marking 10, 50, 100, 500, etc. years ago. Labeled circles mark celestial objects as they pass, for each an event is marked on the moving timeline with a labeled image.]

[Labeled star: Alpha Centauri, 4.2 light years]

[Timeline: 50 years ago, Humans walked on the Moon, image of boot print on the Moon Labeled star: Alderamin, 49 light years]

[Timeline: about 175 years ago, Neptune was discovered, image of Neptune

Labeled star: Nembus, 177 light years]

[Timeline: about 580 years ago, Gutenberg built a printing press, image of printed pages

Labeled group of stars: Praesepe Cluster, 577 light years]

[Begin to leave plane of Milky Way galaxy.]

[Timeline: 4,500 years ago, Pyramids at Giza were built, image of pyramids

Labeled spherical grid: Individual stars visible to unaided eye are within 4500 light years]

[The whole Milky Way galaxy comes into view, with the grid enclosing a small portion of one side of it. Other galaxies enter the view.]

[Timeline: 200,000 years ago, Homo sapiens appeared, image of ancient human skull

Labeled galaxy: Small Magellanic Cloud, 199,000 light years]

[Timeline: 2.5 million years ago, Ice Ages began, image of snowy scene with mammoths

Labeled galaxy: Andromeda Galaxy, 2.5 million light years]

[Galaxy shapes fade into colored points, showing clustering of galaxies.]

[Timeline: 55 million years ago, Early primates appeared, image of Archicebus achilles

Labeled group of galaxies: Virgo Galaxy Cluster, 54 million light years]

[Beyond 1 billion year mark, view is filled with distant galaxies.]

[Timeline: 4.6 billion years ago, Sun formed, image of the Sun]

[Distant galaxies and timeline fade to scene of Sun on the horizon, setting behind mountains. As sun sets, sky darkens, view pans up to stars in the night sky.]

The farther away we look, the farther back in time we see.

However, the starlight we see takes, at most, a few thousand years to reach us, while stars’ lifetimes are millions, billions, or even trillions of years.

As distant as they are, these stars have aged imperceptibly since emitting the light we see tonight.

[credits]