The European Southern Observatory's Very Large Telescope in Chile captured this striking view of the nebula around the star cluster NGC 1929 within the Large Magellanic Cloud, a satellite galaxy of our own Milky Way separated by a mere 179,000 light years.
EnlargeManu Mejias/ESO

The European Southern Observatory's Very Large Telescope in Chile captured this striking view of the nebula around the star cluster NGC 1929 within the Large Magellanic Cloud, a satellite galaxy of our own Milky Way separated by a mere 179,000 light years.

The night sky is a time machine. Look out and you look back in time. But this "time travel by eyesight" is not just the province of astronomy. It's as close as the machine on which you are reading these words. Your present exists at the mercy of many overlapping pasts. So where, then, is "now"?

As almost everyone knows, when you stare into the depths of space you are also looking back in time. Catch a glimpse of a relatively nearby star and you see it as it existed when, perhaps, Lincoln was president (if it's 150 light-years away). Stars near the edge of our own galaxy are only seen as they appeared when the last ice age was in full bloom (30,000 light-years away). And those giant pinwheel assemblies of stars called galaxies are glimpsed, as they existed millions, hundreds of millions or evenbillions of years in the past.

We never see the sky as it is, but only as it was.

Stranger still, the sky we see at any moment defines not a single past but multiple overlapping pasts of different depths. The star's image from 100 years ago and the galaxy image from 100 million years ago reach us at the same time. All of those "thens" define the same "now" for us.

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