With the naked eye you can see 2.5 million years into the past

Find a sky with no moon and no streetlights, in autumn, near the constellation Cassiopeia. That faint, elongated smudge, like breath on a windowpane, is the Andromeda galaxy: a trillion suns two and a half million light-years away.

The light touching your eye left there about 2.5 million years ago. No modern human walked the Earth yet; our genus did not even exist. That light crossed the void through the whole of human history — fire, writing, every war and every song — and ended its journey exactly on your retina.

No instrument required: the naked eye is enough. It is the cheapest time machine there is.

One more detail: Andromeda is coming toward us at about 110 kilometres per second. In some 4.5 billion years, our galaxy and hers will merge into one. What today is a smudge in your eye will one day be the entire sky.