The faint smudge of Andromeda you can see with your naked eye on a dark night is 2.5-million-year-old light — photons that left another galaxy around the time the earliest members of our genus were wa
The Andromeda galaxy is visible to the naked eye as a faint blur, with light that has traveled for 2.5 million years to reach Earth. This timeframe aligns with the early evolutionary history of the genus Homo.
Why it matters
Understanding the lookback time of celestial objects provides a unique perspective on the scale of the universe and the history of human evolution.
Andromeda's faint central glow has travelled for roughly 2.5 million years, placing its departure near the emergence of the genus Homo.
On a clear, dark night, the Andromeda galaxy does not resemble the sweeping spiral in telescope photographs. It appears as a small, pale blur, easy to mistake for a thin cloud. Andromeda is about 2.5 million light-years away, so the photons making that blur have crossed intergalactic space for about 2.5 million years before ending their journey in a human retina.
The evolutionary comparison in the headline is broadly sound. The oldest widely accepted fossil assigned to the genus Homo is around 2.8 million years old. A little later in the record, early species including Homo habilis appear. The light visible from Andromeda tonight began travelling during that formative chapter of the human lineage.
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