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A light-year is a distance, not a length of time: how far light travels in a year, about 9.46 trillion kilometres. The nearest star after the Sun, Proxima Centauri, is about 4.25 light-years away, so the light we see from it set off over four years ago.

Astronomy

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Astronomers define it precisely using a year of 365.25 days, which gives 9,460,730,472,580.8 kilometres. The unit grew out of the first measurement of the distance to another star: in 1838 the German astronomer Friedrich Bessel found that the star 61 Cygni was so far away that its light took about ten years to reach us. The term itself first appeared in 1851, in a German popular astronomy article by Otto Ule, who compared it to measuring a walk in hours. Professional astronomers usually prefer the parsec, about 3.26 light-years. Because light takes time to arrive, looking far away means looking back in time: we see the Andromeda Galaxy by light that left it about 2.5 million years ago.

Türkçe oku
Işık yılı bir zaman değil, uzaklık birimidir: ışığın bir yılda aldığı yol, yani yaklaşık 9,46 trilyon kilometre. Güneş'ten sonra bize en yakın yıldız olan Proxima Centauri yaklaşık 4,25 ışık yılı uzaktadır; ondan bugün gördüğümüz ışık, dört yılı aşkın bir süre önce yola çıktı.

Gökbilimciler bu birimi 365,25 günlük bir yıla göre kesin olarak tanımlar: 9.460.730.472.580,8 kilometre. Birim, başka bir yıldızın uzaklığının ilk kez ölçülmesinden doğdu. 1838'de Alman gökbilimci Friedrich Bessel, 61 Cygni yıldızının uzaklığını ölçtü ve ışığının bize yaklaşık on yılda ulaştığını buldu. “Işık yılı” terimi ilk kez 1851'de Otto Ule'nin Almanca bir popüler astronomi yazısında geçti; Ule bunu bir yürüyüşü saatle ölçmeye benzetmişti. Profesyonel gökbilimciler ise daha çok yaklaşık 3,26 ışık yılına eşit olan parseki kullanır. Işığın yol alması zaman aldığı için uzağa bakmak geçmişe bakmak demektir: Andromeda Galaksisi'ni, oradan yaklaşık 2,5 milyon yıl önce yola çıkmış ışıkla görürüz.

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Background reading: Light-year
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