Happy Light Year?

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A light year isn’t a measure of time, like ‘Happy New Light Year!’ or ‘That’s not gonna happen this Light Year’.

A light year is a way to measure distance. Astronomers deal with such large distances it is more convenient to use light years when talking about how far away astronomical bodies are. It is defined as the distance light travels in one Earth year. The speed of light is about 300,000 kilometers per second or about 186,000 miles per second. At that speed light can cover about 9,500,000,000,000 (9 1/2 trillion) kilometers or about 6,000,000,000,000 (6 trillion) miles in one year.

Astronomers sometimes speak of light seconds, minutes, hours, days, weeks, and months as well.

If we want to get technical a light year is the distance light travels in a vacuum in one Julian year, which is 365 ¼ days. That ¼ is the reason we add a day every four years -- Leap Year. It keeps the calendar right. The reason for measuring the speed of light in a vacuum is that the speed of light can vary depending on what it is passing through. For instance, the speed of light through water is 225,000 kilometers per second (140,000 miles/sec) and in glass about 200,000 kilometers per second (124, 000 miles per second).

So how do we use light years to describe the distance of astronomical bodies? Well, we can say the Earth is 26,000 light years from the center of our galaxy, the Milky Way, which is 100,000 light years across. The nearest star, Proxima Centuri, is 4.22 light years from us. We can also use light minutes to describe the distance to our sun as 8 light minutes. Our Moon is 1 ¼ light seconds away. The visible edge of the universe is around 13 billion light years away.

Another way to look at light years or light minutes is as a way of looking into the past. For example, if it takes light 8 minutes to get to our eyes from the sun, that means we are seeing the sun as it WAS 8 minutes ago. So, if the sun were to explode right now, it would be 8 minutes before we knew it and no sooner, no way, no how.