Space Stories - Astronomy Words: Light Year

0:00It's the 365 days of Astronomy PodGa. Coming in 3, 2, 1. Welcome to the 365 days of Astronomy. I'm Pafifa, your host. And today we are exploring another Astronomy word. Every time you look at the night sky, you are looking into the past. We see the moon as it was a little more than one second ago. We see the sun about 8 minutes in the past. And the light reaching us from the nearest star beyond the sun began its journey more than 4 years ago. The farther we look into space, the further back in time we see. The reason is hidden inside our Astronomy word for today. Lightyear. Although it contains the word year,

1:01a lightyear is not a measurement of time. It is a measurement of distance. Light travels at almost 300,000 km per second. At that speed, it could still call Earth more than 7 times in just 1 second. A lightyear is the distance light travels through space during one year. Approximately 9.46 trillion km. Instead of describing the enormous distances between star with long strings of numbers, astronomers used to distance traveled by light as a cosmic measuring stick. So let's us begin close to home. The moon is about 384,000 km from Earth. Light reflected from its surface takes around 1.3 seconds to reach us. This means we see the moon as it was 1.3 seconds ago. Let's move to the sun.

2:02The sun is about 150,000,000 km away. Its light needs approximately 8 minutes and 20 seconds to reach Earth. When you see the sun, you are seeing light that left it more than 8 minutes earlier. Beyond the solar system, Proxima-Sandory, the nearest star to the sun. It's about 4.24 light years away. Its light has been traveling toward us for more than 40 years. Our galaxy, the Milky Way, is approximately 100,000 light years across. Light would need about 100,000 years to travel from one side of the galaxy to the other. And the Andromeda Galaxy is around 2.5 million light years away. When we observe Andromeda, we see the galaxy as it was about 2.5 million years ago, when early human ancestors were living on Earth.

3:05Andromeda may have changed during the time, but light carrying informations about those change has not reached us yet. This is why telescopes are sometimes described as time machines. They cannot physically carry us into the past, and we cannot change events that have already happened. But telescopes collect old light. They allow us to witness stars and galaxies as they existed years, million of years, or even billions of years ago. By studying objects at different distances, astronomers can explore different periods of cosmic history. Nearby objects shows us the relatively recent universe. Extremely distant galaxy refilled the universe when it was much younger. A light year therefore tells us more than how far away something is. It also tells us how long its light has been traveling, and how far into the past we are seeing.

4:07We are not traveling into the past. The past is traveling across the universe to reach us, carried by delight. And this has been your word in astronomy, light year. Thank you for listening, and this is 365 Days of Astronomy. You are listening to the 365 Days of Astronomy Podcast. Cool. The 365 Days of Astronomy Podcast is produced by the Planetary Science Institute. Audio post-production is by me, Richard Drum. Project management is by Aviva Yamani, and hosting is donated by Libsyn.com. This content is released under a Creative Commons Attribution, Non-Commercial 4.0 International License. Please share what you love, but don't sell what's free.

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