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LIGHT yEAR

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**What type of measuring unit would we need for distances across space?**

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**How far is far? How fast is fast?**

Light Year How far is far? How fast is fast?

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What Is It? A light year is the DISTANCE that light will travel, through a vacuum, in one year. This is not a unit of time, but a unit of distance. Be sure that students know this. Students may not know what a vacuum is. Instruct students to record this information in their notebooks.

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**Calculate Seconds = How many seconds are in a year?**

31,557,600 s in a yr 1 yr days 24 hr 60 min 60 s 1 day 1 hr 1 min = Instruct students to draw a table and add numbers and units in the proper boxes. Inform them that they have to be set up so that the units will cancel. As you view this slide, units will cancel out. Without the units, students can simply multiply the numbers together. OR… Instruct students to use a calculator to find how many seconds there are in a year. This value will calculate on most 4-function calculators.

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**What is the fastest speed we know?**

The Speed of Light

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**Speed of Light c = 299,792,458 meters per second or**

186,000 miles per second The fastest thing that we know of is light which travels at a speed of about 186,000 miles, or 300,000 kilometers, per second in empty space. To get an idea of how fast this is, light can travel about seven times around Earth in one second!

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Speed of Light To get an idea of how fast this is. . . light can travel about seven times around Earth in one second! The fastest thing that we know of is light which travels at a speed of about 186,000 miles, or 300,000 kilometers, per second in empty space. To get an idea of how fast this is, light can travel about seven times around Earth in one second!

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Other Values One light year is approximately trillion miles or 9.5 trillion km. The symbol for light year is “ly”. The miles value is for reference only. Students should know the abbreviation for light year (ly).

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**Why “ly”? Light years are used to measure the vast distances in space.**

In the universe, the kilometer measure is too small to use. In the universe, the kilometer measure is too small to use. For example, the distance to the next nearest big galaxy, the Andromeda Galaxy, is 21 quintillion km. That's 21,000,000,000,000,000,000 km. This is a number so large that it becomes hard to write, to relate to, and to use in calculations. Astronomers use other units of distance.

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**History Seen Every time you look at objects in the night sky or**

the Sun, the light from that object is old. You are looking at history. I

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An Example It takes eight light minutes for light leaving the Sun to reach the Earth. To put this in perspective, if you could drive from the surface of the Sun to the surface of the Earth, it would take you 180 years driving nonstop at 60 mph.

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**Example: Sirius Sirius, in the constellation Canis**

Major, is the sky’s brightest star. It is easy to find on winter and spring evenings. When you look at Sirius, you are looking back in time to see how Sirius looked eight years ago. The light left the star eight years ago. At some point, we may see the death or birth of a star long after the event occurred.

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