1. LIGHT yEAR

2. Light years are units of time used to measure the distance between objects in the universe.
— Why is a light year a unit of distance rather than time?
— Why do we need to use light years as a way of measuring cosmic distances?
— What is the speed of light?
— How are light years used to measure distances in the universe?

3. Why is a light year a unit of distance rather than time? It measures
the distance light travels in a vacuum in one year trillion km or 5.880
trillion miles.

4. Why do we need to use light years as a way of measuring cosmic
distances? If we used conventional measurements, the numbers would be
huge and difficult to calculate.
What is the “speed of light”?

5. What is the “speed of light”? The rate at which light travels through
empty space - 299,792,458 meters/second or 186,000 miles/second.

6. How are light years used to measure distances in the universe? If
we know how fast light can travel from a particular distance (the Sun), we
can apply this to other objects in our universe.

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

8. How far is far? How fast is fast?
Light Year
How far is far?
How fast is fast?

9. 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.

10. 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.

11. What is the fastest speed we know?
The Speed of Light

12. 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!

13. 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!

14. 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).

15. 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.

16. 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

17. 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.

18. 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.

19. After your essay on Thursday and in class on Friday, you will:
Develop a scale drawing that represents the comparative distances in light years between three objects in the universe. Include a statement discussing the advantages and limitations of using a scale drawing to represent large distances.