Light travels at a finite speed, 3x105 km/sec. Given the distances in the Universe, it therefore takes measurable time for it to travel from one object to another-- just like it takes you measurable time to travel between your house and school. In this activity, we will understand just what that means.
Light Travel Activity
In one year, light can travel about 9.5 x 1012 km. Thus, if you are say 10,000,000,000,000 km away from Earth, it takes just over a year for the light that leaves you to arrive on Earth. This means we see you not as you are now, but as you were just over one year ago! In a very real sense, looking out into far distances is looking back into the past. The farther out we look, the further back in time we see.
If given a distance in kilometers, can you convert that distance into "light travel time"? Try it! Below you will find distances from Earth to well-known objects in the Universe. Calculate the "light travel time" for each.
|
Astronomical Object |
Distance in Kilometers |
Light Travel Time |
|
Mars |
78000000 |
Minutes |
|
Sun |
150000000 |
Minutes |
|
Pluto |
5800000000 |
Hours |
|
Next Nearest Star (Proxima Centauri) |
4.1 x 1013 |
Years |
|
Vega |
2.4 x 1014 |
Years |
|
Betelguese |
4.1 x 1015 |
Years |
|
Antares |
5.7 x 1015 |
Years |
|
Orion Nebula |
1.5 x 1016 |
Years |
|
Crab Nebula |
6.0 x 1016 |
Years |
|
Andromeda |
2.4 x 1019 |
Years |
Notice that the Andromeda Galaxy is 2.5 million light years away. Remember, this means that the light our eyes (or telescopes) are receiving right now from Andromeda left that galaxy 2.5 million years ago. What we are actually seeing is an image of what Andromeda looked like 2.5 million years ago. What, you may ask, does Andromeda look like today? Well, unfortunately we will have to wait 2.5 million years to find out! Again we say, looking out in the Universe is like looking back in time.
Time Line Activity
Materials:
List of astronomical objects with distances listed in light units (See Part I, or research your own list using Astronomy textbooks or databases. For example, try the database of Astronomical images from Hubble Space Telescope http://hubblesite.org/newscenter/newsdesk/archive/ . Many of these images contain descriptions which include their distances from Earth in light years.)
Resources for looking up events in the Earth’s history. These resources should include history texts, social studies texts, geology texts, and astronomy texts. You can also try the free online trial from Facts on File http://www.factsonfile.com/or one of these sites:
http://www.sbrowning.com/whowhatwhen/
http://www.hyperhistory.com/online_n2/civil_n2/basictext.html
Anything you choose to use for creating a timeline possibly including: butcher paper, markers, art supplies, rulers, and/or timeline creating software such as found at http://www.tomsnyder.com/products/productextras/TIMV50/
Procedure:
Imagine that you are a being sitting at each of the cosmic locations that you worked with in the Light Travel Activity. As you look towards the Earth, you will observe different eras in history depending upon how far you are from the Earth. This is because light takes time to travel in space, and the farther away you are from the Earth the longer information from Earth takes to get to you. For example, if you are a being sitting on Mars and you look at the Earth, you will see the Earth as it was four minutes ago. This is because Mars is far enough away from Earth that it takes four minutes for light to travel from Earth to Mars. It takes even longer for light to travel between the Earth and other farther away places. For example, if your friend had a 15th birthday party on Earth and you were sitting on the nearest star, your friend would be having their 19th birthday party before you ever got to see the 15th birthday party!
You should have a list of astronomical places and their distances in either km or light-years. In this Activity, you are going to create a timeline which shows Earthly Events as a function of the distance to your astronomical location.
Reminders:
Think About It: How long would your timeline have to be if 1 cm = 100 years?