How+Far+&+How+Big

=How Big and Far Things Are= //Melina Duckett and Oliver Cheng//  

Because our universe is astronomical in size, it is impossible to simply measure the universe in centimeters, meters, kilometers, or miles. Scientists have come up with methods on how to measure the distance between celestial objects, and how big they really are. The concept of measuring space has been around f or centuries, going all the way back to Sumerians who used parallax for navigational purposes. Later, one of the first scientists to begin thinking about measuring space was Aristole in 330 BC. Another scientist who pondered this problem was Galileu Galilei, he noticed that Jupiters moons could be used as a clock in ths sky. However, in 1838 Friedrich Wilhelm Bessel was the first person to use parallax to measure the distance to a star. 

** The Light Year A light year is 9.4605284 × 1015 meters. We simply can’t imagine such a large dist ance, so we don’t understand the significanc   **
e of that. That’s 300 000 kilometres per second. That’s SEVEN times around the Earth every second. In space, we cannot use kilometres to measure distances because the distances are too large and kilometres would be inconvenient to use. Light years are used because even though they are huge compared to our Earth, they are still a relatively small size in the universe. For example, the nearest star “Proxima Centauri” is 4.3 light years away from the sun.   To put this in perspective, the moon is 384,403 km away from the Earth (on average). If we were to bike to the moon at a speed of 10 km/h, it would take 4.4 years. Driving to the moon at a speed of 100 km/h would take 160 days. At the speed of light, it would take us 1.28 seconds.



Another example is the Sun. It is 500 light seconds, or 8 light minutes away. That means if the Sun suddenly exploded, we wouldn’t know for 8 minutes.

Here are the light distances of some things: Our solar system – 10 light hours

The Milky Way - 100 000 light years across

Keep in mind that the Milky Way is only one of millions of galaxies.

 **How is Distance Measured?**   Because all objects in space simply appear as points of light, techniques have been developed to measure these distances. Astronomers use a method called stellar parallax to measure the distance to stars less then 100 light years away. Parallax is essential ly the perceived change in location of an object seen from two different places. The two different positions of the observer, and the position of the object form a triangle, as you can see in the picture below. If the distance between the two points of observation are known, and the direction of the object as seen from each is measured, the distance from the person to the object can be measured. Read more below on Parallax.

**Parallax**
The law of parallax is how things are perceived from 2 different viewpoints.

Here is how you can try it yourself: hold your thumb just in front of your nose, and close one eye. Now use your other eye. The 2 views of your thumb should be different, even though your thumb isn’t moving. Now stretch your arm out, and close one eye and then use the other eye. The thumb still moves, but not as much as when it was closer. Closer objects have a large parallax, while further objects have a smaller parallax. Many animals, including humans have 2 eyes so they can use parallax to gain depth perception. Our brain subconsciously uses data from both eyes to measure distances. Try playing basketball with one eye closed.

We can use parallax to find the distance of stationary objects like stars. Since the Earth is on the opposite side of the Sun every 6 months, we take a picture of the star we’re measuring in February, and then another picture in August. We then see how much the star has moved. If we know the distance between where we were 6 months ago and where we are now, and if we know the parallax angle, then we have enough information to figure out the distance to the object. The more it has moved, the closer it is.

**Kepler’s Third Law**
This law is about the relationship between the time it takes to orbit the sun, and the distance from the planet to the sun. The time it takes to orbit the sun is called a “period”. Kepler’s Third Law states that the orbital period squared is equal to the radius of the orbit cubed.

In simpler words, this means that “the time it takes to go around the sun” ² = “the distance from the planet to the sun” ³

This law is used to project the orbit of an object, which when combined with the distance of the object, can be used to find the mass of the object when you know Newton’s laws of gravity.

“Keplers Third Law”, was discovered by Johannes Kepler. Keplers law of planetary motion is a mathematical relation between distances, and the time it takes for planets to revolve around the sun. This is expressed by the equation Where the square of the time of one orbital period, is equal to the cube of its average radius.

 **T** **2** **= R** **3**


 * How is mass measured?**

Another popular method used to measure distance in space is the “Inverse Square” law. It is a mathematical way of explaining a simple law, light that is closer will shine brighter then light shining farther away. For example, if one star is twice as far away from each other, it will seem 4 times dimmer. The amount of dimming that occurs is relative distance squared. The equation below is used to measure distance, where B is brightness, L is luminosity, and D is distance from the source.

 http://science.jrank.org/pages/5020/Parallax-How-parallax-works.html http://www.hartrao.ac.za/other/howfar/howfar.html http://www.phys.lsu.edu/students/hodges/1109/lab3.pdf http://www.adler-n-subtract.com/qa.html?id=91 http://www.astro.umd.edu/resources/introastro/parallax.html http://en.wikipedia.org/wiki/Kepler%27s_laws_of_planetary_motion http://www.1728.com/kepler3.htm http://www.iki.rssi.ru/mirrors/stern/stargaze/Sappl3rd.htm**
 * Sources:

"Stellar Parallax." __Astronomy__. 30 May 2009 < [|__http://en.mimi.hu/astronomy/stellar_parallax.html__] >. "Parallax." 28 Apr. 2006. 29 May 2009 < [|__http://starchild.gsfc.nasa.gov/docs/StarChild/questions/parallax.html__] >. "Inverse Square Law." __Inverse Square Law__. 18 Feb. 2003. Joshua E. Barnes. 30 May 2009 < [|__http://www.ifa.hawaii.edu/~barnes/ASTR110L_S03/inversesquare.html__] >. __Kepler's 3rd Law__. Software Systems. 30 May 2009 < [|__http://www.1728.com/kepler3.htm__] >.