Scientists on Alpha Centauri have big telescope and observe earth [Plus cool earth-moon transit picture]?

Question

Scientist on Alpha Centauri have built a very large telescope. While observing the planets surrounding "Sol," they observe the short term darkening of a planet they named earth. They surmise that a moon passed in front of the planet. How would they calculate the relative size of the moon to the planet?

Here is the very cool picture of the moon transiting the earth that I promised: http://antwrp.gsfc.nasa.gov/apod/image/0809/epoxi4panel800.jpg

Answer 1

Well the Moon's size was originally determined by the shadow casted on it from the Earth during an eclipse. We didn't know exactly how big the Earth was, but we deduced the ratio between us and the Moon. When looking at the Earth you'll see the Sun's light in a similar arch, and again on the Moon. Here you will be able to determine the ratio between our star the Sun, Earth, and the Moon. One would just need to determine the size of the Sun to complete the equation.

Answer 2

By the percentage of darkening and the rate of light change.

As a first approximation the area of the planet and the area of the moon would define the light reduction percentage.

Answer 3

FYI:

That picture was not taken by the scientist on Alpha Centauri.

That was taken by a probe tha went to the comet Tempel 1.

That probe was called Deep Impact Probe.

Answer 4

Theres no humans on alpha centuari, its a star over 4 light years away.

Answer 5

Poor scientists. They'll probably get eaten by Mind Worms. ;-)

Answer 6

Well, first they'd have to plot the period of the eclipse.  It seems to occur every 28 Earth rotations.  Figuring the mass of both planets to be about equal to each other, they determine that the distance from the Earth to it's moon is about 238,000 miles. (Strange they use 'miles', huh...)  Anyway, they now know that the moon orbits the Earth at:

(2 X 238,000 x pi) / 28 =  1,495,368 / 28 = 53,405 miles per day, or about 2,225 miles per hour (They use hours, too...)

Once they've determined the orbital period of the moon, then they'd have to note when the light from Earth first darkens, to when it becomes completely visible again. From their vantage point, it seems to take about 1 hour for the Earth's light to dim & stablize, then about 3 more hours for the Earth's light to remain dim, then 1 more hour for it's light to begin to regain it's strength.  

So, that means that the moon is about 2000 miles in diameter, as it takes 1 hour to get in front of the Earth, 3 hours to cross, and 1 hour to reveal the Earth.  Since the Moon is about 2000 miles in diameter, and it's moving about 2000 miles per hour, that means that the Earth is about 4 times larger, or around 8000 miles in diameter.  

Answer 7

Like what ChrisCT said that picture was taking by the Deep Impact Probe from 31 million miles away from the Earth.