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### Course: NASA > Unit 2

Lesson 2: Measuring the solar system- A flat earth
- Arc length
- Circumference of Earth
- Occultations
- Occultation vs. transit vs. eclipse
- Size of the moon
- Angular measure 1
- Angular measure 1
- Trigonometric ratios in right triangles
- Angular Measure 2
- Angular Measure 2
- Intro to parallax
- Parallax: distance
- Parallax method
- Solar distance
- Solve similar triangles (advanced)
- Size of the sun
- Scale of solar system

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# Solar distance

## A burning question

Remember Aristarchus of Samos? He was an early Greek astronomer who suggested that the Earth orbited around the Sun (a heliocentric model). He also answered a fascinating question: how far away is the Sun?

He realized that we could figure out the distance to the sun relative to the distance to the moon. He noted that the sun, moon, and earth form a right triangle (right angle at the moon) during the first and last quarter moons.

A quarter moon occurs when the moon appears to be half illuminated from our vantage point. When the moon is seen to be exactly in the first quarter phase the sun-moon-earth angle is exactly 90 degrees. This means a line drawn from Aristarchus’ position to the moon and from the moon to the Sun formed a right angle.

Aristarchus simply needed to calculate the angle x. That is equal to how far he would have to tilt his telescope between the moon and sun (if telescopes had been invented yet). He calculated this angle as 87 degrees, though the true value is closer to

**89.83 degrees**(a difficult measurement to get right with the naked eye!).We already know the distance to the moon is 384,400 km. Finally we have all the information needed! We apply the cosine function to find the

**hypotenuse of the triangle**which is the**distance to sun**:### cos(angle) = adjacent / __hypotenuse__

### cos(89.83) = moon distance / __sun distance__

__sun distance__ = 384,400 / cos(89.83)

__sun distance__ = 129,556,058 km

This is estimate off by 13.3% which is quite good for a rough approximation. Compare this to the actual value:

### Average distance to the Sun: 149,600,000 km

This is the basis for the astronomical unit (abbreviated as AU) which is based on the Earth-Sun distance.

### 1 AU = 149 597 871 km

**Challenge question:**How could we measure the

**size of the sun**using this new information?

hint: we need to review

*similar triangles*. Let's do that now!## Want to join the conversation?

- Are there scientific terms for models of our solar system with, say, Venus or Jupiter or Pluto in the center? Could someone give me a list of names for all the official models?(1 vote)
- I don't think there are already such names, because believe that's true. But such names are made of the Latin name of the planet, connected to centric with an "o" in the middle to connect the parts to one word, like geo-centric, helios-centric becomes heliocentric

so venocentric, ...(1 vote)

- What i couldn't understand is why would you show us th AU in those exercises without defining it ? and instead you're defining it now , there should be an order .(0 votes)
- what if the barrier of earth broke? would the world burn into flames?(0 votes)