Aristarchus' Hypotheses

[Robdebob]

Hello everyone,

My name is Robert, I'm from Brisbane Australia, and I have recently sparked an interest in science. I have been reading about the early astronomers - in particular the work of Aristarchus - and I have a question regarding his third hypothesis:

3) When the moon appears to us halved, the great circle which divides the dark and bright portions of the moon is in the direction of our eye.

I don't understand what Aristarchus meant by "the great circle which devides the dark and bright portions of the moon is in the direction of our eye."

I hope someone can simplify for me this phrase as it is turning out to be quite cryptic in my mind!

Kind regards

Robert

[mikerr]

Robert, Welcome to IIS. Does this explain it?

When the moon appears to us halved, the great circle parallel to the circle which divides the dark and the bright portions in the moon is then in the direction of our eye; that is to say, the great circle parallel to the dividing circle and our eye are in one plane.

From here....

http://perseus.mpiwg-berlin.mpg.de/GreekScience/Students/Kristen/propositions.html

Michael.

[LAW]

The 'terminator', or circumference which is scribed by light and dark, is aligned with us, rather than being perpendicular at 'full' and 'new' moons.

[StutzJr]

An interesting observation I was able to make - having taken the opportunity to subscribe to the "spot the station" email alert from NASA. I was watching the ISS pass overhead on a night with a half moon and it was moving across the sky in a direction toward the moon. As bright as it was, just as the ISS crossed a line level with the light/dark terminator on the moon, it completely disappeared from view.