Earthshine

I saw this article on the BBC website today: Measurements of light reflected off the Earth show how to look for evidence of oceans on exoplanets.

This text is a bit unclear because I was not sure where she was physically located,

...she (Sally Langford of the University of Melbourne in Australia) measured the earthshine reflected from the Indian Ocean and then, as the Earth rotated, from the east coast of Africa.

From a Time magazine article, a little more clearly

Over the course of each evening, Langford watched the earthshine brighten dramatically when sunlight bounced off the Indian Ocean and dim as the African continent rotated into view.

So, sitting in the dark in Melbourne, she watched the dark sides of the moon change brightness as the earth rotated. I imagine that she must have used some geometry to calculate when the sun and the moon were situated at angles at which one could see specular reflection from the ocean. If she is in Australia and the earth rotates to the east, that means that the setting sun which she observes , will be at noon over the Indian ocean. So, when it is dark over Australia, the sun is still shining over the Indian Ocean. And then several hours later, it is shining over Africa. Melbourne is near 140 degrees E of the Greenwich Meridian (GM) . There's about a 6 hour difference from there and the middle of the Indian Ocean (80 degees E). Sahara is about 10 degrees E which is 7 hours further. (Yes, I used a globe to work this out.).

I found other articles which describe what I have written here. I did this mostly to excersize my own thinking.