by Joe Stieber » Thu May 12, 2016 4:34 pm
neufer wrote:
Mercury's disc covers ~4 parts in 100 000 of the sun's disc from Earth's 'close up view'
but it only covers ~1 part in 100 000 of the sun's disc from 40 light years out.
Given that during the transit, Mercury was 12.0 arc seconds in diameter and the sun was 1900.8 arc seconds in diameter as viewed from earth, the sun presented 25,090 times as much area as Mercury, so the nominal 4 parts in 100,000 is correct.
However, from a distance of light years, we could simply compare the apparent area based on their physical diameter. The sun is 695,700 km radius and Mercury is 2,939.7 km radius per Wikipedia, so the sun would present 56,006 times the area of Mercury. That's 1.8 parts in 100,000 for Mercury compared to the sun.
Stewart Coulter wrote:
And if aliens were looking at us from that distance with our current technology they could measure the rotation period of Earth by observing the periodic contrast between the Pacific and Asia.
To do so, they would have to be able to observe the earth as an illuminated object detached from the sun. During a transit, the side of the earth facing them would not be illuminated (just as Mercury was a black dot during the recent transit).
[quote="neufer"]
Mercury's disc covers ~4 parts in 100 000 of the sun's disc from Earth's '[u]close up view[/u]'
but it only covers ~1 part in 100 000 of the sun's disc from 40 light years out.[/quote]
Given that during the transit, Mercury was 12.0 arc seconds in diameter and the sun was 1900.8 arc seconds in diameter as viewed from earth, the sun presented 25,090 times as much area as Mercury, so the nominal 4 parts in 100,000 is correct.
However, from a distance of light years, we could simply compare the apparent area based on their physical diameter. The sun is 695,700 km radius and Mercury is 2,939.7 km radius per Wikipedia, so the sun would present 56,006 times the area of Mercury. That's 1.8 parts in 100,000 for Mercury compared to the sun.
[quote="Stewart Coulter"]
And if aliens were looking at us from that distance with our current technology they could measure the rotation period of Earth by observing the periodic contrast between the Pacific and Asia.[/quote]
To do so, they would have to be able to observe the earth as an illuminated object detached from the sun. During a transit, the side of the earth facing them would not be illuminated (just as Mercury was a black dot during the recent transit).