Starship Asterisk*

by **Joe Stieber** » Thu May 12, 2016 6:11 pm

neufer wrote:
Joe Stieber wrote:
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.
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.

Mercury is 2,439.7 km radius per Wikipedia.

Oops -- I need to clean my reading glasses or something! Anyway, with the correct size of Mercury, that would make the sun about 81,300 times the area of Mercury, or about 1.2 parts in 100,000 -- nominally 1 in 100,00 like Neufer said in the first place

by **neufer** » Thu May 12, 2016 5:46 pm

Joe Stieber wrote:
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.
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.

Mercury is 2,439.7 km radius per Wikipedia.

Joe Stieber wrote:
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).

Saudi Arabia & the Sahara are the biggest (non cloud/ice) brightness features
and can both be observed simultaneously near maximum elongation.

(Saudi Arabia & the Sahara could also be viewed in the 10 micron thermal IR window.)

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).

by **neufer** » Thu May 12, 2016 2:07 pm

Stewart Coulter wrote:
Jimbo Jones wrote:
Would Kepler have been able to detect this transit give a sizeable distance from the Sun? Say 40 LY?
Kepler was sensitive to 1 part in 100 000 and Mercury's disc covered 4 parts in 100 000
of the sun's disc so yes, it would have picked it up.

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.

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.

: [b][color=#FF0000]Sahara DSCOVR/EPIC Earth Polychromatic Imaging Camera[/color][/b]

: [b][color=#0000FF]Pacific DSCOVR/EPIC Earth Polychromatic Imaging Camera[/color][/b]

by **Stewart Coulter** » Thu May 12, 2016 1:39 pm

Jimbo Jones wrote:Curious...Would Kepler have been able to detect this transit give a sizeable distance from the Sun? Say 40 LY?

Kepler was sensitive to 1 part in 100 000 and Mercury's disc covered 4 parts in 100 000 of the sun's disc so yes, it would have picked it up. 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.

by **bls0326** » Thu May 12, 2016 12:08 pm

Such a tiny dot. Based on diameters, something the size of the Earth at the same view would only be 2 1/2 times this tiny dot. Not much bigger.

by **Jimbo Jones** » Thu May 12, 2016 11:21 am

Curious...Would Kepler have been able to detect this transit give a sizeable distance from the Sun? Say 40 LY?

by **Peter Varhegyi** » Thu May 12, 2016 7:09 am

It was the second of 14 transits of the Solar System's innermost planet in the 21st century?
The 3rd was!
http://eclipse.gsfc.nasa.gov/transit/ca ... talog.html

by **APOD Robot** » Thu May 12, 2016 4:08 am

A Transit of Mercury

Explanation: On May 9, the diminutive disk of Mercury spent about seven and a half hours crossing in front of the Sun as viewed from the general vicinity of Earth. It was the second of 14 transits of the Solar System's innermost planet in the 21st century. Captured from Fulham, London, England, planet Earth the tiny silhouette shares the enormous solar disk with prominences, filaments, and active regions in this sharp image. But Mercury's round disk (left of center) appears to be the only dark spot, despite the planet-sized sunspots scattered across the Sun. Made with an H-alpha filter that narrowly transmits the red light from hydrogen atoms, the image emphasizes the chromosphere, stretching above the photosphere or normally visible solar surface. In H-alpha pictures of the chromosphere, normally dark sunspot regions are dominated by bright splotches called plages.

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