488 reputation
7
bio website dotancohen.com
location
age 36
visits member for 11 months
seen 7 hours ago

Sep
16
comment How many astronomers are there in the world today?
Would you consider me an astronomer? I'm a software developer by trade, with no formal astronomy education besides university physics. I'll tell you the names, distances, masses, and other information of most celestial objects with an absolute magnitude 3 or higher. I'll teach you quite a bit about theories including orbital mechanics, astrophysics, gravitation, and also about practicalities such as historical and current spacecraft and missions. If you consider me as an astronomer despite no formal training, then any estimate of amount of astronomers goes way up, there are many more like me.
Sep
4
comment How is space-time bent by black holes?
Exactly. For spherical objects the gravity felt depends on only the mass of the object and the distance from it's center (squared). Black holes are assumed to be spherical.
Sep
4
comment How is space-time bent by black holes?
Actually, you can get nearer to the geometric center of the mass, which accounts for the higher local gravity field (and tidal forces). Note that at the same distance from the (center) of the black hole and the (center) of the star, the black hole's gravity will be lower.
Jul
28
comment Escaping a black hole
Mitchell, you might want to accept the answer as given. It answers your question very well.
Jun
10
comment How does the evolution of a solar system not break the second law of thermodynamics?
The statement "the radiated photons are out there somewhere" really does make it clear.
Jun
10
comment How does the evolution of a solar system not break the second law of thermodynamics?
So where was the corresponding increase in entropy to offset the creation of the solar system? Heat loss?
May
5
comment What would happen if an ice cube is left in space?
This is a terrific answer.
May
1
comment Would a Phobos full impact make Mars temporary habitable?
I am intrigued and I wish to subscribe to your newsletter.
Dec
27
comment How would the night sky look from inside a globular cluster?
The Asimov story Nightfall describes this very situation.
Dec
16
comment Doesn't gravity attract objects in space until they collide?
@Envite: Thanks. I edited the post to remove that simplification with a better, more accurate, simplification.
Dec
9
comment Could any known, living organisms on Earth survive on Mars?
No problem! I'm also hoping to see some other interesting answers.
Nov
24
comment Can I see comet ISON from Saudi Arabia?
Very nice, Astro! Thanks!
Nov
24
comment How would I measure that I'm at a pole?
That is a beautiful last picture, especially how the trails and the roofline are parallel.
Nov
21
comment Doesn't gravity attract objects in space until they collide?
@astromax: Right, but that is in addition to the momentum component of the orbiting body (assuming m<<M). Otherwise, as the OP stated, the objects would collide.
Nov
21
comment Can I see comet ISON from Saudi Arabia?
On what basis do you calculate the absolute magnitude of the cometary system? Surely not as a rocky object I hope! Don't forget that the coma and tail are mostly empty space, thus there is no crescent and the light is very diffuse. You would need to know the amount of particles in the material as well, which we don't know. Even the albedo of the particles we don't know as we don't know which type of particles there will be (ice, methane, etc.).
Nov
21
comment Can I see comet ISON from Saudi Arabia?
@AstroFloyd: Please put that up on Github! I would love to see that, no matter what language it is written in.
Nov
21
comment Can I see comet ISON from Saudi Arabia?
I also highly recommend Stellarium.
Nov
21
comment Can I see comet ISON from Saudi Arabia?
By 'most visible' I infer the greatest visual magnitude difference between light originating from the comet components (nucleus, coma, tail) to between sunlight reflected from out atmosphere or direct from the Sun.
Nov
21
comment How would I measure that I'm at a pole?
Thank you! I have read how Amundsen measured the Sun's altitude while searching for the South Pole, so obviously there exists a daytime method as well. However, I believe that he know which day of the year he was in, so he would know the expected changes in altitude due to the Earth's orbit around the sun.
Nov
17
comment Might Oort cloud comets be exchanged between solar systems?
@Rory: Congrats on the 1000!