Comments on: Beautiful photos…

By: ER

ER — Sat, 04 May 2013 20:13:29 +0000

He probably closed the shutter while he lit a cigarette. Seriously, you often see these gaps in time exposures when the astrophotographer needs to save the picture from temporary light pollution, like a vehicle driving up, or the brief use of a flashlight while adjusting his equipment or consulting a chart.

That gap appears in every star trail, so by overlaying a clear acetate over the image and marking each gap, you would get a star map of the circumpolar regions with recognizable constellation patterns.

For example, if a meteor flashed across the sky during his time exposure, by interrupting it he could get a fairly accurate plot of its path through through the star field.

By: podrock

podrock — Sat, 04 May 2013 19:20:40 +0000

From Boulder's own Bad Astronomer: http://www.slate.com/blogs/bad_astronomy/2013/02/27/spinning_black_hole_scientists_measure_supermassive_black_hole_rotating.html

By: ER

ER — Sat, 04 May 2013 18:22:47 +0000

…problems in angular velocity and momentum are more conveniently done by working in radian measure.

A radian is an arc of a circle equal in length to the radius of that circle. A radian is about 57.3 degrees. It is defined that way so that there are 2 x Pi radians in a full circle. That is, 1 revolution = 360 degrees = 6.28 radians.

If a spinning object of radius 1 gazinga* is spinning at 100 RPM, a point on its edge has a velocity of 628 gazingas
per minute. The relationship is linear, so if you double the size of the object, the linear velocity of the point also doubles.

*The gazinga is the Klingon unit of length.

By: ER

ER — Sat, 04 May 2013 17:31:17 +0000

if you make it big enough the outer edge can be made to go as fast as you like–in theory.

In reality, if something gets moving fast enough, it gets more massive, making it require more energy than you’d expect to make it speed up, due to relativistic reasons. A rotating object spinning at relativistic speeds could not remain rigid,

But gas clouds funneling into black holes have been clocked at enormous velocities.

The earth is 8000 miles in diameter, and rotates in 24 hours.

The sun is 800,000 miles in diameter and rotates in 25 days.

The earth is 193 million miles from the sun and revolves around it in a year.

The sun is 30,000 light years from the galactic nucleus and revolves around it once in 250 million years.

By: RobVG

RobVG — Sat, 04 May 2013 16:32:49 +0000

I’ve always loved the time exposure pics of the circumpolar stars. In “Staring at Polaris” there is a short break about half way through the process. It makes for a more interesting picture and I wounder if it was intentional.

I know it’s elementary, but for some reason I’ve always found the relationship between rpm and and speed fascinating. The stars at the outside of the pic travel farther in the same amount of time than stars near the center. Hence they travel faster.

In fact, if you built a wheel in space with a 5000 mile radius and somehow managed to have enough energy to get it spinning at 360 rpm then the outside of the wheel would be traveling faster than the speed of light.

I posted that idea when I first got here. They shot it down because of some nonsense about the contraption approaching infinite mass as it neared “C”.